THE CONSTITUTION OF NATURE. 13 



that heat is generated. This always occurs in unelastic 

 collision, the heat developed being the exact equivalent 

 of the sensible motion extinguished. This heat virtu- 

 ally declares that the property of elasticity, denied to the 

 masses, exists among their atoms; by the recoil and os- 

 cillation of which the principle of conservation is 

 vindicated. 



But ambiguity in the use of the term ' force ' makes 

 itself more and more felt as we proceed. We have 

 called the attraction of gravity a force, without any 

 reference to motion. A body resting on a shelf is as 

 much pulled by gravity as when, after having been 

 pushed off the shelf, it falls towards the earth. We 

 applied the term force also to that molecular attraction 

 which we called chemical affinity. When, however, we 

 spoke of the conservation of force, in the case of elastic 

 collision, we meant neither a pull nor a push, which, as 

 just indicated, might be exerted upon inert matter, but 

 we meant force invested in motion the vis viva, as it 

 is called, of the colliding masses. 



Force in this form has a definite mechanical mea- 

 sure, in the amount of work that it can perform. The 

 simplest form of work is the raising of a weight. A 

 man walking up-hill, or up-stairs, with a pound weight 

 in his hand, to an elevation say of sixteen feet, performs 

 a certain amount of work, over and above the lifting of 

 his own body. If he carries the pound to a height of 

 thirty-two feet, he does twice the work; if to a height 

 of forty-eight feet, he does three times the work; if to 

 sixty-four feet, he does four times the work, and so on. 

 If, moreover, he carries up two pounds instead of one, 

 other things being equal, he does twice the work; if 

 three, four, or five pounds, he does three, four, or five 

 times the work. In fact, it is plain that the work per- 

 formed depends on two factors, the weight raised and 



