104 THE REALITIES OF MODERN SCIENCE 



is unwound. The uncoiled position is normal for the 

 spring, since to wind it requires work. In this condi- 

 tion the various molecules of which it is composed have 

 definite positions with reference to each other. When, 

 however, we bend the spring, as in winding, we force 

 them to assume a new configuration. Now, whether 

 in the act Of bending we pull two adjacent molecules 

 farther apart or push them closer together we change 

 their separations and potential energy. The proof, 

 however, is not in the separation but in the subsequent 

 ability of the system to do work. 



In the case of the spring the system is molecular. 

 In that of the explosive mixture in the cartridge it 

 consists of a large number of atoms, grouped into 

 molecules of two or more chemical compounds. The 

 atoms which form these compounds are, of course, 

 capable of forming other compounds, the products 

 of the explosion. (If the explosive is a single com- 

 pound it must be an unstable one from which the com- 

 ponent atoms form more stable compounds.) In the 

 case of the pile driver and the earth, we have recog- 

 nized a separation of the parts, not of individual mole- 

 cules or atoms, but of molecules in the bulk. The bulk 

 of molecules forming the weight are separated from 

 the bulk of those which form the earth. 



These are illustrations not only of the energy of 

 separation, but of the three different types of separa- 

 tion. In the system of pile driver and earth the 

 separation is of more than molecular magnitude, that 

 is, too large for forces between molecules to come into 

 play. In the coiled spring the separations are small 

 enough for molecules to exert forces upon each other. 



