THE DYNAMICAL EVIDENCE OF THE 



which indicate that the value of Jflr is to be found for every pair 

 of particles, and the result* added together. 



CUuaius has established this equation by a very simple mathematical pro- 

 CDM ^th w lji c h I need not trouble you, as we are not studying mathematics 

 to-night. We may see, however, that it indicates two causes which may affect 

 the pressure of the fluid on the vessel which contains it: the motion of its 

 particles, which tends to increase the pressure, and the attraction of its particles, 

 which tends to increase the pressure. 



We may therefore attribute the pressure of a fluid either to the motion 

 of its particles or to a repulsion between them. 



Let us test by means of this result of Clausius the theory that the pres- 

 sure of a gas arises entirely from the repulsion which one particle exerts on 

 another, these particles, in the case of gas in a fixed vessel, being really at rest. 



In this case the virial must be negative, and since by Boyle's Law the 

 product of pressure and volume is constant, the virial also must be constant, 

 whatever the volume, in the same quantity of gas at constant temperature. 

 It follows from this that Rr, the product of the repulsion of two particles 

 into the distance between them, must be constant, or in other words that the 

 repulsion must be inversely as the distance, a law which Newton has shewn 

 to be inadmissible in the case of molecular forces, as it would make the action 

 of the distant parts of bodies greater than that of contiguous parts. In fact, 

 we have only to observe that if Rr is constant, the virial of every pair of 

 particles must be the same, so that the virial of the system must be propor- 

 tional to the number of pairs of particles in the system that is, to the 

 square of .the number of particles, or in other words to the square of the 

 quantity of gas in the vessel The pressure, according to this law, would not 

 be the same in different vessels of gas at the same density, but would be 

 greater in a large vessel than in a small one, and greater in the open air than 

 in any ordinary vessel. 



The pressure of a gas cannot therefore be explained by assuming repulsive 

 forces between the particles. 



It must therefore depend, in whole or in part, on the motion of the particles. 



If we suppose the particles not to act on each other at all, there will be 

 no virial, and the equation will be reduced to the form 



