THE DYNAMICAL THEORY OF GASES. 29 



Dr Joule* 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 land of Motion which we call 

 Heat, are a complete exposition of the molecular theory adopted in this paper. 

 After reading his investigationf of the distance described by each molecule 

 between successive collisions, I published some propositions J 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. O. E. Meyer II 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 of gravity of the molecules, and whose magnitude 

 is represented very nearly by some function of the distance of the centres of 

 gravity. I have made this modification of the theory in consequence of the 

 results of my experiments on the viscosity of air at different temperatures, and 

 I have deduced from these experiments that the repulsion is inversely as the 

 fifth power of the distance. 



If we suppose an imaginary plane drawn through a vessel containing a great 

 number of such molecules in motion, then a great many molecules will cross 

 the plane in either direction. The excess of the mass of those which traverse 

 the plane in the positive direction over that of those which traverse it in the 

 negative direction, gives a measure of the flow of gas through the plane in 

 the positive direction. 



* Some Remarks on Heat and the Constitution of Elastic Fluids, Oct. 3, 1848. 



t Phil. Mag. Feb. 1859. 



J "Illustrations of the Dynamical Theory of Gases," Phil. Mag. I860, January and July. 



Poggendorff, Jan. 1862; Phil. Mag. June, 1862. 



|| "Ueber die innere Reibung der Gase" (Poggendorff, Vol. cxxv. 1865). 



