BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 105 



idea of the kinetic origin of pressure. We suppose that the pressure 

 of a gas, for instance, is produced by the impact of the molecules 

 against the walls of the vessel; and we compute the magnitude of the 

 pressure from the total change in one second of the momentum of 

 the molecules striking on unit area of the wall. The velocity with 

 which the molecules strike the walls decreases with falling tempera- 

 ture and vanishes at the absolute zero. The pressure exerted by a 

 liquid also is thought of in .most attempts at a theory of liquids as 

 exerted by the same mechanism and is computed in the same way. 

 But it is entirely obvious that the molecules may exert pressure on 

 the walls in another way. It is inconceivable that at the absolute zero 

 a substance should not resist an attempt to compress it; yet at this 

 temperature there can be no kinetic effect exerted by the molecules 

 as a whole. Under these conditions the molecules transmit pressure in 

 the same way that a compressed spring does; that is, they behave 

 like elastic solids. It does not concern us here to inquire what the 

 ultimate origin of this elasticity is; it may be kinetic within the atom. 

 The point is simply this; from our point of view, which regards the 

 molecule as a whole, we must recognize two possible functions or 

 modes of action; the molecule may behave like a moving centre of 

 mass with kinetic energy and momentum, or it may behave like an 

 elastic solid. The molecule may exert pressure in virtue of either one of 

 these two modes of action. Under ordinary conditions the momentum 

 effect of its motion as a whole is almost the entire effect. But if we 

 examine the rnathematical analysis which justifies us in putting the 

 pressure equal to the total change of momentum in unit time, we 

 shall find that we made certain simplifying assumptions. We assumed 

 that each collision with the wall is unimpeded; that is, the molecule 

 approaches the wall during free flight, has a single encounter and 

 makes a clean get-away. And this assumption was necessary; as soon 

 as the molecule is interfered with during its collision, as it may be 

 by a collision from another molecule from behind, the simple change of 

 momentum relation ceases to give the pressure, and we must treat 

 our molecule during collision as an elastic solid. Now it is evident 

 that as the volume becomes less, that is, as the pressure increases, the 

 total number of collisions with interference will increase, and our 

 kinetic conception of pressure becomes less and less useful. 



A very simple model, which may or may not correspond to the 

 physical facts, is instructive in showing how under different condi- 

 tions we may compute the pressure from the momentum effect alone, 

 or must consider also the elasticity effects. The substance Ave are 



