IN THE LAST HALF- CENTURY. 75 



are concerned, it is ingenerable and inde- 

 structible. In so far as matter may be 

 conceived to exist in a purely passive 

 state, it is, imaginably, older than mo- 

 tion. But, as it must be assumed to be 

 susceptible of motion, a particle of bare 

 matter at rest must be endowed with the 

 potentiality of motion. Such a particle, 

 however, by the supposition, can have 

 no energy, for there is no cause why it 

 should move. Suppose now that it re- 

 ceives an impulse, it will begin to move 

 with a velocity inversely proportional to 

 its mass, on the one hand, and directly 

 proportional to the strength of the im- 

 pulse, on the other, and will possess Jci- 

 netic energy, in virtue of which it will 

 not onlv continue to move for ever if un- 

 impeded, but if it impinges on another 

 such particle, it will impart more or less 

 of its motion to the latter. Let it be con- 

 ceived that the particle acquires a tenden- 

 cy to move, and that nevertheless it does 



