132 Mr. J. C. Maxwell on the Dynamical Theory of Gases. 



ciples of Le Sage to gases and to light. A more extensive ap- 

 plication of the theory of moving molecules was made by Hera- 

 path*. His theory of the collisions of perfectly hard bodies, 

 such as he supposes the molecules to be, is faulty, inasmuch as 

 it makes the result of impact depend on the absolute motion of 

 the bodies, so that by experiments on such hard bodies (if we 

 could get them) we might determine the absolute direction and 

 velocity of the motion of the earth f. This author, however, 

 has applied his theory to the numerical results of experiment in 

 many cases ; and his speculations are always ingenious, and often 

 throw much real light on the questions treated. In particular, 

 the theory of temperature and pressure in gases and the theory 

 of diffusion are clearly pointed out. 



Dr. Joule J has also explained the pressure of gases by the 

 impact of their molecules, and has calculated the velocity which 

 they must have in order to produce the pressure observed in 

 particular gases. 



It is to Professor Clausius, of Zurich, that we owe the most 

 complete dynamical theory of gases. His other researches on 

 the general dynamical theory of heat are well known ; and his 

 memoirs " On the kind of Motion which we call Heat " are a 

 complete exposition of the molecular theory adopted in this 

 paper. After reading his investigation § of the distance described 

 by each molecule between successive collisions, I published some 

 propositions || on the motions and collisions of perfectly elastic 

 spheres, and deduced several properties of gases, especially the 

 law of equivalent volumes, and the nature of gaseous friction. 

 I also gave a theory of diffusion of gases, which I now know to 

 be erroneous ; and there were several errors in my theory of the 

 conduction of heat in gases, which M. Clausius has pointed out 

 in an elaborate memoir on that subject^. 



M. 0. E. Meyer** has also investigated the theory of internal 

 friction, on the hypothesis of hard elastic molecules. 



In the present paper I propose to consider the molecules of a 

 gas, not as elastic spheres of definite radius, but as small bodies 

 or groups of smaller molecules repelling one another with a force 

 whose direction always passes very nearly through the centres 



* Mathematical Physics &c, by John Herapath, Esq. 2 vols. London. 

 Whittaker & Co., and Herapath's Railway Journal Office, 1847. 



f Mathematical Physics &c, p. 134. 



% Some Remarks on Heat and the Constitution of Elastic Fluids, Octo- 

 ber 3, 1848. 



§ Phil. Mag. February 1859. 



|| "Illustrations of the Dynamical Theory of Gases," Phil. Mag. I860, 

 January and July. 



IT Poggendoi-ff, January 1862; Phil. Mag. June 1862. 



** " Ueber die innere Reibung der Gase," Poggcndorff, vol. cxxv. 1865. 



