THE CONSTITUTION OF NATURE 15 



It is sometimes stated tliat gravity is distinguislied from 

 all other forces by the fact of its resisting conversion into 

 other forms of force. Chemical affinity, it is said, can be 

 converted into heat and light, and these again into mag- 

 netism and electricity: but gravity refuses to be so con- 

 verted; being a force maintaining itself under all circum- 

 stances, 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 understood it, 

 to affirm that such a conversion as that here implied oc- 

 curs 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 inde- 

 structible as in the other. Nobody affirms that when a 

 stone rests upon the surface of the earth, the mutual at- 

 traction of the earth and stone is abolished; nobody means 

 to affirm that the mutual attraction of oxygen for hydro- 

 gen 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 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 converted 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 orig- 



