en. 1.] MATTER, MOTION, AND FOLCE. 291 



acts with equal power in all directions. In universal 

 attraction we are accustomed to distinguish three modes, 

 respectively called gravity, cohesion, and chemism or 

 chemical affinity. 



The essential difference between these modes of primary 

 force and the various modes of motion, is illustrated by the 

 familiar facts that gravity causes molar motion while molar 

 motion does not cause gravity ; and that chemism gives rise 

 to the species of molecular motion called heat, while heat 

 cannot give rise to chemism, though it may result in a mole- 

 cular rearrangement which will allow chemism to manifest 

 itself. For example gravity causes a spent rocket to iall 

 to the ground ; but the upward motion of the rocket does not 

 cause gravity, although it results in a position of the rocket 

 which enables gravity to reveal itself by causing downward 

 motion. So when nitrous oxide is decomposed into nitrogen 

 and oxygen, a considerable amount of heat is evolved ; but 

 when all this thermal undulation is restored under appropriate 

 conditions, and the compound is again formed, it is not that 

 the thermal undulation gives rise to the chemism which 

 draws the atoms of nitrogen and oxygen together ; it is only 

 that the thermal undulation results in such a redistribution 

 of the atoms that their progress toward each other is un- 

 impeded, and thus the latent force of chemism is revealed. 



Now the law of the correlation of forces, which perhaps 

 ought rather to be called the law of the transformation of 

 motion, is simply the obverse of that corollary from the per- 

 sistence of force, which affirms that whatever energy has 

 been expended in doing work must reappear as energy. The 

 energy of molar motion which disappears when an arrow 

 sticks in its target is really transformed into the energy of 

 molecular motion which is recognized partly as heat and 

 partly as electricity. That the different modes of motion 

 are transformable into each other, is now one of the common- 

 places of physical science, and needs but little illustration 



u 2 



