## Excerpt

It is the threefold purpose of this essay, first to give a coherent and fairly inclusive account of the well-known and generally accepted portions of Einstein's theory of relativity, second to treat the extension of thermodynamics to special and then to general relativity, and third to consider the applications both of relativistic mechanics and relativistic thermodynamics in the construction and interpretation of cosmological models.

The special theory of relativity will first be developed in the next three chapters, which are devoted respectively to the kinematical, mechanical, and electromagnetic consequences of the two postulates of special relativity. In Chapter II, under the general heading 'The Special Theory of Relativity', the two postulates of the theory will be presented, together with a brief statement of the confirmatory empirical evidence in their favour; their kinematical consequences will then be developed, firstly using the ordinary language which refers kinematical occurrences to some selected set of three Cartesian axes and the set of clocks that can be pictured as moving therewith, and secondly using the more powerful quasi-geometrical language provided by the concept of a four-dimensional space-time continuum. In Chapter III, Special Relativity and Mechanics, we shall develop first the mechanics of a particle and then those of a mechanical continuum from a postulatory basis which is obtained by adding the ideas of the conservation of mass and of the equality of action and reaction to the kinematics of special relativity. No appeal to analogies with electromagnetic results will be needed to obtain the complete treatment, and the considerations will be maintained on a macroscopic level throughout. Finally, in Chapter IV, Special Relativity and Electrodynamics, we shall complete our treatment of the more familiar subject-matter of the special theory, by developing the close relationships between special relativity and electromagnetic theory. The first part of this chapter will be devoted to the incorporation of the Lorentz electron theory in the framework of special relativity, a procedure which tacitly assumes a respectable amount of validity still inherent in classical microscopic considerations in spite of the evident necessity . . .