x x x v i I N T R O D U C T I O N T O V O L U M E 1 4 The letters and writings presented here reveal novel and previously unnoticed connections between seemingly disparate aspects of Einstein’s scientific activities during these two years. In addition to his well-known, fundamental papers on the quantum theory of the ideal gas, we find him deeply involved with scrutinizing the Compton effect and the Bohr-Kramers-Slater theory. The volume also documents his preoccupation with a large number of new experiments designed to test these subjects, as well as an entire series of speculations, theories, and proposed obser- vations meant to interrogate what he called the “electromagnetic bookkeeping” of the Earth (Doc. 87). An idea to test an empirical consequence of his approach to unified field theory based on Eddington’s affine theory engaged his attention in summer and fall of 1923 he collaborated with Hermann Mark on how to design the corresponding ex- periment (Doc. 152). In Berlin, as well as in Kiel and Lautrach, he also worked on the interpretation and experimental confirmation of the Compton effect. After giv- ing up on this approach toward unified field theory, he kept thinking about the re- lationship between quantum theory and classical field theory, respectively. This process resulted by the end of the year in a paper relating quantum aspects to an overdetermination of classical field equations (Doc. 170). While in Berlin from January through April 1924, Einstein continued work on the implications of the Compton effect (Doc. 236). In May 1924, he traveled to Kiel for a month, and upon his return presented his objections to the Bohr-Kramers- Slater theory (Doc. 256) at the Berlin physics colloquium. He embarked on an au- tomobile trip to Göttingen to visit Max and Hedwig Born, and, during the summer, between a meeting of the International Committee on Intellectual Cooperation in Switzerland and his holiday stay once more in Lautrach, he devoted himself with extraordinary alacrity to the translation of S. N. Bose’s paper (Doc. 278) and the composing of a new quantum theory of the ideal gas (Docs. 283, 315, 316). The paper engendered disputes and exchanges, leading Einstein to offer clarifications regarding the revolutionary new quantum statistics in further publications (Docs. 385, 427) before departing for South America, where he lectured on relativ- ity and wrote several articles (Docs. 463, 467, 479, 481, 485). The current volume testifies yet again to the ever-increasing amount of corre- spondence that would be the hallmark of the coming decades in Einstein’s life. For the twenty-six months covered here, the volume presents a total of 1,103 incoming and outgoing letters that Einstein exchanged with almost 500 different correspon- dents (see the Alphabetical List of Correspondence, pp. 783–804). Of the total cor- respondence, 411 letters are presented as full texts, and 692 in abstract. Among the total 411 letters written by Einstein during this period, 261 are presented as full texts, and 149 in abstract.