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



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 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 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 mole- 

 cules. 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 tempera- 

 tures 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 toge- 

 ther, and so introduce a molecular theory of the second order. 

 The doctrines that all matter is extended, and that no two por- 



