THE CONSTITUTION OF NATURE. 17 
earth, as she swings to and fro in her yearly journey round 
the sun, is also true of her minutest atom. We have wheels 
within wheels, and rhythm within rhythm. 
When a body is heated, a change of molecular arrange- 
ment always occurs, and to produce this change heat is 
consumed. Hence, a portion only of the heat communi- 
cated to the body remains as dynamic energy. Looking 
back on some of the statements made at the beginning of 
this article, now that our knowledge is more extensive, we 
see the necessity of qualifying them. When, for example, 
two bodies clash, heat is generated; but the heat, or molec- 
ular dynamic energy, developed at the moment of colli- 
sion, is not the exact equivalent of the sensible dynamic 
energy destroyed. The true equivalent is this heat, plus 
the potential energy conferred upon the molecules by the 
placing of greater distances between them. This molecular 
potential energy is afterward, on the cooling of the body, 
converted into heat. 
Wherever two atoms capable of uniting together by 
their mutual attractions exist separately, they form a 
store of potential energy. Thus our woods, forests, and 
coal-fields on the one hand, and our atmospheric oxygen 
on the other, constitute a vast store of energy of this kind 
vast, but far from infinite. We have, besides our coal- 
fields, metallic bodies more or less sparsely distributed 
through the earth's crust. These bodies can be oxydized; 
and hence they are, so far as they go, stores of energy. 
But the attractions of the great mass of the earth's crust 
are already satisfied, and from them no further energy can 
possibly be obtained. Ages ago the elementary constitu- 
ents of our rocks clashed together and produced the 
motion of heat, which was taken up by the ether and car- 
ried away through stellar space. It is lost forever as far as 
we are concerned. In those ages the hot conflict of carbon, 
oxygen, and calcium produced the chalk and limestone 
hills which are now cold; and from this carbon, oxygen, 
and calcium no further energy can be derived. So it is 
with almost all the other constituents of the earth's crust. 
They took their present form in obedience to molecular 
force; they turned their potential energy into dynamic, 
and yielded it as radiant heat to the universe, ages before 
man appeared upon this planet. For him a residue of 
potential energy remains, vast, truly, in relation to the life 
