THE PROGRESS OF PHYSICS. 171 



per second, or about six times the velocity of sound in 

 air. 



In 1857, in his famous paper " On the Kind of 

 Motion we call Heat," and in his second paper in 

 1859, Clausius greatly advanced the incipient kinetic 

 theory, calculating, for instance, the average length 

 of the path of a molecule in the interval between 

 two " collisions," or near approaches to another 

 molecule. 



In 1859 and 1860, Clerk Maxwell gave his " Illus- 

 trations of the Dynamical Theory of Gases " in which 

 he demonstrated " the laws of motion of an indefinite 

 number of small, hard, and perfectly elastic spheres 

 acting on one another only during impact." 



By the application of an ingenious statistical 

 method and of general dynamical methods to molec- 

 ular problems, Maxwell greatly advanced the theory 

 of gases and the theory of matter. That he was helped 

 by Boltzmann and Clausius and Kelvin and others 

 goes without saying, but it seems legitimate to asso- 

 ciate with his name the coming of age of the molec- 

 ular theory of matter. It matters not a whit for 

 our general purpose how many corrections may have 

 to be made on his computation that the length of the 

 mean free path of molecules of air is irr.Voir of an 

 inch, or that the number of collisions per second ex- 

 perienced by each molecule is about eight thousand 

 millions; the point is rather that he justified a 

 molecular or atomic conception, harmonising the laws 

 of Boyle, Charles, and Avogadro, and suggesting fur- 

 ther developments which are still prompting re- 

 search. 



B. Cauchy's Suggestion of the Heterogeneity 

 of Matter. As a second illustration of the nature 

 of the argument which has resulted in the modern 



