6 MATTER, LIVING FORCE AND HEAT. 



"A body may be endowed with living force in several 

 ways. It may receive it by the impact of another body, 

 Thus, if a perfectly elastic ball be made to strike another 

 similar ball of equal weight at rest, the striking ball will 

 communicate the whole of its living force to the ball struck, 

 and, remaining at rest itself, will cause the other ball to 

 move in the same direction and with the same velocity 

 that it did itself before the collision. Here we see an 

 instance of the facility with which living force may be 

 transferred from one body to another. A body may also 

 be endowed with living force by means of the action of 

 gravitation upon it through a certain distance. If I hold a 

 ball at a certain height and drop it, it will have acquired 

 when it arrives at the ground a degree of living force 

 proportional to its weight and the height from which it has 

 fallen. We see, then, that living force may be produced by 

 the action of gravity through a given distance or space. 

 We may, therefore, say that the former is of equal value, 

 or equivalent, to the latter. Hence, if I raise a weight of 

 i lb. to the height of one foot, so that gravity may act on 

 it through that distance, I shall communicate to it that 

 which is of equal value or equivalent to a certain amount 

 of living force ; if I raise the weight to twice the height, I 

 shall communicate to it the equivalent of twice the quantity 

 of living force. Hence, also, when we compress a spring, 

 we communicate to it the equivalent to a certain amount of 

 living force ; for in that case we produce molecular 

 attraction between the particles of the spring through the 

 distance they are forced asunder, which is strictly analogous 

 to the production of the attraction of gravitation through a 

 certain distance. 



" You will at once perceive that the living force of which 



