THE CONSTITUTION OF NATURE. 11 



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 in- 

 destructible 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 combined 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 con- 

 verted into a motion of vibration, which is heat. The 

 vibration, however, so far from causing the extinction 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 attraction 

 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. 

 To produce motion by gravity space must also intervene 

 between the attracting bodies. When they strike to- 



