48 IDEAL AND ACTUAL GASES 109 



would have to take the influence of this force into account, 

 and as to the change in our views regarding molecules and 

 their motion that are demanded by reason of the cohesion. 

 There are, in fact, two essentially different explanations of 

 the deviations which gases and vapours exhibit from the 

 theoretical laws, and yet both explanations arise at bottom 

 from the same origin. 



In order to understand this double possibility we need 

 only remember that the foundation of our theory contains 

 also several unproved and unprovable assumptions, of which 

 only two come here into account. The first is the assump- 

 tion that gases consist of molecules of invariable mass, the 

 second is the hypothesis that they move in straight lines. 



Of these two hypotheses the latter at first sight seems 

 the more doubtful ; for in any case it is only true with the 

 limitation or exception that at the moment of a collision 

 the motion, till then along a straight line, must experience 

 a sudden change of direction. But the former, too, as we 

 shall see, is not above doubt, and an inexactness in this 

 hypothesis might just as well cause the deviations as an 

 error in the second of the two hypotheses. 



We can frame for ourselves no idea of the cohesion of 

 gases that is essentially different from that of liquids ; we 

 imagine, therefore, forces which act attractively from particle 

 to particle in the direction of the line joining them, and 

 whose strength falls off very quickly as the distance in- 

 creases, so that at a finite or measurable distance the force 

 is infinitely small. It would be as difficult to oppose this 

 customary supposition regarding the nature of cohesion as 

 to contest the essential part of the kinetic theory of gases 

 if we were to ascribe to the forces of cohesion, for a dis- 

 tance of the attracting particles from each other equal to 

 the mean distance apart of those which were the nearest 

 neighbours, a sensible value which comes somehow into 

 consideration. We shall therefore have to assume that 

 attractive forces of any importance are active between two 

 particles only when they actually collide or just graze each 

 other in their paths. 



If we assume this idea of the action of the force of 



