A KINETIC THEORY OF MATTER. 223 



pulsion are not dependent on a peculiar elastic 

 fluid for their existence. . . ." " Heat may be 

 defined as a peculiar motion, probably a vibration, 

 of the corpuscles of bodies, tending to separate 

 them. . . ." " To distinguish this motion from 

 others, and to signify the causes of our sensations 

 of heat, &c., the name repulsive motion has been 

 adopted." x Here we have a most important idea. 

 It would be a somewhat bold figure of speech to 

 say the earth and moon are kept apart by a 

 repulsive motion ; and yet, after all, what is 

 centrifugal force but a repulsive motion, and 

 may it not be that there is no such thing as 

 repulsion, and that it is solely by inertia that 

 what seems to be repulsion is produced ? Two 

 bodies fly together, and, accelerated by mutual 

 attraction, if they do not precisely hit one 

 another, they cannot but separate in virtue of 

 the inertia of their masses. So, after dashing 

 past one another in sharply concave curves round 



1 " Essay on Heat, Light, and the Combinations of Light " ; 

 Collected Works of Sir Humphry Davy, Vol. II. pp. 10, 14, 

 and 20. 



