THE CONSTITUTION OF NATURE 25 



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 translation, 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 oxy- 

 gen of water is divorced from its hydrogen in the leaves 

 of plants. As molecular vis viva the waves disappear, but 

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

 drogen with tension. The atoms are thus enabled to re- 

 combine, 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 

 sunbeams also wrench the atoms of carbonic acid asun- 

 der, and sacrifice themselves in the act; but when the 

 plants are burned, the amount of heat consumed in their 

 production is restored. 



This, then, is the rhythmic play of Nature as regards 

 her forces. Throughout all her regions she oscillates from 

 tension to vis viva, from vis viva to tension. We have the 

 same play in the planetary system. The earth's orbit is 

 an ellipse, one of the foci of which is occupied by the 

 sun. Imagine the earth at the most distant part of the 

 orbit. Her motion, and consequently her vis viva, is then 



a minimum. The planet rounds the curve, and begins its 



SCIENCE 2 



