Unity of the General Forces of Nature. 213 



mechanical motion of the mass as a whole. The one of 

 these motions may be converted into the other. If a leaden 

 bullet be dropped from a considerable height upon an iron 

 plate which arrests and destroys its mechanical motion, the 

 result is a quivering or vibrating of the particles in their 

 molecular spaces, and this is first, heat ; if carried farther, it 

 may become light, or it may give rise to electricity, which 

 again are only other forms of molecular agitation or disturb- 

 ance ; and either of these may, by cooling, which is but 

 the arrest of the molecular agitation, occasion again the 

 mechanical motion from which it originated. 



Accurately conceived, then, all these phenomena are forms, 

 not, as is so often stated, of force, but of motion. Heat is a 

 mode of motion ; light, too, is the vibration of the particles 

 of the elastic medium which fills the inter-planetary spaces 

 around our globe ; it is another and more rapid vibration, 

 propagating itself through the ether by undulations, in other 

 words, it is another mode of motion. The phenomena of 

 electricit}^, also, manifest themselves simply as attractions 

 and repulsions, — that is, as motions of particles and masses 

 of matter, to and from each other. In the same way, every 

 other manifestation of these imponderable agents is simply, 

 and only, a distinct and peculiar mode of molecular motion. 



Now of all these phenomena, the universal law is that no 

 one is fixed or permanent. Each is a transient modification 

 of some other, or of that which is the common ground of 

 them all. Each is called into existence by another; it 

 comes to view solely by the disappearance of another. One 

 is born because a previous phenomenon of the same kind 

 ceases to exist ; each dies in giving birth to its successor. 

 This extinction, too, is absolute and inevitable. The amount 

 of force involved in one of these changes is ever the same, 

 but it cannot exist in two of these forms at the same time ; 

 the preceding form must cease to exist before that into 

 which it is convertible can take its place. The mechanical 

 motion of a body must utterly stop and cease, before the 



