14 PR A GMENTS OF SCIENCE. 



If gravity, instead of being attraction, were repulsion, 

 then, with the particles in contact, the sum' of the tensions 

 between D and F would he a maximum, and the vis viva 

 zero. If, in obedience to the repulsion, D moved away 

 from F, vis viva would be generated; and the farther D 

 retreated from F the greater would be its vis viva, and 

 the less the amount of tension still available for producing 

 motion. Taking repulsion as well as attraction into ac- 

 count, the principle of the conservation of force affirms 

 that the mechanical value of the tensions and vires vivce 

 of the material universe, so far as we know it, is a con- 

 stant quantity. The universe, in short, possesses two 

 kinds of property which are mutually convertible. The 

 diminution of either carries with it the enhancement of 

 the other, the total value of the property remaining un- 

 changed. 



The considerations here applied to gravity apply 

 equally to chemical affinity. In a mixture of oxygen and 

 hydrogen the atoms exist apart, but by the application of 

 proper means they may be caused to rush together across 

 that space that separates them. While this space exists, 

 and as long as the atoms have not begun to move toward. 

 each other, we have tensions and nothing else. During 

 their motion toward each other the tensions, as in the case 

 of gravity, are converted into vis viva. After they clash 

 we have still vis viva, but in another form. It was trans- 

 lation, it is vibration. It was molecular transfer, it is 

 heat. 



It is possible to reverse these processes, to unlock the 

 combined atoms and replace them in their first positions. 

 But, to accomplish this, as much heat would be required 

 as was generated by their union. Such reversals occur 

 daily and hourly in nature. By the solar waves, the 

 oxygen of water is divorced from its hydrogen in the leaves 

 of plants. 4s molecular vis viva the waves disappear, but 

 in so doing they re-endow the atoms of oxygen and 

 hydrogen with tension. The atoms are thus enabled to 

 reeombine, and when they do so they restore the precise 

 amount of heat consumed in their separation. The same 

 remarks apply to the compound of carbon and oxygen, 

 called carbonic acid, which is exhaled from our lungs, 

 produced by our fires, and found sparingly diffused every- 

 where throughout the air. In the leaves of plants the sun- 



