Jeans's primary aim with the first edition of his book, originally published in 1904, was to 'develop the theory of gases upon as exact a mathematical basis as possible'. Twenty years later and those theories were being revolutionised by Quantum Theory. In this fourth edition, Jeans does not attempt to avoid the discoveries of this topical science, but rather exposes the many difficulties that classical theory was experiencing, and how those problems disappeared with Quantum Theory. This edition therefore offers a fascinating insight into a field of physics in transition between two great models of physical science.