236 THERMAL, ELECTRICAL, & RADIATION PHENOMENA Weber's supervision (see Doc. 63, especially notes 2 and 3). After graduation, he hoped to use Weber's laboratory for research on the Thomson effect (see Doc. 74). Around the turn of the century, Riecke and Drude published comprehensive theories of thermal and electrical phenomena in metals that were based on an atomistic conception of electricity.[3] The following letter shows that, by 1899, Einstein also looked upon the study of the relationship between thermal and electrical phenomena as a way to explore the microphysical structure of matter: he had an experimental method to decide "ob die latente Wärme in Metallen auf Bewegung der ponderabeln Materie oder der Elektrizität zurückzuführen ist." In the spring of 1901 Einstein raised the possibility that the internal kinetic energy ("latente kinetische Energie der Wärme") of solids and liquids is "elektrische Resona- torenenergie" (Doc. 93). If it were, there should be some link between the thermal and optical properties of matter. Einstein decided to look for a correlation between spectral properties and deviations from Dulong and Petit's law (see Docs. 93, 94). These speculations are his first known attempt to relate Planck's resonator model (men- tioned in Docs. 96, 97)[4] to the problem of specific heats. He soon came to doubt these speculations (see Doc. 97), but half a decade later he developed the first quantum theory of the specific heat of solids.[5] By 1899 Einstein was pondering the nature of radiation (see Doc. 45).[6] In April 1901 he discussed Planck's work on black body radiation, to which he had an objec- tion (see Docs. 96, 97). These comments seem to concern Planck's classical theory. Einstein probably read one of Planck's quantum papers soon thereafter (see Doc. 97). He noted a parallelism between black body radiation and the kinetic energy of molecules, which led him to raise the possibility of a direct transformation of kinetic energy into light (see Doc. 102). Soon afterwards, Einstein commented enthusiastically on Lenard 1900, a paper on the generation of cathode rays by ultraviolet light (Doc. 111).[7] In 1901 Einstein studied Drude's electron theory of metals (see note 2 and Docs. 96, 97). This kinetic theory of freely moving charge carriers ("Kerne" or "Elektronen") provided explanations for thermoelectric effects and the Wiedemann-Franz law, among other phenomena.[8] Einstein was enthusiastic about Drude's theory, noting that its basic assumption had previously occurred to him, "daß es vorzugsweise nega- [3] Riecke 1898 and Drude 1900c, d. Their theories elaborated earlier ideas of W. Weber, Giese, and F. Kohlrausch (see Riecke 1898, pp. 355, 381, and Drude 1900c, p. 566). [4] Adolf Fisch, a fellow student at the Aargau Kantonsschule and the ETH, Section VI A, is said to have drawn Einstein's atten- tion to Planck's articles (see Lüscher 1944, p. 623). [5] See Einstein 1907. [6] See also the editorial note, "Einstein on the Electrodynamics of Moving Bodies." [7] Later work by Lenard on the photo- electric effect is cited in Einstein 1905a, a basic contribution to the understanding of both black body radiation and the photoelectric effect. [8] Einstein 1905a, p. 133, refers to Drude's explanation of the Wiedemann-Franz law.