214 JSFecessary Limitation of the Doctrine of the 



heat can be developed into which it is resolvable ; and that 

 cessation is absolute. So far as the conversion takes place, 

 so far is the destruction of the previous form of motion utter 

 and complete. 



There is in this common relation of all these phenomena 

 to motion, an obvious reason for their correlation with one 

 another. They are phenomena of the same kind. All are 

 simply forms of motion ; and it is no great novelty to learn 

 that one form of motion may be converted into another. The 

 whole doctrine of the convertibility of the forces resolves 

 itself into the very familiar fact of the communication of 

 motion by impulse. Where one of two billiard balls 

 impinges on another in the precise line of its direction, its 

 motion is imparted to the other, and itself is arrested and 

 stopped. So it is also with the atoms and masses of the 

 physical world. The balls may differ in size, and in the pre- 

 cise mode of their motion, but these are the only differences. 

 One imparts to another its motion ; or a large mass commu- 

 nicates movement to a multitude of minute particles ; in 

 each case it has its own motion destroyed by the change ; 

 and this is the whole of it. The convertibility of the forces 

 implies, then, nothing more than the communication of 

 motion by impulse. 



When, however, we examine the particles of matter for 

 their essential characteristics, we find that something more 

 than mobility enters into their nature. Adopting, with some 

 variation of order, the accurate analysis of Sir William 

 Hamilton, they may be grouped under the two general heads 

 which follow. We may regard matter, first, as included in 

 space, and secondly, as occupying space. 



I. Under the first aspect, as included in space, it has 

 position and mobility, 



(1.) It is known in finite parts and forms ; and these must 

 be recognized as occupying definite places in space ; matter, 

 therefore, has (1) position. 



(2.) Next, it has mobility. In consequence of the rela- 



