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16 FRAGMEN TS OF SCIENCE.

thousand pounds to an elevation of one thousand feet above
the earth’s surface, and let it fall; the energy with which
it would strike the earth would not exceed that of the eight
pounds of oxygen atoms as they dash against one pound
of hydrogen atoms to form water.

It is sometimes stated that the force of gravity is dis-
tinguished from all other forces by the fact of its resisting
conversion into any other. Chemical affinity, it is said,
can be converted into heat and light, and these again into
magnetism and electricity. But gravity refuses to be so
converted; it is a force which maintains itself under all
circumstances, and is not capable of disappearing to give
place to another. If by this is meant that a particle of
matter can never be deprived of its weight, the assertion
is correct; but the law which affirms the convertibility of
natural forces was never meant, in the minds of those who
understood it, to. affirm that such a conversion as that here
implied occurs in any case whatever. As regards converti-
bility into heat, gravity and chemical affinity stand on pre-
cisely the same footing. The attraction in the one case is
as indestructible as in the other. Nobody affirms that
when a stone rests upon the surface of the earth the mutual
attraction of the earth and stone is abolished; nobody
means to affirm that the mutual attraction of oxygen for
hydrogen ceases after the atoms have combined to form
water. What is meant in the case of chemical affinity is,
that the pull of that affinity, acting through a certain space,
imparts a motion of translation of the one atom toward the
other. This motion of translation is not heat, nor is the
force that produces it heat. But when the atoms strike and
recoil, the motion of translation is converted into a motion
of vibration, and this latter motion is heat. But the vibra-
tion, so far from causing the extinction of the original at-
traction, is in part carried on by that attraction. The atoms
recoil in virtue of the elastic force which opposes actual

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