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The first, third, fifth, and sixth lectures present his account of the revolutionary developments occasioned when he first applied the quantum hypothesis to blackbody radiation. The reader is given a valuable opportunity to witness Planck's thought processes both on the level of philosophical principles as well as their application to physical processes on the microscopic and macroscopic scales.

In the second and fourth lectures Planck shows how the new ideas of statistical mechanics transformed the understanding of chemical physics. The seventh lecture discusses the principle of least action, while the final one gives an account of the theory of special relativity, of which Planck had been an early champion.

These lectures are especially important since they reflect Planck's reconsiderations and rethinking of his original discovery of quantum theory. A new Introduction by Peter Pesic places this book in historical perspective among Planck's works and those of his contemporaries. Now available in this inexpensive edition, it will be of particular interest to students of modern physics and of the philosophy and history of science.

Based on Planck's original papers, the book offers a uniform point of view for the entire field. Rejecting the earlier approaches of Helmholtz and Maxwell, Planck makes no assumptions regarding the nature of heat, but begins with only a few empirical facts from which he deduces new physical and chemical laws. He considers fundamental facts and definitions (temperature, molecular weight, quantity of heat), the first and second fundamental principles of thermodynamics (applications to homogeneous and non-homogeneous systems, proof, general deductions), and applications to special states of equilibrium (homogeneous systems, systems in various states of aggregation, system of any number of independent constituents, gaseous systems, dilute solutions, absolute value of the entropy, Nernst’s theorem). Throughout the book numerous examples are worked.

Although Planck originally intended the book to be simply the connected account of ten years of study, he soon expanded it to a treatise which could serve as an introduction to the study of the entire theory of radiant heat in terms of the recently discovered principle of quantum action. He states his point of view in the introduction: "The hypothesis of quanta … may be reduced to the simple proposition that the thermodynamic probability of a physical state is a definite integral number, or, what amounts to the same thing, that the entropy of a state has quite a definite positive value, which, as a minimum, becomes zero, while in contrast therewith, the energy may, according to the classical thermodynamics, decrease without limit to minus infinity." Although several other points of fundamental value in thermodynamics are included, the book is basically a rigorous elaboration of this fundamental idea.

The treatment starts from the simple known experimental laws of optics and advances, by gradual extension and the addition of the results of electrodynamics and thermodynamics, to the problems of spectral distribution of energy and of reversibility.