224 POPULAR LECTURES AND ADDRESSES. 



their common centre of gravity, they fly asunder 

 again. A careless onlooker might imagine they 

 had repelled one another, and might not notice 

 the difference between what he actually sees 

 and what he would see if the two bodies 

 had been projected with great velocity to- 

 wards one another, and either colliding and 

 rebounding, or repelling one another into 

 sharply convex continuous curves, fly asunder 

 again. 



Joule, Clausius, and Maxwell, and no doubt 

 Daniel Bernoulli himself, and I believe every one 

 who has hitherto written or done anything very 

 explicit in the kinetic theory of gases, has taken 

 the mutual action of molecules in collison as 

 repulsive. May it not after all be attractive ? 

 This idea has never left my mind since I first 

 read Davy's Repulsive Motion, about thirty-five 

 years ago, but I never made anything of it, at all 

 events have not done so until to-day (June 16, 

 1884), if this can be said to be making anything 

 of it), when in endeavouring to prepare the present 

 address I notice that Joule's and my own old 



