54 
MR. CLERK MAXWELL ON THE DYNAMICAL THEORY OE OASES. 
This rigidity, however, cannot be directly observed, because the molecules continually 
deflect each other from their rectilinear courses, and so equalize the pressure in all direc- 
tions. The rate at which this equalization takes place is great, but not infinite ; and 
therefore there remains a certain inequality of pressure which constitutes the pheno- 
menon of viscosity. 
I have found by experiment that the coefficient of viscosity in a given gas is indepen- 
dent of the density, and proportional to the absolute temperature, so that if ET be the 
V 
viscosity, ET <x y 
But E =p , therefore T, the time of relaxation, varies inversely as the density and is 
independent of the temperature. Hence the number of collisions producing a given de- 
flection which take place in unit of time is independent of the temperature, that is, of the 
velocity of the molecules, and is proportional to the number of molecules in unit of 
volume. If we suppose the molecules hard elastic bodies, the number of collisions of a 
given kind will be proportional to the velocity, but if we suppose them centres of force, 
the angle of deflection will be smaller when the velocity is greater ; and if the force is 
inversely as the fifth power of the distance, the number of deflections of a given kind will 
be independent of the velocity. Hence I have adopted this law in making my calcu- 
lations. 
The effect of the mutual action of the molecules is not only to equalize the pressure 
in all directions, but, when molecules of different kinds are present, to communicate 
motion from the one kind to the other. I formerly showed that the final result in the 
case of hard elastic bodies is to cause the average vis viva of a molecule to be the same 
for all the different kinds of molecules. Now the pressure due to each molecule is pro- 
portional to its vis viva , hence the whole pressure due to a given number of molecules 
in a given volume will be the same whatever the mass of the molecules, provided the 
molecules of different kinds are permitted freely to communicate motion to each other. 
When the flow of vis viva from the one kind of molecules to the other is zero, the 
temperature is said to be the same. Hence equal volumes of different gases at equal 
pressures and temperatures contain equal numbers of molecules. 
This result of the dynamical theory affords the explanation of the “ law of equivalent 
volumes ” in^ gases. 
We shall see that this result is true in the case of molecules acting as centres of force. 
A law of the same general character is probably to be found connecting the tempera- 
ratures of liquid and solid bodies with the energy possessed by their molecules, although 
our ignorance of the nature of the connexions between the molecules renders it difficult 
to enunciate the precise form of the law. 
The molecules of a gas in this theory are those portions of it which move about as a 
single body. These molecules may be mere points, or pure centres of force endowed 
with inertia, or the capacity of performing work while losing velocity. They may be 
systems of several such centres of force, bound together by their mutual actions, and in 
