"The first edition of this book appeared in 1877, at the time of the most rapid and beautiful development of the kinetic theory of gases. About twenty years before, the founders... Kronig and Clausius, had explained the expansive tendency of gases, and had calculated their pressure on the assumption that the smallest particles of gases do not repel each other, but are in rapid motion. From the theory based on this supposition not only were the laws of gases... deduced... but also new laws, hitherto undreamt of, were discovered... [and] afterwards confirmed... by experiment. These results, which we owe to Maxwell and Clausius, quickly won to the theory many friends and adherents. ...I undertook ...to exhibit the ...theory ...such ...as to be more easily intelligible ...especially to chemists and other natural philosophers to whom mathematics are not congenial. ...I endeavoured ...not only to develop the theory by calculation, but ...to support it by observation and found it on experiment. I... collected... and summarised, the observations by which the admissibility of the theory might be tested and its correctness proved. ...The mathematical discussions form ...an Appendix which ...need not be studied by every reader ..."