THE DYNAMICAL THEORY OF GASES. 33 



be independent of the velocity. Hence I have adopted this law in making my 

 calculations. 



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 shewed 

 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 proportional 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 temperatures 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 this case the different centres may either be 

 separated, so as to form a group of points, or they may be actually coincident, 

 so as to form one point. 



Finally, if necessary, we may suppose them to be small solid bodies of a 

 determinate form ; but in this case we must assume a new set of forces binding 

 the parts of these small bodies together, and so introduce a molecular theory 

 of the second order. The doctrines that all matter is extended, and that no 

 two portions of matter can coincide in the same place, being deductions from 

 our experiments with bodies sensible to us, have no application to the theory 

 of molecules. 



VOL. II. 5 



