20 FRAGMENTS OF SCIENCE. 



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 towards each other, we have tensions and 

 nothing else. During their motion towards each 

 other the tensions, as in the "case of gravity, are con- 

 verted 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 re- 

 versals occur daily and hourly in nature. By the solar 

 waves, the oxygen 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 hydrogen with tension. The 

 atoms are thus enabled to recombine, 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 everywhere throughout 

 the air. In the leaves of plants the sunbeams also 

 wrench the atoms of carbonic acid asunder, and sacrifice 

 themselves in the act ; but when the plants are burnt, 

 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 



