ms 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 towards each
other, we have tensions and nothing else. During their motion towards
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 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 of
the orbit. Her motion, and consequently her _vis viva_, is then a
minimum. The planet rounds the curve, and begins its approach to the
sun. In front it has a store of tensions, which are gradually
consumed, an equivalent amount of _vis viva_ being generated. When
nearest to the sun the motion, and consequently the _vis viva_, reach a
maximum. But here the available tensions have been used up. The
earth rounds this portion of the curve and retreats from the sun.
Tensions are now stored up, but _vis viva_ is lost, to be again restored
at the expense of the compleme