4 MATTER, LIVING FORCE AND HEAT. 



also it possesses the faculty of repulsion, by which force 

 the particles tend to separate further from each other. 

 Wherever these forces exist, they do not supersede the 

 attraction of gravitation. Thus the weight of a piece of 

 iron or steel is in no way affected by imparting to it the 

 magnetic virtue. 



" Besides the force of gravitation, there is another very 

 remarkable property displayed in an equal degree by every 

 kind of matter — its perseverance in any condition, whether 

 of rest or motion, in which it may have been placed. This 

 faculty has received the name of zV/£r^z, signifyingpassiveness, 

 or the inability of any thing to change its own state. It is in 

 consequence of this property that a body at rest cannot be 

 set in motion without the application of a certain amount of 

 force to it, and also that when once the body has been set 

 in motion it will never stop of itself, but continue to move 

 straight forwards with a uniform velocity until acted upon 

 by another force, which, if applied contrary to the direction 

 of motion, will retard it, if in the same direction will accelerate 

 it, and if sideways will cause it to move in a curved direction. 

 In the case in which the force is applied contrary in direction, 

 but equal in degree to that which set the body first in motion, 

 it will be entirely deprived of motion whatever time may have 

 elapsed since the first impulse, and to whatever distance the 

 body may have travelled. 



"From these facts it is obvious that the force expended in 

 setting a body in motion is carried by the body itself, and 

 exists with it and in it, throughout the whole course of its 

 motion. This force possessed by moving bodies is termed by 

 mechanical philosophers vis viva, or living force. The term 

 may be deemed by some inappropriate, inasmuch as there is 

 no life, properly speaking, in question ; but it is useful 



