304 THE HUMAN BODY. 



stone pulled up by a string and left suspended in the air. 

 We know a certain amount of energy was used to lift it; but 

 while it hangs we have neither heat nor light nor mechanical 

 work to represent it. Still the energy is not lost; we know 

 we have only to cut the string and the weight will fall, and 

 striking something give rise to heat. Or we may wind up a 

 spring and keep it so by a catch. In winding it up a certain 

 amount of energy in the form of mechanical work was used 

 'to alter the form of the spring. Until the catch is removed 

 this energy remains stored away as potential energy : but we 

 know it is not lost. Once the spring is let loose again it may 

 drive a clock or a watch, and in so doing will perform again 

 just so much work as was spent in coiling it; and when the 

 watch has run down this energy will all have been turned 

 into other forms mainly heat developed in the friction of 

 the parts of the watch against one another: but partly also 

 in producing movements of the air, a portion of which we 

 ean readily observe in the sound of its ticking. The law of 

 the conservation of energy does not say, then, that either the 

 total potential or the total kinetic energy in the universe is 

 constant in amount: but that the sum of the two is invariable, 

 while constantly undergoing changes from kinetic to potential 

 and vice versa : and from one form of kinetic to another. 



The Energy of Chemical Affinity. Between every two 

 chemical atoms which are capable of entering into combina- 

 tion there exists a certain amount of potential energy; when 

 they unite this energy is liberated, usually in the form of heat, 

 and once they have combined a certain amount of kinetic 

 energy must be spent to pull them apart again; this being 

 exactly the amount which was liberated when they united. 

 The more stable the compound formed the more kinetic 

 energy appears during its formation, and the more must be 

 spent to break it up again. One may imagine the separated 

 atoms as two balls pushed together by springs, the strength 

 of the spring being proportionate to the degree of their 

 chemical affinity. Once they are let loose and permitted to 

 strike together the potential energy previously represented by 

 the compressed springs disappears, and in its place we have 

 the kinetic energy, represented by the heat developed when 

 the balls strike together. To pull them apart again, against 

 the springs, to their original positions, just so much mechani- 

 cal work must be spent as is the equivalent of that amount 



