DOCUMENT 128 NOVEMBER 1908 149 Atherwirbeltheorie der Materie, in welcher Äther und Materie wesensgleich sind, am ehesten zum Ziel führt. Dieses Bild muß dann vor allem die De- formation erklären, die ein Ätherwirbel durch Translation erfährt. Die Poin- caresche Hypothese eines Atherdrucks würde dann aus dem Atherbilde her- aus zu erklären sein. Das scheint mir sehr schwer zu sein. Der einstweilen gewonnene Standpunkt, daß man rein phänomenologisch alles berechnen kann, ist schon ein großer Gewinn. Mit freundlichen Grüßen Ihr ergebener A. H. Bucherer ALS. [6 193]. See Docs. 117 and 119 for more on Bucherer's work on Becquerel rays. Bucherer 1908b, which was presented 22 September at the 80th meeting of the Gesell- schaft Deutscher Naturforscher und Ärzte in Cologne. Probably Einstein 1906e. The argument that follows is also contained in Bucherer 1909, pp. 535-536. The square brackets in the following equation are in the original and denote the vector product. H. A. Lorentz see Lorentz 1895, pp. 24-26. A copy of Lorentz 1906-a reprint of Lorentz 1895-with "Zangger" written in Zangger's hand at the top of the title page, is in Einstein's personal library. In the preceding expression v is the speed of light. A "Tensortripel" corresponds to what is now known as a three-dimensional symmetric tensor. The term was first introduced by Woldemar Voigt (see Voigt 1898, §2). On pp. 27-28 of Lorentz 1895 the problem of stresses in the ether is discussed. Lorentz calculates the Maxwell stresses on a volume of free ether and shows that in general these stresses do not vanish and should set the volume into motion. Since the ether is taken to be immobile as a whole, there is a contradiction, which is then cleared away by the assumption that it is meaningless to speak of forces that work on the ether. This implies that for the ether the principle of action and reaction does not hold. For this conclusion Lorentz was criticized by Henri Poincare (1854-1912) (see Poincare 1900). The introduction of the concept of elec- tromagnetic momentum offered a way out of these difficulties. See Miller 1981, sec. 1.7, for a historical review. See Planck 1908a, in which Planck extends the concept of momentum beyond mechan- ics and electrodynamics by arguing that for every energy flow there exists an associated mo- mentum which is found by dividing the energy flow by c2. A momentum flow is also connected with this momentum. The 80th meeting of the Gesellschaft Deutscher Naturforscher und Ärzte was held from 20 until 26 September in Cologne. "Elektromag" is underscored in red, but part of the red line (underneath the letters "ktro") is deleted in pencil. Cf. Einstein's comments on the desirability of founding special relativity on basic ele- ments in Doc. 73. See Whittaker 1951, pp. 293-303 for a review of theories of vortex atoms. See Poincare 1900 for Poincare's hypothesis.