42 Mr. A. P. Laurie on the Relation betiveen the Heats of 



pulse is then given, sufficient (if the bodies are supposed per- 

 fectly elastic) to cause each to return on its path with a velocity 

 exactly equal to that with which it arrived. This further impulse 

 must also be instantaneous and infinite ; for, force being the 

 cause of motion, if the impulse were finite it would at once 

 cause the bodies to separate through an indefinitely small 

 space, and then, ex hyp., no further action could take place, and 

 the bodies would recede from each other with indefinitely small 

 velocities. If, then, we make these three assumptions^(l) that 

 there is an infinite impulse developed on the collision, which 

 brings the atoms to rest, (2) that there is a further infinite 

 impulse, which separates them, (3) that this further impulse, 

 while infinite, is such as exactly to reverse the previous motion 

 of each particle — then the conservation of energy may still be 

 supposed to hold through the collision. 



It remains to ask whether there are any advantages in the 

 collision theory such as would warrant us in discarding the 

 principle of continuity, and in making the somewhat vio- 

 lent assumptions described above. The advantages specially 

 claimed by its advocates appear to be that it does away with 

 the conception of action at a distance, and also with that of 

 potential energy. The latter supposition, however, is not cor- 

 rect. At the instant when the two atoms are at rest their 

 actual energy is zero, and the energy existing is entirely 

 potential, being due to their capacity of generating a return 

 velocity equal to that of arrival. Of the former supposition I 

 have already spoken; and I may add that I have elsewhere* 

 shown it to be impossible to explain certain elementary facts 

 of physics without the hypothesis of action at a distance. 



VI. Relations between the Heats of Combination of the 

 Elements and their Atomic Weights. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, 



THE periodic variation of the properties of the elements 

 with their atomic weights is now accepted as proved by 

 chemists; but no similar connexion seems yet to have been estab- 

 lished between the atomic weights and heats of combination of 

 the elements. Now, if the atomic weights of the elements are 

 taken as abscissas, and their atomic heats of combination with 

 chlorine as ordinates of a curve, the heats of combination will 

 be seen to be a periodic function of the atomic weights. 



* "On Action at a Distance," Phys. Soc. 1881 ; Phil. Mag. Dec. 1880. 



