One of the most essential books for the understanding of heat in the nineteenth century is Theory of Heat (1872,1888) by James Clerk Maxwell[aut]Maxwell, James Clerk (1831-1879). Maxwell knew the works reviewed in Chaps. 1 – 13 , and presented the results in the language of 1888. He was still confused about many things, but he promoted the microscopic perspective for the understanding of material bodies. There are also rare insights in this book that do not appear anywhere else. This chapter surveys the entire book, and gives detailed discussions of areas where necessary. It is, in effect, the record of the last gasp of nineteenth century understanding and the first movement towards the twentieth century perspective.