14 FRA 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 be 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. 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 every- 
where throughout the air. In the leaves of plants the sun- 
