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30 FRAGMENTS OF SCIENCE.

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. Now this atomic vibration is merely the redistribu-
tion 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
chemical attraction destroyed, or +converted into any thing
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 diminished.

If this be understood it will be at once seen that gravity
may in this sense he said to be convertible into heat; that
it is in reality no more an outstanding and inconvertible
agent, as it is sometimes stated to be, than chemical affin-
ity. 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 thereby developed,
and this is the only sense in which gravity can be said to
be converted into heat. In no case is the force which pro-
duces the motion annihilated or changed into any thing
else. The mutual attraction of the earth and weight exists
when they are in contact as when they were separate;
but the ability of that attraction to employ itself in the
production of motion does not exist.

The transformation, in this case, is easily followed by
the mind’s eye. First, the weight as a whole is set in
motion by the attraction of gravity. This motion . of the
mass is arrested by collision with the earth, being broken up
into molecular tremors, to which we give the name of heat.

And when we reverse the process, and employ those

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