ME. CLEEK MAXWELL ON THE DYNAMICAL THEOEY OF GASES. 
51 
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 * * * § 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 parti- 
cular, the theory of temperature and pressure in gases and the theory of diffusion are 
clearly pointed out. 
Dr. Joule f 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 investiga- 
tionj of the distance described by each molecule between successive collisions, I pub- 
lished 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. O. E. Meyer 5[ has also investigated the theory of internal friction on the hypo- 
thesis 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 th e 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 
* Mathematical Physics, &c., p. 134. 
t Some Bemarks on Heat and the Constitution of Elastic Fluids, Oct. 3, 1848. 
+ Phil. Mag. Feh. 1859. 
§ Illustrations of the Dynamical Theory of Gases, Phil. Mag. I860,, January and July. 
|| Poggendorff, Jan. 1862 ; Phil. Mag. June 1862. 
If IJeber die innere Eeihung der Gase (Poggendorff, vol. cxxv. 1865). 
H 2 
