338 Mr. A. M, Worthington on the 



(it is not at present necessary to distinguish), and with each 

 surface subject to the same pressure. 



If this pressure be increased, we know that the volume 

 of the cube diminishes; and since the diminution is propor- 

 tional to the volume, it follows that, if we have regarded the 

 cube in the first instance as composed of an indefinitely large 

 number of similar separate uniformly distributed molecules 

 the sum of whose masses is the mass of the whole cube and 

 whose masses are independent of their situation, we must 

 now regard the intramolecular distances as uniformly dimi- 

 nished throughout the cube ; and since a definite increase of 

 pressure results in a definite contraction of volume after which 

 the solid again reaches a condition of equilibrium, we may 

 attribute to the molecules a repulsive force which increases as 

 the intramolecular distance is diminished. 



If, keeping the volume constant, we raise the temperature, 

 the pressure exerted by the faces of the cube is increased, 

 and we infer that the repulsive force between the molecules 

 increases with the temperature of the body. 



Since, again, a lowering of the temperature is attended 

 by a uniform contraction of volume even when the external 

 pressure is zero, we infer that the contraction is due to an 

 intramolecular action equivalent to an attractive force, and 

 that the expansion accompanying a rise of temperature is due 

 to the repulsive action overbalancing this attractive force, and 

 since the expansion of the substance results in the attainment 

 of a condition of equilibrium in which the attractive and 

 repulsive actions must be equal, we conclude that the repulsive 

 action diminishes more rapidly with an increase of molecular 

 distance than does the attractive force, and that, conversely, the 

 repulsive action increases with a diminution of distance more 

 rapidly than the attractive. 



It may, however, be urged here that the facts mentioned 

 thus far would be equally well accounted for on the sup- 

 position that the repulsive force increased with an increase in 

 the distance between the molecules, but that the attractive 

 force increased more rapidly. This, however, would require 

 that the repulsive force should diminish as the temperature is 

 raised, a supposition which it would be very difficult to re- 

 concile with what we know of the nature of heat, and which 

 we must therefore reject. Indeed it is important to remember 

 that, in framing a statical theory of molecular equilibrium, our 

 choice of alternative hypotheses may sometimes be guided by 

 information which we possess as to the nature of the dyna- 

 mical phenomena for which we are seeking an equivalent 

 representation. 



