In this concise hardcover edition, Paul Ehrenfest, one of the 20th century's greatest physicists, reformulated the foundations of the statistical approach in mechanics. Originally published in 1912 as an article for the German Encyclopedia of Mathematical Sciences, it has lost little of its scientific and didactic value, and no serious student of statistical mechanics can afford to remain ignorant of this great work. Part One of the book describes the older formulation of statistico-mechanical investigations (kineto-statistics of the molecule). Part Two takes up the modern formulation of kineto-statistics of the gas model, and Part Three explores W. Gibbs's major work, Elementary Principles in Statistical Mechanics and its coverage of such topics as the problem of axiomatization in kineto-statistics, the introduction of canonical and microcanonical distributions, and the analogy to the observable behavior of thermodynamic systems. Unabridged Dover republication of the edition published by Cornell University Press, Ithaca, New York, 1959. 1 illustration. Bibliography. Notes. Appendixes.