218 DOCUMENT 187 NOVEMBER 1909 [3]Perrin 1909 includes experimental results on rotatory Brownian motion, which had been theoretically treated in Einstein 1906b (Vol. 2, Doc. 32), pp. 379-380. [4]Here the expression "Grösse der Moleküle," as well as "Grösse des Atoms," which is used below, refers to Avogadro's number. See Einstein 1904 (Vol. 2, Doc. 5), pp. 358-359, and Einstein 1979, pp. 38, 44, for other instances of this usage. [5]Planck 1906c, p. 162, gives 61.75x1022 (calculated from experimental data and using his radiation law). [6]The value for Avogadro's number characterized by Perrin as "the most probable" ("la plus probable") is in fact 70.5x1022 (Perrin 1909, p. 111). [7]Probably Einstein's most recent papers on radiation theory, Einstein 1909b (Vol. 2, Doc. 56) and Einstein 1909c (Vol. 2, Doc. 60). [8]Pierre Weiss. [9]The Svedberg (1884-1971) was docent in chemistry at the University of Uppsala. His theoretical (and experimental) work on Brownian motion referred to by Einstein is in Svedberg 1906a, 1906b. See Perrin 1909, pp. 72-74, for Perrin's criticism. [10]Einstein 1907c (Vol. 2, Doc.40). See also Vol. 2, the editorial note, "Einstein on Brown- ian Motion," pp. 219-220, for a discussion of Svedberg's work and Einstein's criticism. [11]On p. 26 of Perrin 1909 it is suggested that the osmotic pressure on a semipermeable barrier separating a solution from a quantity of pure solvent is caused by the collisions of the solute molecules against the barrier. [12]This was done in Einstein's dissertation, Einstein 1905j (Vol. 2, Doc. 15). See Vol. 2, the editorial note, "Einstein's Dissertation on the Determination of Molecular Dimensions," pp. 170-182, for more details. [13]In 1910 Jacques Bancelin performed experiments in Perrin's laboratory, in which the viscosity coefficient of a suspension was correlated with the total volume occupied by the sus- pended particles. He found a discrepancy with Einstein's theoretical prediction (Bancelin 1911a, 1911b). See Docs. 239, 241, and 244 for more details and Vol. 2, the editorial note, "Einstein's Dissertation on the Determination of Molecular Dimensions," pp. 179-182, for a discussion. [14]Paul Langevin (1872-1946) was Professor of Experimental Physics at the College de France. 187. To Michele Besso [Zurich, 17 November 1909] Lieber Michele! Ich danke Dir bestens für Deine Karte. Den Aufsatz in der Umschau hatte ich schon gelesen.[1] Es scheint, dass der inverse Prozess zum photochemi- schen von selbst vor sich geht, insbesondere bei den Vorgängen, welche schwacher Belichtung entsprechen. Eine Art dynamisches Gleichgewicht daraus, also nicht Vorschreiten der Reaktion mit wachsender Zeit. Reiz- schwelle warscheinlich scheinbar. Photochemische Prozesse scheinen des- halb ungeeignet zur Prüfung der Quantenhypothese.[2] Ich bin sehr beschäftigt mit den Vorlesungen, sodass meine wirkliche freie Zeit weniger ist als in Bern.[3] Aber man lernt viel dabe. In Lichtquanten we- nig und ohne Erfolg gegrübelt. Habe Stodola einmal gesprochen.[4] Er ist ein