THE CONSTITUTION OF NA TURE. 7 



It is sometimes stated that gravity is distinguished 

 from all other forces by the fact of its resisting conver- 

 sion into other forms of force. Chemical affinity, it is 

 said, can be converted into heat and light, and these again 

 into magnetism and electricity: but gravity refuses to be so 

 converted; being a force maintaining itself under all cir- 

 cumstances, and not capable of disappearing to give place 

 to another. The statement arises from vagueness of 

 thought. If by it be meant that a particle of matter can 

 never be deprived of its weight, the assertion is correct; 

 but the law which affirms the convertibility of natural 

 forces was never intended, in the minds of those who un- 

 derstood it, to affirm that such a conversion as that here 

 implied occurs in any case whatever. As regards convert- 

 ibility 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 toward 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 at- 

 traction. 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 ether; 

 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 



