THE CONSTITUTION OF NATURE. H 



firms the convertibility of natural forces was never in- 

 tended, in the minds of those who understood it, to 

 affirm that such a conversion as that here implied occurs 

 in any case whatever. As regards convertibility into 

 heat, gravity and chemical affinity stand on precisely 

 the same footing. The attraction in the one .case is as 

 indestructible as in the other. Nobody affirms that 

 when a stone rests upon the surface of the earth, the 

 mutual attraction of the earth and stone is abolished; 

 nobody means to affirm that the mutual attraction of 

 oxygen for hydrogen ceases, after the atoms have com- 

 bined to form water. What is meant, in the case of 

 chemical affinity, is, that the pull of that affinity, acting 

 through a certain space, imparts a motion of translation 

 of the one atom towards the other. This motion is not 

 heat, nor is the force that produces it heat. But when 

 the atoms strike and recoil, the motion of translation is 

 converted into a motion of vibration, which is heat. 

 The vibration, however, so far from causing the extinc- 

 tion of the original attraction, is in part carried on by 

 that attraction. The atoms recoil, in virtue of the 

 elastic force which opposes actual contact, and in the 

 recoil they are driven too far back. The original at- 

 traction then triumphs over the force of recoil, and 

 urges the atoms once more together. Thus, like a 

 pendulum, they oscillate, until their motion is imparted 

 to the surrounding aether; or, in other words, until 

 their heat becomes radiant heat. 



In this sense, and in this sense only, is chemical 

 affinity converted into heat. There is, first of all, the 

 attraction between the atoms; there is, secondly, space 

 between them. Across this space the attraction urges 

 them. They collide, they recoil, they oscillate. There 

 is here a change in the form of the motion, but there is 

 no real loss. It is so with the attraction of gravity. 



