248 POPULAR LECTURES AND ADDRESSES. 



model gas. The mutual action at a distance, 

 repulsive or attractive according to the mutual 

 aspect of the two bodies when passing within 

 collisional distance l of one another, suffices to 

 produce the change of direction of motion in 

 collision, which essentially constitutes the 

 foundation of the kinetic theory of gases ; and 

 which, as we have seen before, may as well be 

 due to attraction as to repulsion, so far as we 

 know from any investigation hitherto made in 

 this theory. 



There remains, however, as we have seen 

 before, the difficulty of providing for the case 

 of actual impacts between the solids ; which 

 must be done by giving them massless spring- 



1 According to this view there is no precise distance, or definite 

 condition respecting the distance, between two molecules at which 

 apparently they come to be in collision, or, when receding from one 

 another, they cease to be in collision. It is convenient, however, in 

 the kinetic theory of gases, to adopt arbitrarily a precise definition 

 of collision, according to which two bodies or particles mutually 

 acting at a distance may be said to be in collision when their 

 mutual action exceeds some definite arbitrarily assigned limit, as, 

 for example, when the radius of curvature of the path of either body 

 is less than a stated fraction (i/ioo, for instance) of the distance 

 between them. 





