V O L . 3 , D O C . 1 0 a B O L T Z M A N N ’ S P R I N C I P L E 1 3 Nachlass Zangger]). The meeting took place at the Zunfthaus zur Zimmerleuten, began at 8:15 P.M., and was presided over by Prof. E. Lüdin. Twenty-three members and fourteen guests were present. The minutes of the meeting provide the following abstract: “Der Referent interpretiert zuerst das Boltzmannsche Prinzip. Dasselbe lautet: S=k lg(W), wobei W die Wahrscheinlichkeit eines Zustandes mit dem Entropiewert S eines isolierten Systems bedeutet. Darauf wird gezeigt, dass k eine universelle Konstante ist die durch die Gaskonstante R, die Zahl N der in einem Grammolekül enthaltenen Mole- küle ausgedrückt werden kann, gemäss der Gleichung k=R/N. Die statistische Wahrscheinlichkeit W eines Zustandes kann mittelst der Boltzmannschen Gleichung durch die auf thermodynamischem Wege in jedem einzelnen Falle zu bestimmende Grösse S berechnet werden. Auf diesem Wege wurde die Gesetze der Brownschen Bewegung abgeleitet, es wird der Weg angegeben, auf dem das von durchstrahlten homogenen Flüssigkeiten & Flüssigkeitsgemischen insbesondere in der Nähe des kritischen Zustandes durch Opaleszenz seitlich angestrahlte Licht exakt berechnet werden kann.” The minutes state that the lecture gave rise to a “very lively” (“äusserst lebhaft”) debate, in which participated, among others, Aurel Stodola, Pierre Weiss, and Georg Bredig. The meeting concluded at 10:45 P.M. [2]“Naturwissenschaft” was corrected from “Wissenschaft.” Similarly, at the end of the paragraph, “naturwissenschaftliche” was corrected from “wissenschaftliche.” [3]The words “nach der Erfahrung” were interlineated. [4]Rudolf Clausius (1822–1888) had been Professor of Physics at the Zurich Polytechnic from 1857 to 1867 and was succeeded by Einstein’s teacher Heinrich F. Weber. For a historical discussion of Clausius’s work, see Brush 1976, pp. 160–182, and Jungnickel and McCormmach 1986, pp. 163– 169, 193–202. Weber’s lectures on the theory of heat closely followed Clausius’s approach, and according to later testimony, Einstein was familiar with Clausius’s “general investigations of kinetic theory” (see Vol. 2, the editorial note, “Einstein on the Foundations of Statistical Physics,” p. 42, note 14). The achievements attributed here to Clausius, i.e., the ratio of specific heats, and the relationship between heat conduction, viscosity, and diffusion, are discussed—without mentioning Clausius’s name—and derived within the framework of the molecular theory of heat in Einstein’s lecture notes for his course on the kinetic theory of heat, held at the University of Zurich in the summer of 1910 (Vol. 3, Doc. 4, pp. 179–188). [5]Around the time of this talk, Einstein confessed: “I only wish to add that the road taken by Gibbs in his book, which consists in one’s starting directly from the canonical ensemble, is in my opinion preferable to the road I took. Had I been familiar with Gibbs’s book at that time, I would not have published those papers at all, but would have limited myself to the discussion of just a few points” (“Ich bemerke nur noch, daß der von Gibbs in seinem Buche eingeschlagene Weg, der darin besteht, daß man gleich von einer kanonischen Gesamtheit ausgeht, nach meiner Meinung, dem von mir ein- geschlagenen vorzuziehen ist. Wenn mir das Gibbssche Buch damals bekannt gewesen wäre, hätte ich jene Arbeiten überhaupt nicht publiziert, sondern mich auf die Behandlung einiger weniger Punkte beschränkt” (Einstein 1911c [Vol. 3, Doc. 10]). For Einstein’s reading of Maxwell, Boltzmann, and Gibbs, see Vol. 2, the editorial note, “Einstein on the Foundations of Statistical Physics,” p. 44. [6]The following argument was also made in §5 of Einstein 1905i (Vol. 2, Doc. 14, pp. 158–160). [7]The following problem is also discussed in Einstein’s lecture notes on the kinetic theory of gases, Vol. 3, Doc. 4, pp. 234–236. [8]In Perrin 1908. [9]The relation was derived in §5 of Einstein 1905k (Vol. 2, Doc. 16, p. 234). See Vol. 2, the edito- rial note, “Einstein on Brownian Motion,” pp. 206–222, for further discussion. [10]The following argument was also given in §2 of Einstein 1910d (Vol. 3, Doc. 9). [11]Einstein’s paper on critical opalescence (Einstein 1910d [Vol. 3, Doc. 9], received by Annalen der Physik on 8 October 1910), followed up on an earlier contribution by Marian von Smoluchowski (Smoluchowski 1908). Smoluchowski had shown how the phenomenon of critical opalescence arises as a result of statistical density fluctuations in the vicinity of the critical point. Einstein gave a detailed calculation of the quantity of light given off laterally through opalescence on the basis of Maxwell’s equations. In the first two paragraphs of his paper, Einstein discussed more generally the use and meaning of Boltzmann’s principle for the case in which one does not have a detailed molecular theory at hand. Although Einstein thought the argument in principle to be valid near the critical point to any