Mr. J. C. Maxwell on the Dynamical Theory of Gases. 133 



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 conse- 

 quence of the results of my experiments on the viscosity of air 

 at different tempera hires, and I have deduced from these expe- 

 riments 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. 



If the plane be made to move with such a velocity that there 

 is no excess of flow of molecules in one direction through it, 

 then the velocity of the plane is the mean velocity of the gas 

 resolved normal to the plane. 



There will still be molecules moving in both directions through 

 the plane, and carrying with them a certain amount of momen- 

 tum into the portion of gas which lies on the other side of the 

 plane. 



The quantity of momentum thus communicated to the gas on 

 the other side of the plane during a unit of time is a measure 

 of the force exerted on this gas by the rest. This force is called 

 the pressure of the gas. 



If the velocities of the molecules moving in different direc- 

 tions were independent of one another, then the pressure at any 

 point of the gas need not be the same in all directions, and the 

 pressure between two portions of gas separated by a plane need 

 not be perpendicular to that plane. Hence, to account for the 

 observed equality of pressure in all directions, we must suppose 

 some cause equalizing the motion in all directions. This we 

 find in the deflection of the path of one particle by another when 

 they come near one another. Since, however, this equalization 

 of motion is not instantaneous, the pressures in all directions 

 are perfectly equalized only in the case of a gas at rest, but 

 when the gas is in a state of motion, the want of perfect equality 

 in the pressures gives rise to the phenomena of viscosity or in- 

 ternal friction. The phenomena of viscosity in all bodies may 

 be described, independently of hypothesis, as follows : — 



A distortion or strain of some kind, which, we may call S, is 

 produced in the body by displacement. A state of stress or 

 elastic force, which we may call F, is thus excited. The relation 

 between the stress and the strain may be written F = ES, where 

 E is the coefficient of elasticity for that particular kind of strain. 



