THU CONSTITUTION" OF NATURE. 25 



of proper means they may be caused to rush together 

 across the 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. 

 Durintr 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. The same considerations apply to a mixture of 

 hydrogen and chlorine. When these gases are mingled in 

 the dark they remain separate, but if a sunbeam fall upon 

 the mixture the atoms rush together with detonation. 

 Here also we have tension converted into molecular trans- 

 lation, and molecular translation into heat and sound. 



It is possible to reverse these processes, to unlock the 

 embrace of the atoms and replace them in their first posi- 

 tions. But to accomplish this as much heat would be re- 

 quired 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. As molecular vis viva the waves disappear, but 

 in so doing they reendow the atoms of oxygen and hydro- 

 gen with tension. The atoms are thus enabled to recom- 

 bine, and wdien 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 car- 

 bonic acid, which is exhaled from our lungs, produced by 

 our fires, and found sparingly diffused everywhere through- 

 out the air. In the leaves of plants the sunbeams also 

 wrench these atoms 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 

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