POTENTIAL ENEEGT OF CHEMICAL AFFINITY. 283 



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 

 can 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 inva- 

 riable, 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 combi- 

 nation there exists a certain amount of potential energy; 

 ivhen 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 de- 

 gree of their chemical affinity. Once they are let loose 

 and permitted to strike together the potential energy pre- 

 viously represented by the compressed springs disappears, 

 and in its place we have the kinetic energy, represented by 

 the heat developed whon the balls strike together. To 

 pull them apart again, against the springs, to their original 

 positions, just so much mechanical work must be spent as 

 is the equivalent of that amount of heat which appeared 

 when they struck; and thus kinetic energy will again be- 

 come latent in breaking up the compound represented by 

 the two in contact. The energy liberated in chemical com- 

 bination is the most important source of that used in our 

 machines: and also of that spent by the living Body. 



The Relation between the Matters Removed from the 

 Body daily and the Energy Spent by it. A working 

 locomotive is, we know, constantly losing matter to the 

 -exterior in the form of ashes and gaseous products of com- 



