18 FRAGMENTS OF SCIENCE. 
and wants of an individual, but exceedingly minute in com- 
parison with the earth's primitive store. 
To sum up. The whole stock of energy or working- 
power in the world consists of attractions, repulsions, and 
motions. If the attractions and repulsions be so circum- 
stanced as to be able to produce motion, they are sources of 
working-power, but not otherwise. As stated a moment 
ago, the attraction exerted between the earth and a body 
at a distance from the earth's surface, is a source of work- 
ing-power; because the body can be moved by the attrac- 
tion, and in falling can perform work. When it rests at its 
lowest level it is not a source of power or energy, because 
it can fall no farther. But though it has ceased to be a 
source of enerqy, the attraction of gravity still acts as a 
force, which holds the earth and weight together. 
The same remarks apply to attracting atoms and mole- 
cules. As long as distance separates them, they can move 
across it in obedience to the attraction; and the motion 
thus produced may, by proper appliances, be caused to 
perform mechanical work. When, for example, two atoms 
of hydrogen unite with one of oxygen, to form water, the 
atoms are first drawn toward each other they move, they 
clash, and then by virtue of their resiliency, they recoil and 
quiver. To this quivering motion we give the name of 
heat. This atomic vibration is merely the redistribution 
of the motion produced by the chemical affinity; and this 
is the only sense in which chemical affinity can" be said to 
be converted into heat. We must not imagine the chem- 
ical attraction destroyed, or converted in'to anything else. 
For the atoms, when mutually clasped to form a molecule 
of water, are held together by the very attraction which 
first drew them toward each other. That which has really 
been expended is the pull exerted through the space by 
which the distance between the atoms has been dimin- 
ished. 
If this be understood, it will be at once seen that gravity, 
as before insisted on, may, in this sense, be said to be con- 
vertible into heat; that it is in reality no more an outstand- 
ing and inconvertible agent, as it is sometimes stated to be, 
than is chemical affinity. By the exertion of a certain pull 
through a certain space, a body is caused to clash with a 
certain definite velocity against the earth. Heat is there- 
by developed, and this is the only sense in which gravity 
