THE CONSTITUTION OF NATURE. 15 
beams also wrench the atoms of carbonic acid asunder, 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 
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 complementary 
force on the opposite side of the curve. Thus beats the 
heart of the universe, but without increase or diminution 
of its total stock of force. 
I have thus far tried to steer clear amid confusion, by 
fixing the mind of the reader upon things rather than upon 
names. But good names are essential; and here, as yet, 
we are not provided with such. We have had the force of 
gravity and living force two utterly distinct things. We 
have had pulls and tensions; and we might have had 
the force of heat, the force of light, the force of magnet- 
ism, or the force of electricity all of which terms have 
been employed more or less loosely by writers on physics. 
This confusion is happily avoided by the introduction of 
the term " energy," which embraces both tension and vis 
viva.' Energy is possessed by bodies already in motion; 
it is then actual,, and we agree to call it actual or dynamic 
energy. It is our old vis viva. On the other hand, 
energy is possible to bodies not in motion, but which, in 
virtue of attraction or repulsion, possess a power of motion 
which would realize itself if all hindrances were removed. 
Looking, for example, at gravity; a body on the earth's 
surface in a position from which it cannot fall to a lower 
one possesses no energy. It has neither motion nor power 
