've Immer Bekannten Chemischen Chemie Klassischen Periode Alphabetische Verzeichnis Der Chemischen

've immer bekannten chemischen Chemie klassischen Periode alphabetische Verzeichnis der chemischen Harold C. Urey (1893-1981), FG Brickwedde (1903-1989), GM & Murphy (1903-1968) Ein Wasserstoff-Isotope der Masse 2 Physical Review 39, 164-165 (1932). Copyright 1932 von der American Physical Society, reproduziert mit Erlaubnis. Das Proton-Elektron-Grundstück von bekannten Atomkerne zeigt einige ziemlich starke Regelmäßigkeiten zwischen Atomen niedriger Ordnungszahl. [1] Bis zu einem einfachen Schritt O16weise Figur erscheint, in die die nukleare Arten H2, H3 und He4 könnte sehr gut ausgestattet. Birge und Menzel [2] haben gezeigt, dass die Diskrepanz zwischen der chemischen Atomgewicht des Wasserstoffs und Aston Wert durch die Masse Spektrographen durch die Annahme einer WasserstoffIsotop der Masse 2 zeigen das Ausmaß der 1 Teil in 4500 Teile werden könnte entfielen Wasserstoff der Masse 1. Es ist möglich, mit Zuversicht die Berechnung der Dampfdrücke der reinen Stoffe H1H1, H1H2, H1H3, im Gleichgewicht mit dem reinen festen Phasen. Es ist nur notwendig, anzunehmen, dass in der Debye-Theorie der Festkörper-, θ umgekehrt proportional zur Quadratwurzel der Massen dieser Moleküle und die Rotations-und Schwingungs-Energie der Moleküle nicht in den Prozess der Verdampfung zu ändern. Diese Annahmen werden in Übereinstimmung mit etablierten experimentellen Beweis. Wir finden, dass die Dampfdrücke für diese Moleküle im Gleichgewicht mit den Feststoffen in das Verhältnis von p11: p12: p13 = 1:0.37:0.29 werden sollte. Die Theorie der flüssigen Zustand ist nicht so verstanden vell, aber es scheint vernünftig zu glauben, dass die Unterschiede in der Dampfdruck dieser Moleküle im Gleichgewicht mit ihren whould Flüssigkeiten recht groß zu sein und sollte es ermöglichen, eine rasche Konzentration der schwereren Isotopen, wenn sie vorhanden sind im Rückstand aus der einfachen Verdampfung von flüssigem Wasserstoff in der Nähe der Tripelpunkt. Dementsprechend sind zwei Proben wurden durch Verdampfen von Wasserstoff große Mengen von flüssigem Wasserstoff und sammeln das Gas, das aus dem letzten Teil der letzten Kubikzentimeter eingedampft vorbereitet. Die erste Probe wurde aus dem Endabschnitt von sechs Liter Flüssigkeit bei Atmosphärendruck verdampft gesammelt, und die zweite Probe aus vier Liter bei einem Druck verdampft nur wenige Millimeter über dem Tripelpunkt. Der Prozess der Verflüssigung hat wahrscheinlich keine Wirkung auf die Veränderung der Konzentration der Isotope, da keine merkliche Änderung wurde in der Probe verdampft beobachtet bei atmosphärischem Druck. Diese Proben wurden für die Atom-Spektren von H2 und H3 in einer Wasserstoff-Entladungsrohr in Holz so genannten "Black Stage" mit dem zweiten Ordnung eines 21 Fuß-Gitter mit einer Dispersion von 1,31 Å pro mm ausgeführt werden untersucht. Mit der Probe verdampft bei Siedetemperatur keine Konzentration so hoch geschätzt worden war festgestellt worden war. Dann stiegen die Forderungen, so dass das Verhältnis der Zeit der Exposition gegenüber dem Mindestmaß, um die H1-Linien auf unseren Tellern dich über 4500:1 war. Unter diesen Bedingungen fanden wir in diesem Beispiel als auch in normalen Wasserstoff schwache Linien auf der berechneten Positionen für die Linien H2 begleitenden Hβ, Hγ, Hδ. Diese Zeilen müssen allerdings nicht in der Wellenlänge mit einer molekularen Linien stimmen in der Literatur berichtet. [3] Allerdings waren sie so schwach, dass es schwierig war, sicher sein, dass sie nicht die Geister der stark überbelichtet Atomlinien. Die Probe von Wasserstoff in der Nähe der Tripelpunkt verdampft zeigt diese Linien stark verbessert, bezogen auf die Linien des H1, sowohl über die gewöhnlicher Wasserstoff-und der ersten Probe. Die relativen Intensitäten kann durch die Anzahl und Intensität der symmetrischen Geister auf die Platten gerichtet werden. Die Wellenlängen der H2-Linien, die auf diesen Platten leicht innerhalb von ca. 0,02 A gemessen werden. Die folgende Tabelle gibt den Mittelwert der beobachteten Verschiebungen dieser Linien von denen der H1 und der berechneten Verschiebungen: Line

Hα Hβ Hγ Hδ Δλ calc. 1,793 1,326 1,185 1,119 Δλ obs. Ordentliche Wasserstoff -1,346 1,206 1,145 1. Stichprobe -1,330 1,119 1,103 2. Probe 1,820 1,315 1,176 -Die H2-Linien sind breit, wie es zu einer engen ungelöste Dubletten zu erwarten, aber sie sind nicht so breit und diffus wie die H1 Linien wahrscheinlich aufgrund der geringeren Dopplerverbreiterung. Obwohl ihre relativen Intensitäten, um die Geister der jeweiligen H1 Linien erscheinen nahezu konstant für eine Probe von Wasserstoff, sind sie nicht für ihre Geister Intensitäten im Vergleich zu den bekannten Geister für ihre Intensitäten sind nicht dasselbe im Falle des gewöhnlichen Wasserstoffs und des 1. Stichprobe, wie sie im Falle der zweiten Probe. Sie sind nicht molekularen Linien für sie nicht auf einer Platte mit dem Entladungsrohr in der "weißen Stadium" mit den molekularen Spektrum erweitert (H2γ getroffen wurde, als eine leichte Unregelmäßigkeit auf einem Mikrophotometer Kurve von dieser Platte zu finden). Schließlich die H2α Zeile ist in ein Wams mit einem Abstand von etwa 0,16 Å im Einvernehmen mit den beobachteten Trennung der H1α Linie gelöst. Die relative Häufigkeit in gewöhnlichem Wasserstoff zu urteilen aus der relativen Minimierung der Exposition beträgt etwa 1:4000, oder weniger, im Einvernehmen mit Birge und Menzel schätzt. Eine ähnliche Schätzung der Zahl in der zweiten Probe angegeben einer Konzentration von etwa 1 in 800. So eine spürbare Fraktionierung wurde gesichert, als von der Theorie erwartet. [4] keine Beweise für H3 gesichert ist, aber die Linien würden die Gebiete von unseren Tellern fallen, wenn der Lichthof ist schlecht. Die Destillation wurde auf dem Bureau of Standards durchgeführt von einem von uns (FGB), der die Fortsetzung der Fraktionierung mehr hochkonzentrierten Proben zu sichern. Die spektroskopische

Arbeit wurde an der Columbia University durchgeführt von den beiden anderen (HCU und GMM), die arbeiten an der molekularen Spektrum. Harold C. Urey F. G. Brickwedde G. M. Murphy Columbia University, New York, N. Y. Bureau of Standards, Washington, D. C.

5. Dezember 1931.

[1] Urey, J. Am. Chem. Soc. 53, 2872 (1931), Johnston, ibid., 53, 2866 (1931).

[2] Birge und Menzel, Phys. Rev. 37, 1669 (1931). [3] Gale, Monk und Lee, Astrophys. J. 57, 89 (1928); Finkelnburg, Z. Physik 52, 57 (1928); Connelly, Proc. Phys. Soc. 42, 28 (1929). [4] Keesom und van Dijk, Proc. Acad. Sci. Amsterdam 34, 52 (1931). Zurück zur Liste der ausgewählten historischen Papieren. Back to the top of Classic Chemie.

SIS SUS

batang

Frederick Soddy (1877-1956) Intra-Atomladung.

Nature 92, 399-400 (4. Dezember 1913)

Dass die Intra-Atomladung eines Elements wird durch seinen Platz im Periodensystem bestimmt nicht durch seine Atommasse, wie von A. van der Broek (Nature, 27. November, S. 372) geschlossen, ist stark von der bisherigen Unterstützung Verallgemeinerungen in Bezug auf die Radio-Elemente und die periodische Gesetz. Die aufeinander folgenden Vertreibung ein α-und β zwei Teilchen in drei radioaktiven Änderungen in beliebiger Reihenfolge bringt die Intra-Atomladung des Elements zurück zu ihrem ursprünglichen Wert, und das Element wieder in seinen ursprünglichen Platz in der Tabelle, obwohl seine Atommasse reduziert wird um vier Einheiten. Wir haben vor kurzem erhalten so etwas

wie einen direkten Beweis der Ansicht van der Broek, dass die intra-atomare Ladung des Kerns eines Atoms ist nicht eine rein positive Ladung, wie auf vorläufige Theorie Rutherford. aber ist der Unterschied zwischen einer positiven und einer negativen Ladung kleiner. Fajans, sich in seiner Abhandlung über die periodischen Verallgemeinerung (Physikal. Zeitschr., 1913, vol. Xiv., S. 131), lenkte die Aufmerksamkeit auf die Tatsache, dass die Veränderungen der chemischen Natur auf die Ausweisung von α-und β Teilchen daraus genau der gleichen Art wie in der gewöhnlichen elektrochemischen Veränderungen der Wertigkeit. Er zog daraus den Schluss, dass radio-aktiven

Änderungen müssen in der gleichen Region der atomaren Struktur wie gewöhnliche chemische Veränderungen, anstatt mit einem ausgeprägten inneren Bereich des Bauwerks oder "Kern auftreten", wie bisher angenommen. In meinem Vortrag auf der gleichen Verallgemeinerung, dass unmittelbar nach der Fajans veröffentlicht (Chem. News, 28. Februar), legte ich Wert auf die absolute Identität des chemischen Eigenschaften der verschiedenen Elemente besetzen den gleichen Platz im

Periodensystem. Eine einfache Folgerung aus dieser Sicht versorgte mich mit einem Mittel zur Kontrolle der Richtigkeit der Schlussfolgerung, dass Fajans Radio-Veränderungen und chemischen Veränderungen sind mit der gleichen Region der atomaren Struktur betroffen. Nach meiner Meinung seiner Schlussfolgerung wäre nichts anderes, als dass zum Beispiel, Uran in seiner vierwertigen uranous Verbindungen beteiligt sein müssen chemisch identisch sein mit und nicht trennbar von Thorium-Verbindungen. Bei Uranium-X, aus Uran ich durch Ausweisung eines α Teilchen gebildet, ist chemisch identisch mit Thorium, wie dies auch Ionium in der gleichen Weise vom Uran II gebildet. Uran X verliert zwei β Teilchen und geht zurück in Uran II, chemisch identisch mit Uran. Uranous Salze verlieren auch zwei Elektronen und in die mehr sechswertiges Uranylverbindungen weiterzugeben. Wenn diese Elektronen aus der gleichen Region des Atoms uranous Salze kommen sollte chemisch nicht trennbar von Thoriumsalze. Aber sie sind es nicht. Es besteht eine starke Ähnlichkeit in der chemischen Charakter zwischen uranous und Thoriumsalze, und ich fragte Herr Fleck zu prüfen, ob sie auf chemischem Wege trennen könnte, wenn gemischt werden, wobei das Uran unverändert bleiben überall in der uranous oder vierwertigen Zustand. Herr Fleck wird die Experimente gesondert bekannt geben, und ich bin ihm zu Dank verpflichtet für das Ergebnis, dass die zwei Klassen von Verbindungen können leicht durch Fraktionierung getrennt werden. Dies, so denke ich, beläuft sich auf ein Beweis dafür, dass die Elektronen als β-Strahlen aus einem Kern nicht liefern kann Elektronen oder zurückzukaufen, um sie aus dem Ring kommen vertrieben, obwohl dieser Ring ist zu gewinnen oder zu verlieren Elektronen von außen während der gewöhnlichen elektrochemischen Veränderungen fähig der Wertigkeit.

Ich halte Ansicht van der Broek, dass die Zahl, die die positive Netto-Ladung des Kerns der Zahl der der Stelle, die das Element befindet sich im Periodensystem, wenn alle möglichen Orte, von Wasserstoff bis Uran in der Reihenfolge angeordnet sind, als praktisch erwiesen, so ist Was den relativen Wert der Ladung für die Mitglieder des Ende der Sequenz, von Thallium, Uran, betroffen ist. Wir sind im Unklaren gelassen, um den absoluten Wert der Ladung, wegen der Zweifel über die genaue Zahl der "Seltenen Erden", die es gibt. Wenn wir davon ausgehen, dass all diese bekannt sind, den Wert für die positive Ladung des Kerns des Uran-Atoms ist etwa 90. Der Erwägung, dass, wenn wir die eher zweifelhaft Annahme, dass die periodischen fährt regelmäßig zu machen, hinsichtlich der Zahl der Orte, durch die Selten-Erd-Gruppe, und dass zwischen Barium und Radium, zum Beispiel, zwei komplette lange Zeit vorhanden ist, wird die Zahl 96. In jedem Fall ist es deutlich weniger als 120, die

Anzahl der Gebühr in Höhe eines halben Atomgewicht, wie es wäre, wenn der Kern der α-Partikel nur ausgefertigt. Sechs nukleare Elektronen sind dafür bekannt, in der Uran-Atom, das in seiner Veränderungen sechs β-Strahlen vertreibt existieren. Sind der Kern, der sich aus α Teilchen muss es dreißig oder vierundzwanzig nuklearen bzw. Elektronen, verglichen mit sechsundneunzig oder 102 bzw. in den Ring. Wenn, wie vorgeschlagen wurde, ist Wasserstoff eine zweite Komponente der atomaren Struktur, es muss mehr als diese. Aber es kann kein Zweifel, dass es einige werden muss, und dass die zentrale Ladung des Atoms auf die Theorie Rutherford kann nicht eine reine positive Ladung, sondern müssen Elektronen enthalten, wie van der Broek abschließt.

Soweit ich persönlich betroffen bin, hat dies in einer großen Klärung meiner Ideen geführt, und es kann hilfreich sein, andere, obwohl kein Zweifel daran gibt es wenig Originalität in ihm. Das gleiche algebraische Summe der positiven und negativen Ladungen im Kern, als die arithmetische Summe unterschiedlich ist, gibt, was ich "Isotope" oder "Isotopen-Elemente" nennen, weil sie den gleichen Platz im Periodensystem zu besetzen. Sie sind chemisch identisch, und speichern Sie nur im Hinblick auf die relativ wenigen physikalischen Eigenschaften, die auf Atommasse hängen direkt, auch körperlich identisch. Unit Veränderungen dieser Kernladung, so algebraisch rechnen, geben die nachfolgenden Plätze im Periodensystem. Für ein "Ort", oder ein Kernladung, mehr als eine Anzahl von Elektronen in der äußeren Ring-System bestehen können und in einem solchen Fall das Element weist variable Wertigkeit. Aber solche Veränderungen der Anzahl oder der Wertigkeit, beziehen sich nur auf den Ring und externen Umfeld. Es gibt keinen In-und Out-Gehen von Elektronen zwischen Ring und Kern. Frederick Soddy Laboratorium für Physikalische Chemie, University of Glasgow.

-------------------------------------------------- -----------------------------Zurück zur Liste der ausgewählten historischen Papieren. Back to the top of Classic Chemistry.The Streuung von α-und β Teilchen durch Materie und der Struktur des Atoms E. Rutherford, F.R.S. * Philosophical Magazine Series 6, vol. 21 Mai 1911, S. 669-688 -------------------------------------------------- -----------------------------669 § 1. Es ist bekannt, dass die α-und β die Teilchen Ablenkungen leiden unter ihrer geradlinigen Pfaden durch Begegnungen mit den Atomen der Materie. Diese Streuung ist viel stärker ausgeprägt als für die β für die α Teilchen wegen der viel kleiner Impuls und Energie der ehemaligen Teilchen. Es scheint keinen Zweifel daran, dass diese sich schnell bewegenden Teilchen durchlaufen die Atome in den Weg, und dass die Ablenkungen beobachtet werden durch das starke elektrische Feld im atomaren System durchlaufen. Es wurde allgemein angenommen, dass die Streuung der ein Büschel von α oder βStrahlen beim Durchgang durch eine dünne Platte der Materie das Ergebnis einer Vielzahl von kleinen Streuungen von den Atomen der Materie durchzogen ist. Die Beobachtungen zeigen jedoch, der Geiger und Marsden ** über die Streuung von α-Strahlen, dass einige der α Teilchen, etwa 1 in 20.000 bis einem mittleren Winkel von 90 Grad wurden im Vorbeigehen sich aber eine Schicht aus Gold-Folie über 0,00004 cm . dick, das entspricht in Anhalten-Macht der α Partikel bis 1,6 Millimeter Luft war. Geiger *** zeigte später, dass die wahrscheinlichste Winkel der Ablenkung für ein Büschel von α Teilchen abgelenkt über 90 Grad ist verschwindend klein. Darüber hinaus wird es später sehen, daß die Verteilung der Teilchen α für verschiedene Winkel der großen Ablenkung nicht folgen die Wahrscheinlichkeit Recht zu erwarten, wenn so große Ablenkung von einer großen Anzahl von kleinen Abweichungen nach oben vorgenommen werden. Es scheint vernünftig, anzunehmen, dass die Ablenkung durch einen großen Winkel zu einer einzelnen atomaren Begegnung zurückzuführen ist, für die Chance auf eine zweite Begegnung der Art, dass eine große Ablenkung produzieren, müssen in den meisten Fällen sehr gering sein. Eine einfache Rechnung zeigt, dass das Atom muss ein Sitz eines starken elektrischen Feldes, um eine so große Ablenkung in einer einzigen Begegnung zu produzieren. Kürzlich JJ Thomson **** hat einen Theorie * Mitgeteilt durch den Autor. Ein kurzer Bericht über dieses Papier wurde auf der Manchester Literary and Philosophical Society im Februar mitgeteilt, 1911. ** Proc. Roy. Soc. LXXXII, S. 495 (1909) *** Proc. Roy. Soc. LXXXIII, S. 492 (1910) **** Camb. Lit. & Phil Soc. xv pt. 5 (1910) 670 erklären, die Streuung von geladenen Teilchen beim Durchgang durch geringen Dicken der Materie. Das Atom soll der eine Anzahl N von negativ geladenen Blutkörperchen, begleitet von einer gleichen Menge von positiver Elektrizität aus gleichmäßig über eine Kugel verteilt. Die Ablenkung eines negativ geladenen Teilchen beim Durchgang durch das Atom ist auf zwei Ursachen zurückzuführen - (1) die Abstoßung der Blutkörperchen durch das Atom verteilt, und (2) die Anziehungskraft der positiven

Elektrizität im Atom. Die Ablenkung der Teilchen beim Durchgang durch das Atom soll klein sein, während die durchschnittliche Ablenkung, nachdem eine große Zahl m der Begegnung als übernahm [die Quadratwurzel] m · θ, wenn θ ist der durchschnittliche Ablenkung durch einen einzigen Atom. Es konnte gezeigt werden, dass die Anzahl N der Elektronen im Atom aus der Beobachtung der Streuung ableiten konnte, war experimentell untersucht Crowther * in einer späteren Arbeit werden. Seine Ergebnisse offenbar bestätigt, der die wichtigsten Schlussfolgerungen der Theorie, und er leitete, auf der Annahme, daß die positive Elektrizität Zeitpunkt unterbrochen war, dass die Zahl der Elektronen in einem Atom etwa dreimal sein Atomgewicht wurde. Die Theorie von JJ Thomson ist auf der Annahme, dass die Streuung durch eine einzige atomare Begegnung ist klein, und die besondere Struktur für das Atom nicht für eine sehr große Ablenkung der Durchmesser der Kugel von positiver Elektrizität zugeben, ist winzig im Vergleich mit dem Durchmesser des Einflussbereichs des Atoms. Da die α-und β Teilchen durchqueren das Atom, sollte es möglich sein, von einer engen Studie über die Art der Ablenkung, um eine Vorstellung von der Konstitution des Atoms, die Effekte zu produzieren Form beobachtet. In der Tat, die Streuung von High-Speed-geladene Teilchen durch die Atome der Materie ist eine der vielversprechendsten Methoden des Angriffs dieses Problems. Die Entwicklung der Szintillationsmethode zählen einzigen α Teilchen bietet außergewöhnliche Vorteile der Untersuchung, und die Untersuchungen von H. Geiger mit dieser Methode haben schon viel zu unserem Wissen über die Streuung von α-Strahlen von der Materie hat. § 2. Wir werden zunächst theoretisch untersucht die einzelnen Begegnungen ** mit einem Atom von einfacher Struktur, die in der Lage ist, * Crowther, Proc. Roy. Soc. LXXXIV. S. 226 (1910) ** Die Abweichung eines Teilchens in einem beträchtlichen Winkel von einer Begegnung mit einem einzigen Atom wird in diesem Papier die Bezeichnung "Single"-Streuung. Die Abweichung eines Teilchens, die sich aus einer Vielzahl von kleinen Abweichungen wird man als "Verbindung" Streuung. 671 produzieren große Verformungen eines α Teilchen, und vergleichen Sie dann die Folgerungen aus der Theorie mit den experimentellen Daten zur Verfügung. EnglishIndonesian—Detect language— AfrikaansAlbanianArabicBelarusianBulgarianCatalanChineseCroatianCzechDanishDutchEnglishEston ianFilipinoFinnishFrenchGalicianGermanGreekHebrewHindiHungarianIcelandicIndonesianIrishItalian JapaneseKoreanLatvianLithuanianMacedonianMalayMalteseNorwegianPersianPolishPortugueseRoma nianRussianSerbianSlovakSlovenianSpanishSwahiliSwedishThaiTurkishUkrainianVietnameseWelshYi ddish > GermanEnglish—AfrikaansAlbanianArabicBelarusianBulgarianCatalanChinese (Simplified)Chinese (Traditional)CroatianCzechDanishDutchEnglishEstonianFilipinoFinnishFrenchGalicianGermanGreek HebrewHindiHungarianIcelandicIndonesianIrishItalianJapaneseKoreanLatvianLithuanianMacedonian MalayMalteseNorwegianPersianPolishPortugueseRomanianRussianSerbianSlovakSlovenianSpanishS wahiliSwedishThaiTurkishUkrainianVietnameseWelshYiddish swap Translate web pages directly from your browser!Download Google Toolbar Contribute a better translation

Harold C. Urey (1893-1981), F. G. Brickwedde (1903-1989), & G. M. Murphy (1903-1968) A Hydrogen Isotope of Mass 2 Physical Review 39, 164-165 (1932). Copyright 1932 by the American Physical Society; reproduced with permission. The proton-electron plot of known atomic nuclei shows some rather marked regularities among atoms of lower atomic number.[1] Up to O16 a simple step-wise figure appears into which the nuclear species H2, H3 and He4 could be fitted very nicely. Birge and Menzel[2] have shown that the discrepancy between the chemical atomic weight of hydrogen and Aston's value by the mass spectrograph could be accounted for by the assumption of a hydrogen isotope of mass 2 present to the extent of 1 part in 4500 parts of hydrogen of mass 1. It is possible to calculate with confidence the vapor pressures of the pure substances H1H1, H1H2, H1H3, in equilibrium with the pure solid phases. It is only necessary to assume that in the Debye theory of the solid state, θ is inversely proportional to the square root of the masses of these molecules and that the rotational and vibrational energies of the molecules do not change in the process of vaporization. These assumptions are in accord with well-established experimental evidence. We find that the vapor pressures for these molecules in equilibrium with their solids should be in the ratio of p11:p12:p13 = 1:0.37:0.29. The theory of the liquid state is not so vell understood but it seems reasonable to believe that the differences in vapor pressure of these molecules in equilibrium with their liquids whould be rather large and should make possible a rapid concentration of the heavier isotopes, if they exist, in the residue from the simple evaporation of liquid hydrogen near its triple point. Accordingly two samples of hydrogen were prepared by evaporating large quantities of liquid hydrogen and collecting the gas which evaporated from the last fraction of the last cubic centimeter. The first sample was collected from the end portion of six liters of liquid evaporated at atmospheric pressure, and the second sample from four liters evaporated at a pressure only a few millimeters above the triple point. The process of liquefaction has probably no effect in changing the concentration of the isotopes since no appreciable change was observed in the sample evaporated at atmospheric pressure. These samples were investigated for the atomic spectra of H2 and H3 in a hydrogen discharge tube run in Wood's so-called "black stage" by using the second order of a 21 foot grating with a dispersion of 1.31 Å per mm. With the sample evaporated at the boiling point no concentration so high as had been estimated was detected. We then increased the exposures so that the ratio of the time of exposure to the minimum required to get the H1 lines on our plates was about 4500:1. Under these conditions we found in this sample as well as in ordinary hydrogen faint lines at the calculated positions for the lines of H2 accompanying Hβ, Hγ, Hδ. These lines do not agree in wavelength with any molecular lines reported in the literature.[3] However they were so weak that it was difficult to be sure that they were not ghosts of the strongly overexposed atomic lines. The sample of hydrogen evaporated near the triple point shows these lines greatly enhanced, relative to the lines of H1, over both those of ordinary hydrogen and of the first sample. The relative intensities can be judged by the number and intensity of the symmetrical ghosts on the plates. The wave-lengths of the H2 lines appearing on these plates could be easily measured within about 0.02 Å. The following table gives the mean of the observed displacements of these lines from those of H1 and the calculated displacements:

Line

Hα

Hβ

Hγ

Hδ

Δλ calc. 1.793 1.326 1.185 1.119 Δλ obs. Ordinary hydrogen -1.346 1.206 1.145 1st sample -1.330 1.119 1.103 2nd sample 1.820 1.315 1.176 -2 The H lines are broad, as is to be expected for close unresolved doublets, but they are not as broad and diffuse as the H1 lines probably due to the smaller Döppler broadening. Although their intensities relative to the ghosts of the respective H1 lines appear nearly constant for any one sample of hydrogen, they are not ghosts for their intensities relative to the known ghosts for their intensities are not the same in the case of ordinary hydrogen and of the 1st sample as they are in the case of the second sample. They are not molecular lines for they do not appear on a plate taken with the discharge tube in the "white stage" with the molecular spectrum enhanced (H2γ was found as a slight irregularity on a microphotometer curve of this plate). Finally the H2α line is resolved into a doublet with a separation of about 0.16 Å in agreement with the observed separation of the H1α line. The relative abundance in ordinary hydrogen, judging from relative minimum exposure time is about 1:4000, or less, in agreement with Birge and Menzel's estimate. A similar estimate of the abundance in the second sample indicated a concentration of about 1 in 800. Thus an appreciable fractionation has been secured as expected from theory.[4] No evidence for H3 has been secured, but its lines would fall on regions of our plates where the halation is bad. The distillation was carried out at the Bureau of Standards by one of us (F.G.B.), who is continuing the fractionation to secure more highly concentrated samples. The spectroscopic work was done at Columbia University by the other two (H.C.U. and G.M.M.) who are working on the molecular spectrum. Harold C. Urey F. G. Brickwedde G. M. Murphy Columbia University, New York, N. Y. Bureau of Standards, Washington, D. C. December 5, 1931. [1]Urey, J. Am. Chem. Soc. 53, 2872 (1931); Johnston, ibid., 53, 2866 (1931). [2]Birge and Menzel, Phys. Rev. 37, 1669 (1931). [3]Gale, Monk and Lee, Astrophys. J. 57, 89 (1928); Finkelnburg, Z. Physik 52, 57 (1928); Connelly, Proc. Phys. Soc. 42, 28 (1929). [4]Keesom and van Dijk, Proc. Acad. Sci. Amsterdam 34, 52 (1931). Back to the list of selected historical papers.

Back to the top of Classic Chemistry. Frederick Soddy (1877-1956) Intra-atomic Charge. Nature 92, 399-400 (December 4, 1913) That the intra-atomic charge of an element is determined by its place in the periodic table rather than by its atomic weight, as concluded by A. van der Broek (NATURE, November 27, p. 372), is strongly supported by the recent generalisation as to the radio-elements and the periodic law. The successive expulsion of one α and two β particles in three radio-active changes in any order brings the intra-atomic charge of the element back to its initial value, and the element back to its original place in the table, though its atomic mass is reduced by four units. We have recently obtained something like a direct proof of van der Broek's view that the intra-atomic charge of the nucleus of an atom is not a purely positive charge, as on Rutherford's tentative theory. but is the difference between a positive and a smaller negative charge. Fajans, in his paper on the periodic law generalisation (Physikal. Zeitsch., 1913, vol. xiv., p. 131), directed attention to the fact that the changes of chemical nature consequent upon the expulsion of α and β particles are precisely of the same kind as in ordinary electrochemical changes of valency. He drew from this the conclusion that radio-active changes must occur in the same region of atomic structure as ordinary chemical changes, rather than with a distinct inner region of structure or "nucleus," as hitherto supposed. In my paper on the same generalisation, published immediately after that of Fajans (Chem. News, February 28), I laid stress on the absolute identity of chemical properties of different elements occupying the same place in the periodic table. A simple deduction from this view supplied me with a means of testing the correctness of Fajans's conclusion that radio-changes and chemical changes are concerned with the same region of atomic structure. On my view his conclusion would involve nothing else than that, for example, uranium in its tetravalent uranous compounds must be chemically identical with and non-separable from thorium compounds. For uranium X, formed from uranium I by expulsion of an α particle, is chemically identical with thorium, as also is ionium formed in the same way from uranium II. Uranium X loses two β particles and passes back into uranium II, chemically identical with uranium. Uranous salts also lose two electrons and pass into the more hexavalent uranyl compounds. If these electrons come from the same region of the atom uranous salts should be chemically non-separable from thorium salts. But they are not. There is a strong resemblance in chemical character between uranous and thorium salts, and I asked Mr. Fleck to examine whether they could be separated by chemical methods when mixed, the uranium being kept unchanged throughout in the uranous or tetravalent condition. Mr. Fleck will publish the experiments separately, and I am indebted to him for the result that the two classes of compounds can readily be separated by fractionation methods. This, I think, amounts to a proof that the electrons expelled as β rays come from a nucleus not capable of supplying electrons to or withdrawing them from the ring, though this ring is capable of gaining or losing electrons from the exterior during ordinary electrochemical changes of valency.

I regard van der Broek's view, that the number representing the net positive charge of the nucleus is the number of the place which the element occupies in the periodic table when all the possible places from hydrogen to uranium are arranged in sequence, as practically proved so far as the relative value of the charge for the members of the end of the sequence, from thallium to uranium, is concerned. We are left uncertain as to the absolute value of the charge, because of the doubt regarding the exact number of rare-earth elements that exist. If we assume that all of these are known, the value for the positive charge of the nucleus of the uranium atom is about 90. Whereas if we make the more doubtful assumption that the periodic table runs regularly, as regards numbers of places, through the rare-earth group, and that between barium and radium, for example, two complete long periods exist, the number is 96. In either case it is appreciably less than 120, the number were the charge equal to one-half the atomic weight, as it would be if the nucleus were made out of α particles only. Six nuclear electrons are known to exist in the uranium atom, which expels in its changes six β rays. Were the nucleus made up of α particles there must be thirty or twenty-four respectively nuclear electrons, compared with ninety-six or 102 respectively in the ring. If, as has been suggested, hydrogen is a second component of atomic structure, there must be more than this. But there can be no doubt that there must be some, and that the central charge of the atom on Rutherford's theory cannot be a pure positive charge, but must contain electrons, as van der Broek concludes. So far as I personally am concerned, this has resulted in a great clarification of my ideas, and it may be helpful to others, though no doubt there is little originality in it. The same algebraic sum of the positive and negative charges in the nucleus, when the arithmetical sum is different, gives what I call "isotopes" or "isotopic elements," because they occupy the same place in the periodic table. They are chemically identical, and save only as regards the relatively few physical properties which depend on atomic mass directly, physically identical also. Unit changes of this nuclear charge, so reckoned algebraically, give the successive places in the periodic table. For any one "place," or any one nuclear charge, more than one number of electrons in the outer-ring system may exist, and in such a case the element exhibits variable valency. But such changes of number, or of valency, concern only the ring and its external environment. There is no in- and out-going of electrons between ring and nucleus. FREDERICK SODDY Physical Chemistry Laboratory, University of Glasgow.

-------------------------------------------------------------------------------Back to the list of selected historical papers. Back to the top of Classic Chemistry.The Scattering of α and β Particles by Matter and the Structure of the Atom E. Rutherford, F.R.S.* Philosophical Magazine

Series 6, vol. 21 May 1911, p. 669-688 -------------------------------------------------------------------------------669 § 1. It is well known that the α and the β particles suffer deflexions from their rectilinear paths by encounters with atoms of matter. This scattering is far more marked for the β than for the α particle on account of the much smaller momentum and energy of the former particle. There seems to be no doubt that such swiftly moving particles pass through the atoms in their path, and that the deflexions observed are due to the strong electric field traversed within the atomic system. It has generally been supposed that the scattering of a pencil of α or β rays in passing through a thin plate of matter is the result of a multitude of small scatterings by the atoms of matter traversed. The observations, however, of Geiger and Marsden** on the scattering of α rays indicate that some of the α particles, about 1 in 20,000 were turned through an average angle of 90 degrees in passing though a layer of gold-foil about 0.00004 cm. thick, which was equivalent in stopping-power of the α particle to 1.6 millimetres of air. Geiger*** showed later that the most probable angle of deflexion for a pencil of α particles being deflected through 90 degrees is vanishingly small. In addition, it will be seen later that the distribution of the α particles for various angles of large deflexion does not follow the probability law to be expected if such large deflexion are made up of a large number of small deviations. It seems reasonable to suppose that the deflexion through a large angle is due to a single atomic encounter, for the chance of a second encounter of a kind to produce a large deflexion must in most cases be exceedingly small. A simple calculation shows that the atom must be a seat of an intense electric field in order to produce such a large deflexion at a single encounter. Recently Sir J. J. Thomson**** has put forward a theory to * Communicated by the Author. A brief account of this paper was communicated to the Manchester Literary and Philosophical Society in February, 1911. ** Proc. Roy. Soc. lxxxii, p. 495 (1909) *** Proc. Roy. Soc. lxxxiii, p. 492 (1910) **** Camb. Lit. & Phil Soc. xv pt. 5 (1910) 670 explain the scattering of electrified particles in passing through small thicknesses of matter. The atom is supposed to consist of a number N of negatively charged corpuscles, accompanied by an equal quantity of positive electricity uniformly distributed throughout a sphere. The deflexion of a negatively electrified particle in passing through the atom is ascribed to two causes -- (1) the repulsion of the corpuscles distributed through the atom, and (2) the attraction of the positive electricity in the atom.

The deflexion of the particle in passing through the atom is supposed to be small, while the average deflexion after a large number m of encounters was taken as [the square root of] m · θ, where θ is the average deflexion due to a single atom. It was shown that the number N of the electrons within the atom could be deduced from observations of the scattering was examined experimentally by Crowther* in a later paper. His results apparently confirmed the main conclusions of the theory, and he deduced, on the assumption that the positive electricity was continuous, that the number of electrons in an atom was about three times its atomic weight. The theory of Sir J. J. Thomson is based on the assumption that the scattering due to a single atomic encounter is small, and the particular structure assumed for the atom does not admit of a very large deflexion of diameter of the sphere of positive electricity is minute compared with the diameter of the sphere of influence of the atom. Since the α and β particles traverse the atom, it should be possible from a close study of the nature of the deflexion to form some idea of the constitution of the atom to produce the effects observed. In fact, the scattering of high-speed charged particles by the atoms of matter is one of the most promising methods of attack of this problem. The development of the scintillation method of counting single α particles affords unusual advantages of investigation, and the researches of H. Geiger by this method have already added much to our knowledge of the scattering of α rays by matter. § 2. We shall first examine theoretically the single encounters** with an atom of simple structure, which is able to * Crowther, Proc. Roy. Soc. lxxxiv. p. 226 (1910) ** The deviation of a particle throughout a considerable angle from an encounter with a single atom will in this paper be called 'single' scattering. The deviation of a particle resulting from a multitude of small deviations will be termed 'compound' scattering. 671 produce large deflections of an α particle, and then compare the deductions from the theory with the experimental data available. Consider an atom which contains a charge ±Ne at its centre surrounded by a sphere of electrification containing a charge ±Ne [N.B. in the original publication, the second plus/minus sign is inverted to be a minus/plus sign] supposed uniformly distributed throughout a sphere of radius R. e is the fundamental unit of charge, which in this paper is taken as 4.65 x 10¯10 E.S. unit. We shall suppose that for distances less than 10¯12 cm. the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point. It will be shown that the main deductions from the theory are independent of whether the central charge is supposed to be positive or negative. For convenience, the sign will be assumed to be positive. The question of the stability of the atom proposed need not be considered at this stage, for this will obviously depend upon the minute structure of the atom, and on

the motion of the constituent charged parts. In order to form some idea of the forces required to deflect an alpha particle through a large angle, consider an atom containing a positive charge Ne at its centre, and surrounded by a distribution of negative electricity Ne uniformly distributed within a sphere of radius R. The electric force X and the potential V at a distance r from the centre of an atom for a point inside the atom, are given by

Suppose an α particle of mass m and velocity u and charge E shot directly towards the centre of the atom. It will be brought to rest at a distance b from the centre given by

It will be seen that b is an important quantity in later calculations. Assuming that the central charge is 100 e, it can be calculated that the value of b for an α particle of velocity 2.09 x 109 cms. per second is about 3.4 x 10¯12 cm. In this calculation b is supposed to be very small compared with R. Since R is supposed to be of the order of the radius of the atom, viz. 10¯8 cm., it is obvious that the α particle before being turned back penetrates so close to 672 the central charge, that the field due to the uniform distribution of negative electricity may be neglected. In general, a simple calculation shows that for all deflexions greater than a degree, we may without sensible error suppose the deflexion due to the field of the central charge alone. Possible single deviations due to the negative electricity, if distributed in the form of corpuscles, are not taken into account at this stage of the theory. It will be shown later that its effect is in general small compared with that due to the central field. Consider the passage of a positive electrified particle close to the centre of an atom. Supposing that the velocity of the particle is not appreciably changed by its passage through the atom, the path of the particle under the influence of a repulsive force varying inversely as the square of the distance will be an hyperbola with the centre of the atom S as the external focus. Suppose the particle to enter the atom in the direction PO (fig. 1), and that the direction of motion

on escaping the atom is OP'. OP and OP' make equal angles with the line SA, where A is the apse of the hyperbola. p = SN = perpendicular distance from centre on direction of initial motion of particle.

673 Let angle POA = θ. Let V = velocity of particle on entering the atom, ν its velocity at A, then from consideration of angular momentum pV = SA . ν. From conservation of energy (1/2)mV2 = (1/2)mν2 - (NeE / SA), ν2 = V2 (1 - (b / SA)). Since the eccentricity is sec θ, SA = SO + OA = p cosec θ(1 + cos θ) = p cot θ / 2 p2 = SA(SA - b) = p cot θ/2(p cot θ/2 - b), therefore b = 2p cot θ. The angle of deviation θ of the particles is π - 2θ and cot θ / 2 = (2p / b) * . . . . (1) This gives the angle of deviation of the particle in terms of b, and the perpendicular distance of the direction of projection from the centre of the atom. For illustration, the angle of deviation f for different values of p / b are shown in the following table: -p / b . . . . . 10 5 2 1 0.5 0.25 0.125

f . . . . . . . 5°.7 11°.4 28° 53° 90° 127° 152° § 3. Probability of single deflexion through any angle Suppose a pencil of electrified particles to fall normally on a thin screen of matter of thickness t. With the exception of the few particles which are scattered through a large angle, the particles are supposed to pass nearly normally through the plate with only a small change of velocity. Let n = number of atoms in unit volume of material. Then the number of collisions of the particle with the atom of radius R is πR2nt in the thickness t. * A simple consideration shows that the deflexion is unaltered if the forces are attractive instead of repulsive. 674 The probability m of entering an atom within a distance p of its center is given by m = πp2nt. Chance dm of striking within radii p and p + dp is given by dm = 2πpnt . dp = (π / 4)ntb2 cot f/2 cosec2 f/2 df . . . . (2) since cot f/2 = 2p / b The value of dm gives the fraction of the total number of particles which are deviated between the angles f and f + df. The fraction p of the total number of particles which are deflected through an angle greater than f is given by p = (π / 4)ntb2 cot2 f/2 . . . . . . (3) The fraction p which is deflected between the angles f1 and f2 is given by

p = (π / 4)ntb2 (cot2 f1/2 - cot2 f2/2) . . . . . . . . . . . . . (4) It is convenient to express the equation (2) in another form for comparison with experiment. In the case of the α rays, the number of scintillations appearing on the constant area of the zinc sulphide screen are counted for different angles with the direction of incidence of the particles. Let r = distance from point of incidence of α rays on scattering material, then if Q be the total number of particles falling on the scattering material, the number y of α particles falling on unit area which are deflected through an angle f is given by y = Qdm / 2πr2 sin f . df = (ntb2 . Q . cosec4 f/2) / 16r2 . . . . . . . (5) Since b = 2NeE / mu2, we see from this equation that the number of α particles (scintillations) per unit area of zinc sulphide screen at a given distance r from the point of 675 Incidence of the rays is proportional to (1) cosec4 f/2 or 1/f4 if f be small; (2) thickness of scattering material t provided this is small; (3) magnitude of central charge Ne; (4) and is inversely proportional to (mu2)2, or to the fourth power of the velocity if m be constant. In these calculations, it is assumed that the α particles scattered through a large angle suffer only one large deflexion. For this to hold, it is essential that the thickness of the scattering material should be so small that the chance of a second encounter involving another large deflexion is very small. If, for example, the probability of a single deflexion f in passing through a thickness t is 1/1000, the probability of two successive deflexions each of value f is 1/106 , and is negligibly small. The angular distribution of the α particles scattered from a thin metal sheet affords one of the simplest methods of testing the general correctness of this theory of single scattering. This has been done recently for α rays by Dr. Geiger,* who found that the distribution for particles deflected between 30° and 150° from a thin gold-foil was in substantial agreement with the theory. A more detailed account of these and other experiments to test the validity of the theory will be published later. § 4. Alteration of velocity in an atomic encounter It has so far been assumed that an α or β particle does not suffer an appreciable change of velocity as the result of a single atomic encounter resulting in a large deflexion of the particle. The effect of such

an encounter in altering the velocity of the particle can be calculated on certain assumptions. It is supposed that only two systems are involved, viz., the swiftly moving particle and the atom which it traverses supposed initially at rest. It is supposed that the principle of conservation of momentum and of energy applies, and that there is no appreciable loss of energy or momentum by radiation. * Manch. Lit. & Phil. Soc. 1910. 676 Let m be mass of the particle, ν1 = velocity of approach, ν2 = velocity of recession, M= mass of atom, V = velocity communicated to atom as result of encounter. Let OA (fig. 2) represent in magnitude and direction the momentum mν1 of the entering particle, and OB the momentum of the receding particle which has been turned through an angle AOB = f. Then BA represents in magnitude and direction the momentum MV of the recoiling atom. (MV)2 = (mν1)2 + (mν2)2 - 2m2ν1ν2 cos f . . . (1) By conservation of energy MV2 = mν12 - mν22 . . . . .(2) Suppose M/m = K and ν2 = pν1, where p < 1. From (1) and (2),

Consider the case of an α particle of atomic weight 4, deflected through an angle of 90° by an encounter with an atom of gold of atomic weight 197. Since K= 49 nearly,

or the velocity of the particle is reduced only about 2 per cent. by the encounter. In the case of aluminium K=27/4 and for f = 90° p = 0.86. It is seen that the reduction of velocity of the α particle becomes marked on this theory for encounters with the lighter atoms. Since the range of an α particle in air or other matter is approximately proportional to the cube of the velocity, it follows that an α particle of range 7 cms. has its range reduced to 4.5 cms. after incurring a single 677 deviation of 90° in traversing an aluminium atom. This is of a magnitude to be easily detected experimentally. Since the value of K is very large for an encounter of a β particle with an atom, the reduction of velocity on this formula is very small. Some very interesting cases of the theory arise in considering the changes of velocity and the distribution of scattered particles when the α particle encounters a light atom, for example a hydrogen or helium atom. A discussion of these and similar cases is reserved until the question has been examined experimentally.

§ 5. Comparison of single and compound scattering Before comparing the results of theory with experiment, it is desirable to consider the relative importance of single and compound scattering in determining the distribution of the scattered particles. Since the atom is supposed to consist of a central charge surrounded by a uniform distribution of the opposite sign through a sphere of radius R, the chance of encounters with the atom involving small

deflexions is very great compared with the change of a single large deflexion. This question of compound scattering has been examined by Sir J. J. Thomson in the paper previously discussed (§1). In the notation of this paper, the average deflexion f1 due to the field of the sphere of positive electricity of radius R and quantity Ne was found by him to be

The average deflexion f2 due to the N negative corpuscles supposed distributed uniformly throughout the sphere was found to be

The mean deflexion due to both positive and negative electricity was taken as (f12 + f22)1/2

In a similar way, it is not difficult to calculate the average deflexion due to the atom with a central charge discussed in this paper. Since the radial electric field X at any distance r from the 678 centre is given by

it is not difficult to show that the deflexion (supposed small) of an electrified particle due to this field is given by

Where p is the perpendicular from the center on the path of the particles and b has the same value as before. It is seen that the value of θ increases with diminution of p and becomes great for small value of f. Since we have already seen that the deflexions become very large for a particle passing near the center of the atom, it is obviously not correct to find the average value by assuming θ is small. Taking R of the order 10-8 cm., the value of p for a large deflexions is for α and β particles of the order 10-11 cm. Since the chance of an encounter involving a large deflexion is small compared with the chance of small deflexions, a simple consideration shows that the average small deflexion is practically unaltered if the large deflexions are omitted. This is equivalent to integrating over that part of the cross section of the atom where the deflexions are small and neglecting the small central area. It can in this way be simply shown that the average small deflexion is given by

This value of f1 for the atom with a concentrated central charge is three times the magnitude of the average deflexion for the same value of Ne in the type of atom examined by Sir J. J. Thomson. Combining the deflexions due to the electric field and to the corpuscles, the average deflexion is

It will be seen later that the value of N is nearly proportional to the atomic weight, and is about 100 for gold. The effect due to scattering of the individual corpuscles expressed by the second term of the equation is consequently small for heavy atoms compared with that due to the distributed electric field. 679 Neglecting the second term, the average deflexion per atom is 3πb / 8R. We are now in a position to consider the relative effects on the distribution of particles due to single and to compound scattering. Following J. J. Thomson's argument, the average deflexion θ after passing through a thickness t of matter is proportional to the square root of the number of encounters and is given by

where n as before is equal to the number of atoms per unit volume. The probability p1 for compound scattering that the deflexion of the particle is greater than f is equal to e-f2/ θt2. Consequently

Next suppose that single scattering alone is operative. We have seen (§3) that the probability p2 of a deflexion greater than f is given by p = (π / 4)b2 . n . t (cot2 f / 2) . By comparing these two equations p2 log p1= - 0.181f2 cot2 f / 2 , f is sufficiently small that tan f/2 = f/2, p2 log p1= -0.72

If we suppose that p2 = 0.5, then p1 = 0.24 If p2 = 0.1, then p1 = 0.0004 It is evident from this comparison, that the probability for any given deflexion is always greater for single than for compound scattering. The difference is especially marked when only a small fraction of the particles are scattered through any given angle. It follows from this result that the distribution of particles due to encounters with the atoms is for small thicknesses mainly governed by single scattering. No doubt compound scattering produces some effect in equalizing the distribution of the scattered particles; but its effect becomes relatively smaller, the smaller the fraction of the particles scattered through a given angle. 680 §6. Comparison of Theory with Experiments On the present theory, the value of the central charge Ne is an important constant, and it is desirable to determine its value for different atoms. This can be most simply done by determining the small fraction of α or β particles of known velocity falling on a thin metal screen, which are scattered between f and f + df where f is the angle of deflexion, The influence of compound scattering should be small when this fraction is small. Experiments in these directions are in progress, but it is desirable at this stage to discuss in the light of the present theory the data already published on scattering of α and β particles, The following points will be discussed: -(a) The 'diffuse reflexion' of α particles, i.e. the scattering of α particles through large angles (Geiger and Marsden.)

(b) The variation of diffuse reflexion with atomic weight of the radiator (Geiger and Marsden.) (c) The average scattering of a pencil of α rays transmitted through a thin metal plate (Geiger.) (d) The experiments of Crowther on the scattering of β rays of different velocities by various metals.

(a) In the paper of Geiger and Marsden (loc.cit.) on the diffuse reflexion of α particles falling on various substances it was shown that about 1/8000 of the α particles from radium C falling on a thick plate of platinum are scattered back in the direction of the incidence. This fraction is deduced on the assumption that the α particles are uniformly scattered in all directions , the observation being made for a deflexion of about 90°. The form of experiment is not very suited for accurate calculation, but from

the data available it can be shown that the scattering observed is about that to be expected on the theory if the atom of platinum has a central charge of about 100 e. In their experiments on this subject, Geiger and Marsden gave the relative number of α particles diffusely reflected from thick layers of different metals, under similar conditions . The numbers obtained by them are given in the table below, where z represents the relative number of scattered particles, measured by the of scintillations per minute on a zinc sulphide screen.

681 Metal Atomic weight z z / A3/2 Lead 207 62 208 Gold 197 67 242 Platinum 195 63 232 Tin 119 34 226 Silver 108 27 241 Copper 64 14.5 225 Iron 56 10.2 250 Aluminium 27 3.4 243 Average 233

On the theory of single scattering, the fraction of the total number of α particles scattered through any given angle in passing through a thickness t is proportional to n . A2t , assuming that the central charge is proportional to the atomic weight A. In the present case, the thickness of matter from which the scattered α particles are able to emerge and affect the zinc sulphide screen depends on the metal. Since Bragg has shown that the stopping power of an atom for an α particle is proportional to the square root of its atomic weight, the value of nt for different elements is proportional to 1 / [square root of] A . In

this case t represents the greatest depth from which the scattered α particles emerge. The number z of α particles scattered back from a thick layer is consequently proportional to A3/2 or z / A3/2 should be a constant. To compare this deduction with experiment, the relative values of the latter quotient are given in the last column . Considering the difficulty of the experiments, the agreement between theory and experiment is reasonably good.*

The single large scattering of α particles will obviously affect to some extent the shape of the Bragg ionization curve for a pencil of α rays. This effect of large scattering should be marked when the α rays have traversed screens of metals of high atomic weight, but should be small for atoms of light atomic weight.

(c) Geiger made a careful determination of the scattering of α particles passing through thin metal foils, by the scintillation method, and deduced the most probable angle

* The effect of change of velocity in an atomic encounter is neglected in this calculation. 682

through which the α particles are deflected in passing through known thickness of different kinds of matter. A narrow pencil of homogeneous α rays was used as a source. After passing through the scattering foil , the total number of α particles are deflected through different angles was directly measured. The angle for which the number of scattered particles was a maximum was taken as the most probable angle. The variation of the most probable angle with thickness of matter was determined, but calculation from these data is somewhat complicated by the variation of velocity of the α particles in their passage through the scattering material. A consideration of the curve of distribution of the α particles given in the paper (loc.cit. p. 498) shows that the angle through which half the particles are scattered is about 20 per cent greater than the most probable angle. We have already seen that compound scattering may become important when about half the particles are scattered through a given angle, and it is difficult to disentangle in such cases the relative effects due to the two kinds of scattering. An approximate estimate can be made in the following ways: -From (§5) the relation between the probabilities p1 and p2 for compound and single scattering respectively is given by p2 log p1= -0.721. The probability q of the combined effects may as a first approximation be taken as q = (p12 +p22)1/2.

If q = 0.5, it follows that p1 = 0.2 and p2 = 0.46 We have seen that the probability p2 of a single deflexion greater than f is given by p2 = (π / 4)n . t . b2 (cot2 f / 2) . Since in the experiments considered f is comparatively small

Geiger found that the most probable angle of scattering of the α rays in passing through a thickness of gold equivalent in stopping power to about 0.76 cm. of air was 1° 40'. The angle f through which half the α particles are tuned thus corresponds to 2° nearly. t = 0.00017 cm.; n = 6.07 x 1022; u (average value) = 1.8 x 109. E/m = 1.5 x 1014 E.S. units; e = 4.65 x 10-10, 683 Taking the probability of single scattering = 0.46 and substituting the above value in the formula, the value of N for gold comes out to be 97. For a thickness of gold equivalent in stopping power to 2.12 cms, of air, Geiger found the most probable angle to be 3° 40'. In this case, t = 0.00047, f = 4°.4, and average u =1.7 x 109, and N comes out to be 114. Geiger showed that the most probable angle of deflexion for an atom was nearly proportional to its atomic weight. It consequently follows that the value for N for different atoms should be nearly proportional to their atomic weights, at any rate for atomic weights between gold and aluminum.

Since the atomic weight of platinum is nearly equal to that of gold, it follows from these considerations that the magnitude of the diffuse reflexion of α particles through more than 90° from gold and the magnitude of the average small angle scattering of a pencil of rays in passing through gold-foil are both explained on the hypothesis of single scattering by supposing the atom of gold has a central charge of about 100 e. (d) Experiments of a Crowther on scattering of α rays. -- We shall now consider how far the experimental results of Crowther on scattering of β particles of different velocities by various materials can be explained on the general theory of single scattering. On this theory, the fraction of β particles p turned through an angel greater than f is given by p = (π / 4)n . t . b2 (cot2 f / 2) . In most of Crowther's experiments f is sufficiently small that tan f/2 may be put equal to f/2 without much error. Consequently f2 = 2πn . t . b2 if p =1/2 On the theory of compound scattering, we have already seen that the chance p1 that the deflexion of the particles is greater than f is given by

Since in the experiments of Crowther the thickness t of matter was determined for which p1 = 1/2, f2 = 0.96π n t b2. For the probability of 1/2, the theories of single and compound 684 scattering are thus identical in general form, but differ by a numerical constant. It is thus clear that the main relations on the theory of compound scattering of Sir J. J. Thomson, which were verified experimentally by Crowther, hold equally well on the theory of single scattering. For example, it tm be the thickness for which half the particles are scattered through an angle f, Crowther showed that f / [square root of] tm and also mu2 / E times [square root of] tm were constants for a given material when f was fixed. These relations hold also on the theory of single scattering.

Notwithstanding this apparent similarity in form, the two theories are fundamentally different. In one case, the effects observed are due to cumulative effects of small deflexion, while in the other the large deflexions are supposed to result from a single encounter. The distribution of scattered particles is entirely different on the two theories when the probability of deflexion greater than f is small. We have already seen that the distribution of scattered α particles at various angles has been found by Geiger to be in substantial agreement with the theory of single scattering, but can not be explained on the theory of compound scattering alone. Since there is every reason to believe that the laws of scattering of α and β particles are very similar, the law of distribution of scattered β particles should be the same as for α particles for small thicknesses of matter. Since the value of mu2 / E for β particles is in most cases much smaller than the corresponding value for the α particles, the chance of large single deflexions for β particles in passing through a given thickness of matter is much greater than for α particles. Since on the theory of single scattering the fraction of the number of particles which are undeflected through this angle is proportional to kt, where t is the thickness supposed small and k a constant, the number of particles which are undeflected through this angle is proportional to 1 - kt. From considerations based on the theory of compound scattering, Sir J.J. Thomson deduced that the probability of deflexion less than f is proportional to 1 - em / t where m is a constant for any given value of f. The correctness of this latter formula was tested by Crowther by measuring electrically the fraction I / Io of the scattered β particles which passed through a circular opening subtending an angle of 36° with the scattering material. If I / Io = 1 - 1 - em / t, the value of I should decrease very slowly at first with 685 increase of t. Crowther, using aluminium as scattering material, states that the variation of I / Io was in good accord with this theory for small values of t. On the other hand, if single scattering be present, as it undoubtedly is for α rays, the curve showing the relation between I / Io and t should be nearly linear in the initial stages. The experiments of Marsden* on scattering of β rays, although not made with quite so small a thickness of aluminium as that used by Crowther, certainly support such a conclusion. Considering the importance of the point at issue, further experiments on this question are desirable. From the table given by Crowther of the value f / [square root of] tm for different elements for β rays of velocity 2.68 x 10-10 cms. per second, the value of the central charge Ne can be calculated on the theory of single scattering. It is supposed, as in the case of the α rays, that for given value of f / [square root of] tm the fraction of the β particles deflected by single scattering through an angle greater than f is 0.46 instead of 0.5

The value of N calculated from Crowther's data are given below. Element Atomic weight f / [square root of] tm N Aluminium 27 4.25 22 Copper 63.2 10.0 42 Silver 108 29 138 Platnium 194 29 138 It will be remembered that the values of N for gold deduced from scattering of the α rays were in two calculations 97 and 114. These numbers are somewhat smaller than the values given above for platinum (viz. 138), whose atomic weight is not very different from gold. Taking into account the uncertainties involved in the calculation from the experimental data, the agreement is sufficiently close to indicate that the same general laws of scattering hold for the α and β particles, notwithstanding the wide differences in the relative velocity and mass of these particles. As in case of the α rays, the value of N should be most simply determined for any given element by measuring * Phil. Mag. xviii. p. 909 (1909) 686 the small fraction of the incident β particles scattered through a large angle. In this way, possible errors due to small scattering will be avoided. The scattering data for the β rays, as well as for the α rays indicate that the central charge in an atom is approximately proportional to its atomic weight. This falls in with the experimental deductions of Schmidt.* In his theory of absorption of β rays, he supposed that in traversing a thin sheet of matter, a small fraction α of the particles are stopped, and a small fraction β are reflected or scattered back in the direction of incidence. From comparison of the absorption curves of different elements, he deduced that the value of the constant β for different elements is proportional to nA2 where n is the number of atoms per unit volume and A the atomic weight of the element. This is exactly the relation to be expected on the theory of single scattering if the central charge on an atom is proportional to its atomic weight. §7. General Considerations In comparing the theory outlined in this paper with the experimental results, it has been supposed that the atom consists of a central charge supposed concentrated at a point, and that the large single deflexions of the α and β particles are mainly due to their passage through the strong central field. The

effect of the equal and opposite compensation charge supposed distributed uniformly throughout a sphere has been neglected. Some of the evidence in support of these assumptions will now be briefly considered. For concreteness, consider the passage of a high speed α particle through an atom having a positive central charge Ne, and surrounded by a compensating charge of N electrons. Remembering that the mass, momentum, and kinetic energy of the α particle are very large compared with the corresponding values of an electron in rapid motion, it does not seem possible from dynamic considerations that an α particle can be deflected through a large angle by a close approach to an electron, even if the latter be in rapid motion and constrained by strong electrical forces. It seems reasonable to suppose that the chance of single deflexions through a large angle due to this cause, if not zero, must be exceedingly small compared with that due to the central charge. It is of interest to examine how far the experimental evidence throws light on the question of extent of the Annal. d. Phys. iv. 23. p. 671 (1907) 687 distribution of central charge. Suppose, for example, the central charge to be composed of N unit charges distributed over such a volume that the large single deflexions are mainly due to the constituent charges and not to the external field produced by the distribution. It has been shown (§3) that the fraction of the α particles scattered through a large angle is proportional to (NeE)2, where Ne is the central charge concentrated at a point and E the charge on the deflected particles, If, however, this charge is distributed in single units, the fraction of the α particles scattered through a given angle is proportional of Ne2 instead of N2e2. In this calculation, the influence of mass of the constituent particle has been neglected, and account has only been taken of its electric field. Since it has been shown that the value of the central point charge for gold must be about 100, the value of the distributed charge required to produce the same proportion of single deflexions through a large angle should be at least 10,000. Under these conditions the mass of the constituent particle would be small compared with that of the α particle, and the difficulty arises of the production of large single deflexions at all. In addition, with such a large distributed charge, the effect of compound scattering is relatively more important than that of single scattering. For example, the probable small angle of deflexion of pencil of α particles passing through a thin gold foil would be much greater than that experimentally observed by Geiger (§ b-c). The large and small angle scattering could not then be explained by the assumption of a central charge of the same value. Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents. At the same time, the experimental evidence is not precise enough to negative the possibility that a small fraction of the positive charge may be carried by satellites extending some distance from the centre. Evidence on this point could be obtained by examining whether the same central charge is required to explain the large single deflexions of α and β particles; for the α particle must approach much closer to the center of the atom than the β particle of average speed to suffer the same large deflexion. The general data available indicate that the value of this central charge for different atoms is

approximately proportional to their atomic weights, at any rate of atoms heavier than aluminium. It will be of great interest to examine 688 experimentally whether such a simple relation holds also for the lighter atoms. In cases where the mass of the deflecting atom (for example, hydrogen, helium, lithium) is not very different from that of the α particle, the general theory of single scattering will require modification, for it is necessary to take into account the movements of the atom itself (see § 4). It is of interest to note that Nagaoka* has mathematically considered the properties of the Saturnian atom which he supposed to consist of a central attracting mass surrounded by rings of rotating electrons. He showed that such a system was stable if the attracting force was large. From the point of view considered in his paper, the chance of large deflexion would practically be unaltered, whether the atom is considered to be disk or a sphere. It may be remarked that the approximate value found for the central charge of the atom of gold (100 e) is about that to be expected if the atom of gold consisted of 49 atoms of helium, each carrying a charge of 2 e. This may be only a coincidence, but it is certainly suggestive in view of the expulsion of helium atoms carrying two unit charges from radioactive matter. The deductions from the theory so far considered are independent of the sign of the central charge, and it has not so far been found possible to obtain definite evidence to determine whether it be positive or negative. It may be possible to settle the question of sign by consideration of the difference of the laws of absorption of the β particles to be expected on the two hypothesis, for the effect of radiation in reducing the velocity of the β particle should be far more marked with a positive than with a negative center. If the central charge be positive, it is easily seen that a positively charged mass if released from the center of a heavy atom, would acquire a great velocity in moving through the electric field. It may be possible in this way to account for the high velocity of expulsion of α particles without supposing that they are initially in rapid motion within the atom. Further consideration of the application of this theory to these and other questions will be reserved for a later paper, when the main deductions of the theory have been tested experimentally. Experiments in this direction are already in progress by Geiger and Marsden. University of Manchester April 1911 Nagaoka, Phil. Mag. vii. p. 445 (1904). Pierre Curie (1859-1906) and Marie Sklodowska Curie (18671934) On a New Radioactive Substance Contained in Pitchblende[1] note by M. P. Curie and Mme. S. Curie, presented by M. Becquerel Comptes Rendus 127, 175-8 (1898) translated and reprinted in Henry A. Boorse and Lloyd Motz, eds.,

The World of the Atom, Vol. 1 (New York: Basic Books, 1966) Certain minerals containing uranium and thorium (pitchblende, chalcolite, uranite) are very active from the point of view of the emission of Becquerel rays. In a previous paper, one of us has shown that their activity is even greater than that of uranium and thorium, and has expressed the opinion that this effect was attributable to some other very active substance included in small amounts in these minerals.[2] The study of uranium and thorium compounds has shown in fact that the property of emitting rays which make the air conducting and which affect photographic plates, is a specific property of uranium and thorium that occurs in all compounds of these metals, being weaker in proportion as the active metal in the compound is diminished. The physical state of the substances appears to have an entirely secondary importance. Various experiments have shown that the state of mixture of these substances seems to act only to vary the proportions of the active bodies and the absorption produced by the inert substances. Certain causes (such as the presence of impurities) which have so great an effect on the phosphorescence or fluorescence are here entirely without effect. It is therefore very probable that if certain minerals are more active than uranium and thorium, it is because they contain a substance more active than these metals. We have sought to isolate this substance in pitchblende and experiment has just confirmed the preceding conjectures. Our chemical researches have been guided constantly by a check of the radiant activity of the separated products in each operation. Each product was placed on one of the plates of a condenser and the conductivity acquired by the air was measured with the aid of an electrometer and a piezoelectric quartz, as in the work cited above. One has thus not only an indication but a number which gives a measure of the strength of the product in the active substance. The pitchblende which we have analysed was approximately two and a half times more active than uranium in our plate apparatus. We have treated it with acids and have treated the solutions obtained with hydrogen sulfide. Uranium and thorium remain in solution. We have verified the following facts: The precipitated sulphides contain a very active substance together with lead, bismuth, copper, arsenic, and antimony. This substance is completely insoluble in the ammonium sulphide which separates it from arsenic and antimony. The sulphides insoluble in ammonium sulphide being dissolved in nitric acid, the active substance may be partially separated from lead by sulphuric acid. On washing lead sulfate with dilute sulphuric acid, most of the active substance entrained with the lead sulphate is dissolved. The active substance present in solution with bismuth and copper is precipitated completely by ammonia which separates it from copper. Finally the active substance remains with bismuth. We have not yet found any exact procedure for separating the active substance from bismuth by a wet

method. We have, however, effected incomplete separations as judged by the following facts: When the sulphides are dissolved by nitric acid, the least soluble portions are the least active. In the precipitation of the salts from water the first portions precipitated are by far the most active. We have observed that on heating pitchblende one obtains by sublimation some very active products. This observation led us to a separation process based on the difference in volatility between the active sulphide and bismuth sulphide. The sulphides are heated in vacuum to about 700° in a tube of Bohemian glass. The active sulphide is deposited in the form of a black coating in those regions of the tube which are at 250° to 300°, while the bismuth sulphide stays in the hotter parts. More and more active products are obtained by repetition of these different operations. Finally we obtained a substance whose activity is about four hundred times greater than that of uranium. We have sought again among the known substances to determine if this is the most active. We have examined compounds of almost all the elementary substances; thanks to the kindness of several chemists we have had samples of the rarest substances. Uranium and thorium only are naturally active, perhaps tantalum may be very feebly so. We believe therefore that the substance which we have removed from pitchblende contains a metal not yet reported close to bismuth in its analytical properties. If the existence of this new metal is confirmed, we propose to call it polonium from the name of the country of origin of one of us. M. Demarçay has been kind enough to examine the spectrum of the substance which we studied. He was not able to distinguish any characteristic line apart from those ascribable to impurities. This fact is not favourable to the idea of the existence of a new metal. However, M. Demarçay called our attention to the fact that uranium, thorium, and tantalum exhibit spectra formed of innumerable very fine lines difficult to resolve.[3,4] Allow us to note that if the existence of a new element is confirmed, this discovery will be uniquely attributable to the new method of detection that Becquerel rays provide.

-------------------------------------------------------------------------------[1]This work was done at the Municipal School of Industrial Physics and Chemistry. We particularly thank M. Bémont, head of chemical operations, for his advice and the assistance he willingly provided. --original note [2]Mme. P. Curie, Comptes Rendus, vol. 126, p. 1101. --original note [3]The peculiarity of these three spectra is described in the fine work of M. Demarçay, Electric Spectra (1895). --original note

[4]The excerpt in Boorse and Motz ends here. The remainder of the paper was translated by Carmen Giunta, as were the original footnotes.--CJG

-------------------------------------------------------------------------------Back to the list of selected historical papers. Back to the top of Classic Chemistry.Ernest Rutherford (1871-1937) & Frederick Soddy (1877-1956) The Cause and Nature of Radioactivity from Philosophical Magazine 4, 370-96 (1902) [as abridged and reprinted in Henry A. Boorse & Lloyd Motz, The World of the Atom, Vol. 1 (New York: Basic Books, 1966)] Introduction The following papers give the results of a detailed investigation of the radioactivity of thorium compounds which has thrown light on the questions connected with the source and maintenance of the energy dissipated by radioactive substances. Radioactivity is shown to be accompanied by chemical changes in which new types of matter are being continuously produced. These reaction products are at first radioactive, the activity diminishing regularly from the moment of formation. Their continuous production maintains the radioactivity of the matter producing them at a definite equilibrium-value. The conclusion is drawn that these chemical changes must be sub-atomic in character. The present researches had as their starting-point the facts that had come to light with regard to thorium radioactivity. Besides being radioactive in the same sense as the uranium compounds, the compounds of thorium continuously emit into the surrounding atmosphere a gas which it has been named, is the source of rays, which ionize gases and darken the photographic film.[1] The most striking property of the thorium emanation is its power of exciting radioactivity on all surfaces with which it comes into contact. A substance after being exposed for some time in the presence of the emanation behaves as if it were covered with an invisible layer of an intensely active material. If the thoria is exposed in a strong electric field, the excited radioactivity is entirely confined to the negatively charged surface. In this way it is possible to concentrate the excited radioactivity on a very small area. The excited radioactivity can be removed by rubbing or by the action of acids, as, for example, sulphuric, hydrochloric, and hydrofluoric acids. If the acids be then evaporated, the radioactivity remains on the dish. The emanating power of thorium compounds is independent of the surrounding atmosphere, and the excited activity it produces is independent of the nature of the substance on which it is manifested. These properties made it appear that both phenomena were caused by minute quantities of special kinds of matter in the radioactive state, produced by the thorium compound.

The next consideration in regard to these examples of radioactivity, is that the activity in each case diminishes regularly with the lapse of time, the intensity of radiation at each instant being proportional to the amount of energy remaining to be radiated. For the emanation a period of one minute, and for the excited activity a period of eleven hours, causes the activity to fall to half its value. ... The radioactivity of thorium at any time is the resultant of two opposing processes-The production of fresh radioactive material at a constant rate by the thorium compound; The decay of the radiating power of the active material with time. The normal or constant radioactivity possessed by thorium is an equilibrium value, where the rate of increase of radioactivity due to the production of fresh active material is balanced by the rate of decay of radioactivity of that already formed. It is the purpose of the present paper to substantiate and develope this hypothesis. The Rates of Recovery and Decay of Thorium Radioactivity A quantity of the pure thorium nitrate was separated from ThX ... by several precipitations with ammonia. The radioactivity of the hydroxide so obtained was tested at regular intervals to determine the rate of recovery of its activity. For this purpose the original specimen of .5 gram was left undisturbed throughout the whole series of measurements on the plate over which it had been sifted, and was compared always with .5 gram of ordinary de-emanated thorium oxide spread similarly on a second plate and also left undisturbed. The emanation from the hydroxide was prevented from interfering with the results by a special arrangement for drawing a current of air over it during the measurements. The active filtrate from the preparation was concentrated and made up to 100 c.c. volume. One quarter was evaporated to dryness and the ammonium nitrate expelled by ignition in a platinum dish, and the radioactivity of the residue tested at the same intervals as the hydroxide to determine the rate of decay of its activity. The comparison in this case was a standard sample of uranium oxide kept undisturbed on a metal plate, which repeated work has shown to be a perfectly constant source of radiation. The remainder of the filtrate was used for other experiments. The following table gives an example of one of a numerous series of observations made with different preparations at different times. The maximum value obtained by the hydroxide and the original value of the ThX are taken as 100: Time in days Activity of Hydroxide Activity of ThX 0 44 100 1 37 117 2 48 100 3 54 88 4 62 72 5 68 -6 71 53 8 78 --

9 -- 29.5 10 83 25.2 13 -- 15.2 15 -- 11.1 17 96.5 -21 99 -28 100 --

[Figure 1] shows the curves obtained by plotting the radioactivities as the ordinates, and the time in days as abscissae. Curve II. illustrates the rate of recovery of the activity of thorium, curve I. the rate of decay of the activity of ThX. It will be seen that neither of the curves is regular for the first two days. The activity of the hydroxide at first actually diminished and was at the same value after two days as when first prepared. The activity of the ThX, on the other hand, at first increases and does not begin to fall below the original value till after the lapse of two days. ... These results cannot be ascribed to errors of measurement, for they have been regularly observed whenever similar preparations have been tested. The activity of the residue obtained from thorium oxide by the second method of washing decayed very similarly to that of ThX, as shown by the above curve. If for present purposes the initial periods of the curve are disregarded and the later portions only considered, it will be seen at once that the time taken for the hydroxide to recover one half of its lost activity is about equal to the time taken by the ThX to lose half its activity, viz., in each case about 4 days, and speaking generally the percentage proportion of the lost activity regained by the hydroxide over any given interval is approximately equal to the percentage proportion of the activity lost by the ThX during the same interval. If the recovery curves is produced backwards in the normal direction to cut the vertical axis, it will be seen to do so at a minimum of about 25 per cent., and the above result holds even more accurately if the recovery is assumed to start from this constant minimum, as indeed, it has been shown to do under suitable conditions. ...

This is brought out by [Figure 2], which represents the recovery curve of thorium in which the percentage amounts of activity recovered, reckoned from this 25 per cent. minimum, are plotted as ordinates. In the same figure the decay curve after the second day is shown on the same scale. The activity of ThX decreases very approximately in a geometrical progression with the time, i.e. if I0 represent the initial activity and It the activity after time t, (1) It/I0 = e-lt , where l is a constant and e the base of natural logarithms. The experimental curve obtained with the hydroxide for the rate of rise of its activity from a minimum

to a maximum value will therefore be approximately expressed by the equation (2) It/I0 = 1- e-lt , where I0 represents the amount of activity recovered when the maximum is reached, and It the activity recovered after time t, l being the same constant as before. Now this last equation has been theoretically developed in other places to express the rise of activity to a constant maximum of a system consisting of radiating particles in which The rate of supply of fresh radiating particles is constant. The activity of each particle dies down geometrically with the time according to equation (1). It therefore follows that if the initial irregularities of the curves are disregarded and the residual activity of thorium is assumed to possess a constant value, the experimental curve obtained for the recovery of activity will be explained if two processes are supposed to be taking place: That the active constituent ThX is being produced at a constant rate; That the activity of the ThX decays geometrically with time. Without at first going into the difficult questions connected with the initial irregularities and the residual activity, the main result that follows from the curves given can be put to experimental test very simply. The primary conception is that the major part of the radioactivity of thorium is not due to the thorium at all, but to the presence of a non-thorium substance in minute amount which is being continuously produced. Chemical Properties of ThX The fact that thorium on precipitation from its solutions by ammonia leaves the major part of its activity in the filtrate does not of itself prove that a material constituent responsible for this activity has been chemically separated. It is possible that the matter constituting the non-thorium part of the solution is rendered temporarily radioactive by its association with thorium, and this property is retained through the processes of precipitation, evaporation, and ignition, and manifests itself finally on the residue remaining. This view, however, can be shown to be quite untenable, for upon it any precipitate capable of removing thorium completely from its solution should yield active residues similar to those obtained from ammonia. Quite the reverse, however, holds. When thorium nitrate is precipitated by sodium or ammonium carbonate, the residue from the filtrate by evaporation and ignition is free from activity, and the thorium carbonate possesses the normal value for its activity. The same holds true when oxalic acid is used as the precipitant. This reagent even in strongly acid solution precipitates almost all of the thorium. When the filtrate is rendered alkaline by ammonia, filtered, evaporated, and ignited, the residue obtained is inactive.

In the case where sodium phosphate is used as the precipitant in ordinary acid solution, the part that comes don is more or less free from ThX. On making the solution alkaline with ammonia, the remainder of the thorium is precipitated as phosphate, and carries with it the whole of the active constituent, so that the residue from the filtrate is again inactive. In fact ammonia is the only reagent of those tried capable of separating ThX from thorium. The result of Sir William Crookes with uranium, which we have confirmed with the electrical method, may be here mentioned. UrX is completely precipitated by ammonia together with uranium, and the residue obtained by the evaporation of the filtrate is quite inactive. There can thus be no question that both ThX and UrX are distinct types of matter with definite chemical properties. Any hypothesis that attempts to account for the recovery of activity of thorium and uranium with time must of necessity start from this primary conception. The Continuous Production of ThX If the recovery of the activity of thorium with time is due to the production of ThX, it should be possible to obtain experimental evidence of the process. The first point to be ascertained is how far the removal of ThX by the method given reduces the total radioactivity of thorium. A preliminary trial showed that the most favourable conditions for the separation are by precipitating in hot dilute solutions by dilute ammonia. A quantity of 5 grams of thorium nitrate, as obtained from the maker, was so precipitated by ammonia, the precipitate being redissolved in nitric acid and reprecipitated under the same conditions successively without lapse of time. The removal of ThX was followed by measuring the activity of the residues obtained from the first filtrate was equivalent to 4.25 grams of thoria, from the second to 0.33 gram, and from the third to 0.07 gram. It will be seen that by two precipitations practically the whole of the ThX is removed. The radioactivity of the separated hydroxide was 48 per cent. of that of the standard de-emanated sample of thoria. Rate of Production of ThX A quantity of thorium nitrate solution that had been freed from ThX about a month before, was again subjected to the same process. The activity of the residue from the filtrate in an experiment in which 10 grams of this nitrate had been employed was equivalent to 8.3 grams of thorium oxide. This experiment was performed on the same day as the one recorded above, in which 5 grams of new nitrate had been employed, and it will be seen that there is no difference in the activity of the filtrate in the two cases. In one month the activity of the ThX in a thorium compound again possesses its maximum value. If a period of 24 hours is allowed to elapse between the successive precipitations, the activity of the ThX formed during that time corresponds to about one-sixth of the maximum activity of the total thorium employed. In three hours the activity of the amount produced is about one-thirtieth. The rate of production of ThX worked out from those figures well agrees with the form of the curve obtained for the recovery of activity of thorium, if the latter is taken to express the continuous production of ThX at

a constant rate and the diminution of the activity of the product in geometrical progression with time. By using the sensitive electrometer, the course of production of ThX can be followed after extremely short intervals. Working with 10 grams of thorium nitrate, the amount produced in the minimum time taken to carry out the successive precipitations is as much as can be conveniently measured. If any interval is allowed to lapse the effect is beyond the range of the instrument, unless the sensitiveness is reduced to a fraction of its ordinary value by the introduction of capacities into the system. Capacities of .01 and .02 microfarad, which reduce the sensitiveness to less than one two-hundredth of the normal, were frequently employed in dealing with these active residues. The process of the production of ThX is continuous, and no alteration was observed in the amount produced in a given time after repeated separations. In an experiment carried out for another purpose after 23 successive precipitations extending over 9 days, the amount formed during the last interval was as far as could be judged no less than what occurred at the beginning of the process. The phenomena of radioactivity, by means of the electrometer as its measuring instrument, thus enables us to detect and measure changes occurring in matter after a few minutes' interval, which have never yet been detected by the balance or suspected of taking place. The Cause and Nature of Radioactivity The foregoing conclusions enable a great generalization to be made in the subject of radioactivity. Energy considerations require that the intensity of radiation from any source should die down with time unless there is a constant supply of energy to replace that dissipated. This has been found to hold true in the case of all known types of radioactivity with the exception of the "naturally" radioactive elements-to take the best established cases, thorium, uranium, and radium. It will be shown later that the radioactivity of the emanation produced by thorium compounds decays geometrically with the time under all conditions, and is not affected by the most drastic chemical and physical treatment. The same has been shown by one of us to hold for the excited radioactivity produced by the thorium emanation. This decays at the same rate whether on the wire on which it is originally deposited, or in solution of hydrochloric or nitric acid. The excited radioactivity produced by the radium emanation appears analogous. All these examples satisfy energy considerations. In the case of the three naturally occurring radioactive elements, however, it is obvious that there must be a continuous replacement of the dissipated energy, and no satisfactory explanation has yet been put forward. The nature of the process becomes clear in the light of the foregoing results. The material constituent responsible for the radioactivity, when it is separated from the thorium which produces it, then behaves in the same way as the other types of radioactivity cited. Its activity decays geometrically with the time, and the rate of decay is independent of the molecular conditions. The normal radioactivity is, however, maintained at a constant value by a chemical change which produces fresh radioactive material at a rate also independent of the conditions. The energy required to maintain the radiations will be accounted for if we suppose that the energy of the system after the change has occurred is less than it was before. The work of Crookes and Becquerel on the separation of UrX and the recovery of the activity of the uranium with time, makes it appear extremely probable that the same explanation holds true for this

element. The work of M. and Mme. Curie, the discoverers of radium, goes to show that this body easily suffers a temporary decrease of its activity by chemical treatment, the normal value being regained after the lapse of time, and this can be well interested on the new view. All known types of radioactivity can thus be brought under the same category. Summary of Results The foregoing experimental results may be briefly summarized. The major part of the radioactivity of thorium--ordinarily about 54 percent.--is due to a non-thorium type of matter, ThX, possessing distinct chemical properties, which is temporarily radioactive, its activity falling to half value in about four days. The constant radioactivity of thorium is maintained by the production of this material at a constant rate. Both the rate of production of the new material and the rate of decay of its activity appear to be independent of the physical and chemical condition of the system. The ThX further possesses the property of exciting radioactivity on surrounding inactive matter, and about 21 per cent. of the total activity under ordinary circumstances is derived from this source. Its rate of decay and other considerations make it appear probable that it is the same as the excited radioactivity produced by the thorium emanation, which is in turn produced by ThX . There is evidence that, if from any cause the emanation is prevented from escaping in the radioactive state, the energy of its radiation goes to augment the proportion of excited radioactivity in the compound. [The sections on which the following conclusions were based have been omitted. --Boorse & Motz.] Thorium can be freed by suitable means from both ThX and the excited radioactivity which the latter produces, and then possesses an activity about 25 per cent. of its original value, below which it has not been reduced. This residual radiation consists entirely of rays non-deviable by the magnetic field, whereas the other two components comprise both deviable and non-deviable radiation. Most probably this residual activity is caused by a second non-thorium type of matter produced in the same change as ThX, and it should therefore prove possible to separate it by chemical methods. General Theoretical Considerations Turning from the experimental results to their theoretical interpretation, it is necessary to first consider the generally accepted view of the nature of radioactivity. It is well established that this property is the function of the atom and not of the molecule. Uranium and thorium, to take the most definite cases, possess the property in whatever molecular condition they occur, and the former also in the elementary state. So far as the radioactivity of different compounds of different density and states of division can be compared together, the intensity of the radiation appears to depend only on the quantity of active element present. It is not possible to explain the phenomena by the existence of impurities associated with the radioactive elements, even if any advantage could be derived from the assumption. For these impurities must necessarily be present always to the same extent in different specimens derived from the most widely different sources, and, moreover, they must persist in altered amount after the most refined processes of purification. This is contrary to the accepted meaning of the term impurity. All the most prominent workers in this subject are agreed in considering radioactivity an atomic phenomenon. M. and Mme. Curie, the pioneers in the chemistry of the subject, have recently put forward their views. They state that this idea underlies their whole work fro the beginning and created

their methods of research. M. Becquerel, the original discoverer of the property for uranium, in his announcement of the recovery of the activity of the same element after the active constituent had been removed by chemical treatment, points out the significance of the fact that uranium is giving out cathode-rays. These, according to the hypothesis of Sir William Crookes and Prof. J. J. Thomson, are material particles of mass one thousandth of the hydrogen atom. Since, therefore, radioactivity is at once an atomic phenomenon and accompanied by chemical changes in which new types of matter are produced, these changes must be occurring within the atom, and the radioactive elements must be undergoing spontaneous transformation. The results that have so far been obtained, which indicate that the velocity of the reaction is unaffected by the conditions, make it clear that the changes in question are different in character from any that have been before dealt with in chemistry. It is apparent that we are dealing with phenomena outside the sphere of known atomic forces. Radioactivity may therefore be considered as a manifestation of subatomic chemical change. The changes brought to knowledge by radioactivity, although undeniably material and chemical in nature, are of a different order of magnitude from any that have before been dealt with in chemistry. The course of the production of new matter which can be recognized by the electrometer, by means of the property of radioactivity, after the lapse of a few hours or even minutes, might conceivably require geological epochs to attain to quantities recognized by the balance. However, the well-defined chemical properties of both ThX and UrX are not in accordance with the view that the actual amounts involved are of this extreme order of minuteness. On the other hand, the existence of radioactive elements at all in the earth's crust is an à priori argument against the magnitude of the change being anything but small. Radioactivity as a new property of matter capable of exact quantitative determination thus possesses an interest apart from the peculiar properties and powers which the radiations themselves exhibit. Mme. Curie, who isolated from pitchblende a new substance, radium, which possessed distinct chemical properties and spectroscopic lines, used the property as a means of chemical analysis. An exact parallel is to be found in Bunsen's discovery and separation of caesium and rubidium by means of the spectroscope. The present results show that radioactivity can also be used to follow chemical changes occurring in matter. The properties of matter that fulfil the necessary conditions for the study of chemical change without disturbance to the reacting system are few in number. It seems not unreasonable to hope, in the light of the foregoing results, that radioactivity, being such a property, affords the means of obtaining information of the processes occurring within the chemical atom, in the same way as the rotation of the plane of polarization and other physical properties have been used in chemistry for the investigation of the course of molecular change.

-------------------------------------------------------------------------------[1]If thorium oxide be exposed to a white heat its power of giving an emanation is to a large extent

destroyed. Thoria that has been so treated is referred to throughout as "de-emanated." -------------------------------------------------------------------------------Back to the list of selected historical papers. Back to the top of Classic Chemistry.Antoine Henri Becquerel (1852-1908) On the rays emitted by phosphorescence [read before the French Academy of Science 24 Feb. 1896 (Comptes Rendus 122, 420 (1896)) translated by Carmen Giunta] In an earlier session, M. Chairman Henry announced that phosphorescent zinc sulfide placed in the path of rays emanating from a Crookes tube augmented the intensity of rays passing through the aluminum. Elsewhere, M. Niewenglowski recognized that commercial phosphorescent calcium sulfide emits rays which pass through opaque bodies. This fact extends to various phosphorescent bodies, and in particular to uranium salts whose phosphorescence has a very brief duration. With the double sulfate of uranium and potassium, of which I have a few crystals forming a thin transparent crust, I was able to perform the following experiment: One wraps a Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative. One can repeat the same experiments placing a thin pane of glass between the phosphorescent substance and the paper, which excludes the possibility of chemical action due to vapors which might emanate from the substance when heated by the sun's rays. One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduces silver salts.

-------------------------------------------------------------------------------On the invisible rays emitted by phosphorescent bodies. [read before the French Academy of Science 2 March 1896 (Comptes Rendus 122, 501 (1896)) translated by Carmen Giunta] In the previous session, I summarized the experiments which I had been led to make in order to detect the invisible rays emitted by certain phosphorescent bodies, rays which pass through various bodies that are opaque to light. I was able to extend these observations, and although I intend to continue and to elaborate upon the study of these phenomena, their outcome leads me to announce as early as today the first results I obtained. The experiments which I shall report were done with the rays emitted by crystalline crusts of the double sulfate of uranyl and potassium [SO4(UO)K+H2O], a substance whose phosphorescence is very vivid and persists for less than 1/100th of a second. The characteristics of the luminous rays emitted by this material have been studied previously by my father, and in the meantime I have had occasion to point out some interesting peculiarities which these luminous rays manifest. One can confirm very simply that the rays emitted by this substance, when it is exposed to sunlight or to diffuse daylight, pass through not only sheets of black paper but also various metals, for example a plate of aluminum and a thin sheet of copper. In particular, I performed the following experiment: A Lumière plate with a silver bromide emulsion was enclosed in an opaque case of black cloth, bounded on one side by a plate of aluminum; if one exposed the case to full sunlight, even for a whole day, the photographic plate would not become clouded; but, if one came to attach a crust of the uranium salt to the exterior of the aluminum plate, which one could do, for example, by fastening it with strips of paper, one would recognize, after developing the photographic plate in the usual way, that the silhouette of the crystalline crust appears in black on the sensitive plate and that the silver salt facing the phosphorescent crust had been reduced. If the layer of aluminum is a bit thick, then the intensity of the effect is less than that through two sheets of black paper. If one places between the crust of the uranium salt and the layer of aluminum or black paper a screen formed of a sheet of copper about 0.10 mm thick, in the form of a cross for example, then one sees in the image the silhouette of that cross, a bit fainter yet with a darkness indicative nonetheless that the rays passed through the sheet of copper. In another experiment, a thinner sheet of copper (0.04 mm) attenuated the active rays much less. Phosphorescence induced no longer by the direct rays of the sun, but by solar radiation reflected in a metallic mirror of a heliostat, then refracted by a prism and a quartz lens, gave rise to the same phenomena.

I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts, arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images. Here is how I was led to make this observation: among the preceding experiments, some had been prepared on Wednesday the 26th and Thursday the 27th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt. Since the sun did not come out in the following days, I developed the photographic plates on the 1st of March, expecting to find the images very weak. Instead the silhouettes appeared with great intensity. I immediately thought that the action had to continue in darkness, and I arranged the following experiment: At the bottom of a box of opaque cardboard I placed a photographic plate; then, on the sensitive side I put a crust of the uranium salt, a convex crust which only touched the bromide emulsion at a few points; then, alongside, I placed on the same plate another crust of the same salt but separated from the bromide emulsion by a thin pane of glass; this operation was carried out in the darkroom, then the box was shut, then enclosed in another cardboard box, and finally put in a drawer. I did the same with the case closed by a plate of aluminum in which I put a photographic plate and then on the outside a crust of the uranium salt. The whole was enclosed in an opaque box, and then in a drawer. After five hours, I developed the plates, and the silhouettes of the crystalline crusts appeared in black as in the previous experiments and as if they had been rendered phosphorescent by light. For the crust placed directly on the emulsion, there was scarcely a difference in effect between the points of contact and the parts of the crust which remained about a millimeter away from the emulsion; the difference can be attributed to the different distance from the source of the active rays. The effect from the crust placed on a pane of glass was very slightly attenuated, but the shape of the crust was very well reproduced. Finally, through the sheet of aluminum, the effect was considerably weaker, but nonetheless very clear. It is important to observe that it appears this phenomenon must not be attributed to the luminous radiation emitted by phosphorescence, since at the end of 1/100th of a second this radiation becomes so weak that it is hardly perceptible any more. One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies. However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion. I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena.

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Back to the list of selected historical papers. Back to the top of Classic Chemistry.

Selected Classic Papers from the History of Chemistry The nucleus: isotopes and radioactivity • Corbin Allardice and Edward R. Trapnell (1949): An eyewitness account of the first selfsustaining nuclear chain reaction. This paper is at the ChemTeam site. • Francis Aston (1919): early report of mass spectra, suggesting that isotopes have integer masses. (Link to a photo of his apparatus or biographical sketch of Aston.) • Francis Aston (1920): early report of mass spectra showing isotopes of stable elements. This paper is at the ChemTeam site, as is this picture. • Henri Becquerel: two brief reports about radioactivity read to the French Academy of Sciences one week apart in 1896. In between the two reports, Becquerel realized that he was not dealing with ordinary phosphorescence! (Link to a biographical sketch of Becquerel and view a picture of a photographic plate from which he made his discovery. • Niels Bohr (1939): liquid drop model of fission in wake of Meitner-Frisch paper. This paper is at the ChemTeam site. • Harriet Brooks: 1904 description of a volatile radioactive product from radium: Marie Curie was not the only woman active in early research on radioactivity! This paper is at the UCLA site on contributions of women to physics, as is this biographical information on Brooks. • James Chadwick: 1932 letter and subsequent detailed paper explaining experimental observations by invoking a new particle, the neutron. These papers are at the ChemTeam site. (View part of his apparatus or biographical information on Chadwick.) • John Cockcroft and Ernest Walton: 1932 paper on the disintegration of lithium by fast protons: artificial transmutation. This paper is at Nature's physics portal. (Link to a biographical sketch of Cockcroft and one of Walton.) • Marie Curie: 1898 paper surveying the material world for radioactivity, finding it in uranium and thorium minerals, and suggesting that a new radioactive element may be found in pitchblende. (Link to a biographical information on Curie.) • Pierre and Marie Curie: 1898 announcement of a new radioactive element, polonium. • Pierre and Marie Curie and G. Bémont: December 1898 announcement of a new strongly radioactive element, radium. • Kasimir Fajans: 1913 paper on the radioactive displacement law and isotopes. This paper is at the ChemTeam site as is this photo. • Enrico Fermi: 1934 note suspects (incorrectly) production of transuranic elements by bombarding thorium and uranium with neutrons. Noddack critiqued this conclusion on chemical grounds. Meitner and Frisch later explained these results as nuclear fission. Fermi's paper is at the ChemTeam site. View biographical information on Fermi.

• Otto Frisch: brief 1939 note follows up paper with Lise Meitner on fission of uranium This paper is at the ChemTeam site. View a biographical sketch of Frisch. • Hans Geiger and Ernest Marsden: 1909 paper reporting unexpected backscatter of alpha particles; interpretation of this phenomenon led to the nuclear model of the atom. This paper is at the ChemTeam site. (Link to a biographical sketch of Geiger • Hans Geiger: from 1910 paper on scattering of alpha particles from gold foil. This paper is at the ChemTeam site. • Hans Geiger and Ernest Marsden: 1913 paper comparing backscatter of alpha particles to the predictions of Rutherford's nuclear model of the atom. This paper is at the ChemTeam site. • Otto Hahn and Fritz Strassmann: 1939 paper reporting a result they barely believe themselves: barium, lanthanum, and cerium obtained from the bombardment of uranium by neutrons, then a more definite announcement of uranium fission. These papers are at the ChemTeam site. View biographical information on Hahn and Strassmann • Lise Meitner and Otto Frisch: 1939 paper invokes fission of uranium to explain neutron bombardment results. This paper is at the Nature's physics portal. Link to a biographical sketch of Meitner. • Ida Noddack: 1934 note critiques Fermi's conclusion that he had produced transuranic elements by bombarding thorium and uranium with neutrons. Noddack's paper is at the ChemTeam site. Link to biographical information on Noddack. • William Ramsay & Frederick Soddy: 1903 investigation of the inert nature of radium emanation and the observation that helium is evolved from both radium and its emanation. • Theodore W. Richards & Max E. Lembert: 1914 paper on atomic weights of lead found different atomic weights for lead of radioactive origin compared to "ordinary" lead; authors cautiously interpret the results as consistent with the concept of isotopes. Link to a biographical sketch of Richards. • Wilhelm Röntgen: "On a New Kind of Rays", 1895 paper first describing X-rays. X-ray photographs of a human hand (fig. 1) and a compass card (fig. 2) accompanied the paper. [More information about Röntgen and X-rays: The New Light: Discovery and Introduction] • Ernest Rutherford: 1899 paper distinguishes between two types of radioactivity, which he labels alpha and beta. This paper is at the ChemTeam site. (Link to a biographical sketch Rutherford.) • Ernest Rutherford: 1900 paper introduces concept of radioactive half-life and measures half-life of "thorium emanation" (now known as 220Rn). This paper is at the ChemTeam site. • Ernest Rutherford & Frederick Soddy: 1902 paper that concludes, "radioactive elements must be undergoing spontaneous transformation." (This conclusion is found in the paper's final section.) • Ernest Rutherford and T. Royds: 1909 paper identifying the α particle with doubly-charged helium. The paper is worth reading for the careful marshalling of one last conclusive piece of evidence about the nature of the particles Rutherford and his co-workers had been studying for a decade. • Ernest Rutherford: abstract of a 1911 paper proposing the nuclear model of the atom to explain results of scattering experiments. This paper is at the ChemTeam site. • Ernest Rutherford: 1911 paper proposing the nuclear model of the atom to explain results of scattering experiments. This paper is at the ChemTeam site. • Ernest Rutherford: 1914 paper on the nuclear model of the atom, including reference to Moseley's work on atomic number. This paper is at the ChemTeam site, as is Moseley's. • Ernest Rutherford: 1919 paper describing the bombardment of nitrogen by alpha particles.

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Rutherford concludes the nitrogen atoms are disintegrated in the process. So atoms are not indestructable after all, and the alchemists' dreams of transmutation are realized. Ernest Rutherford: 1920 lecture describing the state of knowledge of nuclear structure at a time after the discovery of isotopy and atomic number but before the neutron; the standard picture included electrons in the nucleus. M. L. Oliphant, P. Harteck, and Ernest Rutherford: 1934 note from the Rutherford lab describes fusion ('transmutation') of deuterium. These papers (preliminary note and more detailed paper) are at the ChemTeam site. Frederick Soddy: 1913 paper which gives the rules for chemical transformations accompanying α and β decay; its discussion of "non-separable" elements all but defines (but does not name) isotopy, including a speculation that they are not limited to radioactive elements. (Link to a biographical sketch of Soddy.) Frederick Soddy: 1913 paper which introduces the term "isotopes" for atoms which have the same nuclear charge but different mass. Frederick Soddy: from 1913 review article; discusses isotopes and the displacement law Silvanus Thompson: Thompson thought of performing the same sorts of experiments as Becquerel at about the same time; comparison of this paper with Becquerel's highlights the luck and genius of Becquerel. This article also illustrates the confusion immediately following the discovery of X-rays and radioactivity: the former were not believed to be electromagnetic and the latter was! Harold Urey, Ferdinand Brickwedde, and George Murphy: 1932 paper announcing detection of a heavy isotope of hydrogen, H2 (reprinted with permission of the American Physical Society). View biographical sketches of Urey, Brickwedde, and Murphy.

Organic chemistry • Jean-Baptiste-André Dumas: paper on acetic acid and trichloroacetic acid (1839) supporting Dumas' notion of chemical types. Link to a biographical sketch of Dumas. • Jean-Baptiste-André Dumas and Justus von Liebig (1837): organic chemistry differs from inorganic in that the radicals in the latter are simple, but in the former are compound. (View a biographical sketch of Liebig.) • Emil Fischer: 1891 paper on the structure of glucose and other simple sugars, a landmark of stereochemical reasoning from wet chemistry. This paper [pdf] is at J. Michael McBride's site at Yale. Link to a biographical sketch of Fischer. • Edward Frankland: complete 1852 paper on organometallic compounds; it contains an early and clear statement of the concept of valence. (Thanks to John Park for transcription.) Link to further information on Frankland. • Charles Gerhardt: 1853 excerpt on organic types. Link to biographical sketch of Gerhardt. • August Wilhelm von Hofmann: 1851 excerpt on amines and organic ammonium compounds, relevant to type theory. Link to a biographical sketch of Hofmann. • August Kekule: excerpt of 1865 paper on the structure of aromatic compounds. This paper is on Rod Beavon's chemistry site. (Link to further information on Kekule.) • Joseph Achille Le Bel: tetrahedral geometry of carbon (1874). This paper is at the ChemTeam site as is this photo. • Jacobus van't Hoff: optical activity and the tetrahedral geometry of carbon (1874). This paper is at the ChemTeam site. Link to a biographical sketch of van't Hoff.

• A. D. Walsh, R. Robinson, C. A. McDowell, and J.W. Linnett separately discuss Walsh's proposed description of bonding in cyclopropane, 1947. These papers are at Daniel Berger's Walsh cyclopropane pages at Bluffton College. • Alexander Williamson: synthesis of ether and structure of ethers and alcohols (1850). Link to further information about Williamson. • Friedrich Wöhler: synthesis of urea from inorganic materials, conventionally regarded as fatal to the idea that organic compounds could only be produced through a "vital force." This paper is at the ChemTeam site, as is this picture. • Charles-Adolphe Wurtz on amines and their relation to ammonia (1849). (Link to a photo of Wurtz or a biographical paragraph.)

Periodic table and periodic law • Humphry Davy: 1812 paper searching for analogies among elements. • Johann Wolfgang Döbereiner: 1829 paper on triads of analogous elements and their classification. Link to a biographical sketch of Döbereiner . • Dmitrii Mendeleev, (1869): his first published periodic table and the abstract by which it was first known in Western Europe. (Link to a collection of on-line material about Mendeleev.) • Dmitrii Mendeleev, (1871): table from Annalen, suppl. VIII, 133 (1871). This item is posted at Chris Heilman's Pictorial Periodic Table site. • Dmitrii Mendeleev: excerpt from 1871 paper on periodicity of the elements focuses on the properties of the predicted element eka-boron, now known as scandium. This paper is on Rod Beavon's chemistry site. • Dmitrii Mendeleev, (1889): Faraday lecture on the Periodic Law, 20 years after Mendeleev's first work on the subject • Julius Lothar Meyer, (1870). A table of most of the known elements arranged to show family resemblances and a figure showing atomic volumes varying periodically. (Link to further information on Meyer.) • Julius Lothar Meyer: excerpt from 1870 paper on periodicity of the elements. This paper is on Rod Beavon's chemistry site. • Henry Moseley (1913, 1914): X-ray spectra of the elements reveal integers characteristic of each element, namely the atomic number. This paper is at the ChemTeam site, as is this picture and this essay on Moseley and his work. • J. A. R. Newlands, (1863, 1864, 1865, and 1866): his first attempts to find relationships among the atomic weights ("equivalents") of families of elements and accounts of his "law of octaves". The 1863 and 1864 papers are a long way from the periodic table, and even from his later law of octaves (1865 and 1866 items). (Link to a biographical sketch or view his picture in the Edgar Fahs Smith collection.) • J. A. R. Newlands, On the discovery of the periodic law: and on relations among the atomic weights: 1884 monograph that collects all of Newlands' papers on the subject (This book is at Google Books). • William Ramsay, (1897): expands periodic table to make a new column for noble gases; predicts discovery and properties of neon. (Link to a biographical sketch of Ramsay.) • A. van den Broek: two letters on numbering the elements (1911 and 1913). These papers are at the ChemTeam site: 1 and 2.

Thermodynamics • Francis Bacon (1620): Before caloric and the kinetic theory, Bacon reviewed a wide range of observations about heat and related phenomena to illustrate his inductive scientific method, and suggested that heat is related to motion. There is even a mention of triboluminescent candy (in Table II, number 11). (Link to a biographical sketch of Bacon or to a complete text of Novum Organum.) • Joseph Black: 1803 (posthumous) paper on heat distinguished between heat and temperature and described specific heat and latent heat, even though treating heat as matter. • Sadi Carnot: Reflections on the Motive Power of Fire (or "of heat" as this translation has it), 1824. This book is at the on-line Steam Engine Library at the University of Rochester. (Link to a biographical sketch of Carnot.) • Rudolf Clausius: excerpts from two papers on entropy. The first (1850) notes that heat is not indestructible, and examines how it can be converted to work with the flow of heat from a warm body to a cold; the second (1865) coins the term entropy and states the second law of thermodynamics. (Link to a biographical sketch of Clausius.) • Rudolf Clausius: 1857 paper on the kinetic theory of gases; derives expressions for the pressure of a gas based from analysis of collisions for average molecular speeds. • John Dalton, excerpts from A New System of Chemistry (1808). Describes how heat (caloric) was believed to combine with matter, especially gases. (See also Lavoisier excerpt in this section.) Heat capacity of gases proposed to vary inversely with atomic weight (like law of Dulong & Petit). • Humphry Davy: from Davy's first scientific publication (1799), some insightful ideas and dubious experiments on the nature of heat and friction. (Link to a biographical sketch of Davy.) • Daniel Gabriel Fahrenheit: 1724 paper observing several liquids to boil at constant temperatures. (Link to biographical information on Fahrenheit.) • Cato Guldberg and Peter Waage: "Studier over Affiniteten", describing law of mass action to the Norwegian Academy of Sciences and Letters in 1864. This paper is at the ChemTeam site, as is this picture. • Josiah Willard Gibbs: preface to Statistical Mechanics, published in 1902. This paper [pdf] is at the American Institute of Physics website. (Link to biographical information on Gibbs.) • John Herapath: excerpt of 1821 paper on kinetic theory of gases: heat is motion, and there need not be repulsive forces between gaseous atoms. Link to biographical information on Herapath. • Germain Henri Hess: 1840 paper on heats of reaction (Hess's law) This paper is at the ChemTeam site. (Link to a biographical sketch of Hess.) • James Prescott Joule: 1845 note on the relationship between heat and mechanical energy (the mechanical equivalent of heat). This paper is at the ChemTeam site. (View Joule's apparatus, or link to a biographical sketch of Joule.) • James Prescott Joule: 1851 estimate of the speed of a gas molecule. • Antoine Lavoisier: on caloric and its role in the three states of matter, from Elements of Chemistry (1789) • Antoine Lavoisier: Oeuvres, (Paris, 1862-1893, 6 vols.): page images at Panopticon Lavoisier includes complete Traité élémentaire de chimie • Henri Louis le Chatelier (1884): enunciates his principle concerning chemical equilibrium. Link to a biographical sketch of le Chatelier • James Clerk Maxwell: introduces Maxwell's "demon" and its implications for the second law of thermodynamics (1872).

• Julius Robert Mayer: on the conservation or interconvertability of energy (or force or vis viva, as the paper says). Click here for a biographical sketch of Mayer. • Alexis-Thérèse Petit & Pierre-Louis Dulong: complete 1819 paper on heat capacities of elements, that contains the law of Dulong & Petit. (Link to biographical information on Dulong and Petit.) • Agnes Pockels: letter on surface properties of water, sent to Lord Rayleigh and later published in Nature. This paper is at the Contributions of 20th Century Women to Physics site at UCLA. (Link to biographical information on Pockels.) • François-Marie Raoult: 1882 paper on freezing point depression. (Link to biographical sketch of Raoult.) • François-Marie Raoult: 1887 paper on the lowering of vapor pressure. • Benjamin Thomson (Count Rumford): 1798 paper on the quantity and nature of the heat generated in boring a cannon. This paper is at the ChemTeam site. (Link to a biographical sketch of Rumford.) • William Thomson (Baron Kelvin of Largs): some thoughts (not all correct--see Keith J. Laidler, The World of Physical Chemistry, pp. 99-100) on an absolute thermodynamic scale of temperature (1848). (Link to a biographical sketch of Kelvin.) • William Thomson (Baron Kelvin of Largs): 1852 formulation of the second law of thermodynamics and description of an absolute temperature scale. • William Thomson (Baron Kelvin of Largs) (1865): an application of heat transfer to geology, leading Thomson to believe the earth is relatively young. • Jacobus van't Hoff: osmosis and the analogy between solutions and gases (1887). This paper is at the ChemTeam site.

Others • Justus Liebig: Familiar Letters on Chemistry (1843). This monograph on chemistry and some of its applications to agriculture and industry in the middle 19th century is available courtesy of Peter Childs, Limerick, Ireland. • Philosophical Transactions of the Royal Society, volumes 50-67 (1757-77). This resource is available as page images at the Internet Library of Early Journals, Bodelian Library, Oxford University. The entire volumes are posted, so this resource spans the range of natural philosophy. • Louis-Jacques Thenard: 1819 paper announces discovery of hydrogen peroxide and describes some of its properties (including some painful tests: don't try this at home)

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Selected Classic Papers

from the History of Chemistry The nucleus: isotopes and radioactivity • Corbin Allardice and Edward R. Trapnell (1949): An eyewitness account of the first self-sustaining nuclear chain reaction. This paper is at the ChemTeam site. • Francis Aston (1919): early report of mass spectra, suggesting that isotopes have integer masses. (Link to a photo of his apparatus or biographical sketch of Aston.) • Francis Aston (1920): early report of mass spectra showing isotopes of stable elements. This paper is at the ChemTeam site, as is this picture. • Henri Becquerel: two brief reports about radioactivity read to the French Academy of Sciences one week apart in 1896. In between the two reports, Becquerel realized that he was not dealing with ordinary phosphorescence! (Link to a biographical sketch of Becquerel and view a picture of a photographic plate from which he made his discovery. • Niels Bohr (1939): liquid drop model of fission in wake of Meitner-Frisch paper. This paper is at the ChemTeam site. • Harriet Brooks: 1904 description of a volatile radioactive product from radium: Marie Curie was not the only woman active in early research on radioactivity! This paper is at the UCLA site on contributions of women to physics, as is this biographical information on Brooks. • James Chadwick: 1932 letter and

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subsequent detailed paper explaining experimental observations by invoking a new particle, the neutron. These papers are at the ChemTeam site. (View part of his apparatus or biographical information on Chadwick.) John Cockcroft and Ernest Walton: 1932 paper on the disintegration of lithium by fast protons: artificial transmutation. This paper is at Nature's physics portal. (Link to a biographical sketch of Cockcroft and one of Walton.) Marie Curie: 1898 paper surveying the material world for radioactivity, finding it in uranium and thorium minerals, and suggesting that a new radioactive element may be found in pitchblende. (Link to a biographical information on Curie.) Pierre and Marie Curie: 1898 announcement of a new radioactive element, polonium. Pierre and Marie Curie and G. Bémont: December 1898 announcement of a new strongly radioactive element, radium. Kasimir Fajans: 1913 paper on the radioactive displacement law and isotopes. This paper is at the ChemTeam site as is this photo. Enrico Fermi: 1934 note suspects (incorrectly) production of transuranic elements by bombarding thorium and uranium with neutrons. Noddack critiqued this conclusion on chemical grounds. Meitner and Frisch later explained these results as nuclear fission. Fermi's paper is at the ChemTeam site. View biographical information on Fermi. Otto Frisch: brief 1939 note follows up paper with Lise Meitner on fission of uranium This paper is at the ChemTeam site. View a biographical sketch of Frisch.

• Hans Geiger and Ernest Marsden: 1909 paper reporting unexpected backscatter of alpha particles; interpretation of this phenomenon led to the nuclear model of the atom. This paper is at the ChemTeam site. (Link to a biographical sketch of Geiger • Hans Geiger: from 1910 paper on scattering of alpha particles from gold foil. This paper is at the ChemTeam site. • Hans Geiger and Ernest Marsden: 1913 paper comparing backscatter of alpha particles to the predictions of Rutherford's nuclear model of the atom. This paper is at the ChemTeam site. • Otto Hahn and Fritz Strassmann: 1939 paper reporting a result they barely believe themselves: barium, lanthanum, and cerium obtained from the bombardment of uranium by neutrons, then a more definite announcement of uranium fission. These papers are at the ChemTeam site. View biographical information on Hahn and Strassmann • Lise Meitner and Otto Frisch: 1939 paper invokes fission of uranium to explain neutron bombardment results. This paper is at the Nature's physics portal. Link to a biographical sketch of Meitner. • Ida Noddack: 1934 note critiques Fermi's conclusion that he had produced transuranic elements by bombarding thorium and uranium with neutrons. Noddack's paper is at the ChemTeam site. Link to biographical information on Noddack. • William Ramsay & Frederick Soddy: 1903 investigation of the inert nature of radium emanation and the observation that helium is evolved from both radium and its emanation.

• Theodore W. Richards & Max E. Lembert: 1914 paper on atomic weights of lead found different atomic weights for lead of radioactive origin compared to "ordinary" lead; authors cautiously interpret the results as consistent with the concept of isotopes. Link to a biographical sketch of Richards. • Wilhelm Röntgen: "On a New Kind of Rays", 1895 paper first describing X-rays. X-ray photographs of a human hand (fig. 1) and a compass card (fig. 2) accompanied the paper. [More information about Röntgen and X-rays: The New Light: Discovery and Introduction] • Ernest Rutherford: 1899 paper distinguishes between two types of radioactivity, which he labels alpha and beta. This paper is at the ChemTeam site. (Link to a biographical sketch Rutherford.) • Ernest Rutherford: 1900 paper introduces concept of radioactive half-life and measures half-life of "thorium emanation" (now known as 220Rn). This paper is at the ChemTeam site. • Ernest Rutherford & Frederick Soddy: 1902 paper that concludes, "radioactive elements must be undergoing spontaneous transformation." (This conclusion is found in the paper's final section.) • Ernest Rutherford and T. Royds: 1909 paper identifying the α particle with doubly-charged helium. The paper is worth reading for the careful marshalling of one last conclusive piece of evidence about the nature of the particles Rutherford and his coworkers had been studying for a decade. • Ernest Rutherford: abstract of a 1911 paper proposing the nuclear model of the atom to explain results of

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scattering experiments. This paper is at the ChemTeam site. Ernest Rutherford: 1911 paper proposing the nuclear model of the atom to explain results of scattering experiments. This paper is at the ChemTeam site. Ernest Rutherford: 1914 paper on the nuclear model of the atom, including reference to Moseley's work on atomic number. This paper is at the ChemTeam site, as is Moseley's. Ernest Rutherford: 1919 paper describing the bombardment of nitrogen by alpha particles. Rutherford concludes the nitrogen atoms are disintegrated in the process. So atoms are not indestructable after all, and the alchemists' dreams of transmutation are realized. Ernest Rutherford: 1920 lecture describing the state of knowledge of nuclear structure at a time after the discovery of isotopy and atomic number but before the neutron; the standard picture included electrons in the nucleus. M. L. Oliphant, P. Harteck, and Ernest Rutherford: 1934 note from the Rutherford lab describes fusion ('transmutation') of deuterium. These papers (preliminary note and more detailed paper) are at the ChemTeam site. Frederick Soddy: 1913 paper which gives the rules for chemical transformations accompanying α and β decay; its discussion of "nonseparable" elements all but defines (but does not name) isotopy, including a speculation that they are not limited to radioactive elements. (Link to a biographical sketch of Soddy.) Frederick Soddy: 1913 paper which introduces the term "isotopes" for atoms which have the same nuclear

charge but different mass. • Frederick Soddy: from 1913 review article; discusses isotopes and the displacement law • Silvanus Thompson: Thompson thought of performing the same sorts of experiments as Becquerel at about the same time; comparison of this paper with Becquerel's highlights the luck and genius of Becquerel. This article also illustrates the confusion immediately following the discovery of X-rays and radioactivity: the former were not believed to be electromagnetic and the latter was! • Harold Urey, Ferdinand Brickwedde, and George Murphy: 1932 paper announcing detection of a heavy isotope of hydrogen, H2 (reprinted with permission of the American Physical Society). View biographical sketches of Urey, Brickwedde, and Murphy.

Organic chemistry • Jean-Baptiste-André Dumas: paper on acetic acid and trichloroacetic acid (1839) supporting Dumas' notion of chemical types. Link to a biographical sketch of Dumas. • Jean-Baptiste-André Dumas and Justus von Liebig (1837): organic chemistry differs from inorganic in that the radicals in the latter are simple, but in the former are compound. (View a biographical sketch of Liebig.) • Emil Fischer: 1891 paper on the structure of glucose and other simple sugars, a landmark of stereochemical reasoning from wet chemistry. This paper [pdf] is at J. Michael McBride's site at Yale. Link to a biographical sketch of Fischer. • Edward Frankland: complete 1852 paper on organometallic compounds; it contains an early and clear

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statement of the concept of valence. (Thanks to John Park for transcription.) Link to further information on Frankland. Charles Gerhardt: 1853 excerpt on organic types. Link to biographical sketch of Gerhardt. August Wilhelm von Hofmann: 1851 excerpt on amines and organic ammonium compounds, relevant to type theory. Link to a biographical sketch of Hofmann. August Kekule: excerpt of 1865 paper on the structure of aromatic compounds. This paper is on Rod Beavon's chemistry site. (Link to further information on Kekule.) Joseph Achille Le Bel: tetrahedral geometry of carbon (1874). This paper is at the ChemTeam site as is this photo. Jacobus van't Hoff: optical activity and the tetrahedral geometry of carbon (1874). This paper is at the ChemTeam site. Link to a biographical sketch of van't Hoff. A. D. Walsh, R. Robinson, C. A. McDowell, and J.W. Linnett separately discuss Walsh's proposed description of bonding in cyclopropane, 1947. These papers are at Daniel Berger's Walsh cyclopropane pages at Bluffton College. Alexander Williamson: synthesis of ether and structure of ethers and alcohols (1850). Link to further information about Williamson. Friedrich Wöhler: synthesis of urea from inorganic materials, conventionally regarded as fatal to the idea that organic compounds could only be produced through a "vital force." This paper is at the ChemTeam site, as is this picture. Charles-Adolphe Wurtz on amines and their relation to ammonia (1849).

(Link to a photo of Wurtz or a biographical paragraph.)

Periodic table and periodic law • Humphry Davy: 1812 paper searching for analogies among elements. • Johann Wolfgang Döbereiner: 1829 paper on triads of analogous elements and their classification. Link to a biographical sketch of Döbereiner . • Dmitrii Mendeleev, (1869): his first published periodic table and the abstract by which it was first known in Western Europe. (Link to a collection of on-line material about Mendeleev.) • Dmitrii Mendeleev, (1871): table from Annalen, suppl. VIII, 133 (1871). This item is posted at Chris Heilman's Pictorial Periodic Table site. • Dmitrii Mendeleev: excerpt from 1871 paper on periodicity of the elements focuses on the properties of the predicted element eka-boron, now known as scandium. This paper is on Rod Beavon's chemistry site. • Dmitrii Mendeleev, (1889): Faraday lecture on the Periodic Law, 20 years after Mendeleev's first work on the subject • Julius Lothar Meyer, (1870). A table of most of the known elements arranged to show family resemblances and a figure showing atomic volumes varying periodically. (Link to further information on Meyer.) • Julius Lothar Meyer: excerpt from 1870 paper on periodicity of the elements. This paper is on Rod Beavon's chemistry site. • Henry Moseley (1913, 1914): X-ray spectra of the elements reveal

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integers characteristic of each element, namely the atomic number. This paper is at the ChemTeam site, as is this picture and this essay on Moseley and his work. J. A. R. Newlands, (1863, 1864, 1865, and 1866): his first attempts to find relationships among the atomic weights ("equivalents") of families of elements and accounts of his "law of octaves". The 1863 and 1864 papers are a long way from the periodic table, and even from his later law of octaves (1865 and 1866 items). (Link to a biographical sketch or view his picture in the Edgar Fahs Smith collection.) J. A. R. Newlands, On the discovery of the periodic law: and on relations among the atomic weights: 1884 monograph that collects all of Newlands' papers on the subject (This book is at Google Books). William Ramsay, (1897): expands periodic table to make a new column for noble gases; predicts discovery and properties of neon. (Link to a biographical sketch of Ramsay.) A. van den Broek: two letters on numbering the elements (1911 and 1913). These papers are at the ChemTeam site: 1 and 2.

Thermodynamics • Francis Bacon (1620): Before caloric and the kinetic theory, Bacon reviewed a wide range of observations about heat and related phenomena to illustrate his inductive scientific method, and suggested that heat is related to motion. There is even a mention of triboluminescent candy (in Table II, number 11). (Link to a biographical sketch of Bacon or to a complete text of Novum Organum.) • Joseph Black: 1803 (posthumous)

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paper on heat distinguished between heat and temperature and described specific heat and latent heat, even though treating heat as matter. Sadi Carnot: Reflections on the Motive Power of Fire (or "of heat" as this translation has it), 1824. This book is at the on-line Steam Engine Library at the University of Rochester. (Link to a biographical sketch of Carnot.) Rudolf Clausius: excerpts from two papers on entropy. The first (1850) notes that heat is not indestructible, and examines how it can be converted to work with the flow of heat from a warm body to a cold; the second (1865) coins the term entropy and states the second law of thermodynamics. (Link to a biographical sketch of Clausius.) Rudolf Clausius: 1857 paper on the kinetic theory of gases; derives expressions for the pressure of a gas based from analysis of collisions for average molecular speeds. John Dalton, excerpts from A New System of Chemistry (1808). Describes how heat (caloric) was believed to combine with matter, especially gases. (See also Lavoisier excerpt in this section.) Heat capacity of gases proposed to vary inversely with atomic weight (like law of Dulong & Petit). Humphry Davy: from Davy's first scientific publication (1799), some insightful ideas and dubious experiments on the nature of heat and friction. (Link to a biographical sketch of Davy.) Daniel Gabriel Fahrenheit: 1724 paper observing several liquids to boil at constant temperatures. (Link to biographical information on Fahrenheit.) Cato Guldberg and Peter Waage: "Studier over Affiniteten", describing

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law of mass action to the Norwegian Academy of Sciences and Letters in 1864. This paper is at the ChemTeam site, as is this picture. Josiah Willard Gibbs: preface to Statistical Mechanics, published in 1902. This paper [pdf] is at the American Institute of Physics website. (Link to biographical information on Gibbs.) John Herapath: excerpt of 1821 paper on kinetic theory of gases: heat is motion, and there need not be repulsive forces between gaseous atoms. Link to biographical information on Herapath. Germain Henri Hess: 1840 paper on heats of reaction (Hess's law) This paper is at the ChemTeam site. (Link to a biographical sketch of Hess.) James Prescott Joule: 1845 note on the relationship between heat and mechanical energy (the mechanical equivalent of heat). This paper is at the ChemTeam site. (View Joule's apparatus, or link to a biographical sketch of Joule.) James Prescott Joule: 1851 estimate of the speed of a gas molecule. Antoine Lavoisier: on caloric and its role in the three states of matter, from Elements of Chemistry (1789) Antoine Lavoisier: Oeuvres, (Paris, 1862-1893, 6 vols.): page images at Panopticon Lavoisier includes complete Traité élémentaire de chimie Henri Louis le Chatelier (1884): enunciates his principle concerning chemical equilibrium. Link to a biographical sketch of le Chatelier James Clerk Maxwell: introduces Maxwell's "demon" and its implications for the second law of thermodynamics (1872). Julius Robert Mayer: on the conservation or interconvertability of

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energy (or force or vis viva, as the paper says). Click here for a biographical sketch of Mayer. Alexis-Thérèse Petit & Pierre-Louis Dulong: complete 1819 paper on heat capacities of elements, that contains the law of Dulong & Petit. (Link to biographical information on Dulong and Petit.) Agnes Pockels: letter on surface properties of water, sent to Lord Rayleigh and later published in Nature. This paper is at the Contributions of 20th Century Women to Physics site at UCLA. (Link to biographical information on Pockels.) François-Marie Raoult: 1882 paper on freezing point depression. (Link to biographical sketch of Raoult.) François-Marie Raoult: 1887 paper on the lowering of vapor pressure. Benjamin Thomson (Count Rumford): 1798 paper on the quantity and nature of the heat generated in boring a cannon. This paper is at the ChemTeam site. (Link to a biographical sketch of Rumford.) William Thomson (Baron Kelvin of Largs): some thoughts (not all correct--see Keith J. Laidler, The World of Physical Chemistry, pp. 99100) on an absolute thermodynamic scale of temperature (1848). (Link to a biographical sketch of Kelvin.) William Thomson (Baron Kelvin of Largs): 1852 formulation of the second law of thermodynamics and description of an absolute temperature scale. William Thomson (Baron Kelvin of Largs) (1865): an application of heat transfer to geology, leading Thomson to believe the earth is relatively young. Jacobus van't Hoff: osmosis and the analogy between solutions and gases

(1887). This paper is at the ChemTeam site.

Others • Justus Liebig: Familiar Letters on Chemistry (1843). This monograph on chemistry and some of its applications to agriculture and industry in the middle 19th century is available courtesy of Peter Childs, Limerick, Ireland. • Philosophical Transactions of the Royal Society, volumes 50-67 (175777). This resource is available as page images at the Internet Library of Early Journals, Bodelian Library, Oxford University. The entire volumes are posted, so this resource spans the range of natural philosophy. • Louis-Jacques Thenard: 1819 paper announces discovery of hydrogen peroxide and describes some of its properties (including some painful tests: don't try this at home) Back to the top of the classic papers list Back to the top of the Classic Chemistry site Back to the top of the Classic Chemistry site