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THE CONSTITUTION OF NATURE. 15

otherwise arranged. Here we have atoms between which
a strong attraction is exercised, and also atoms between
which a weak: attraction is exercised. One atom can jostle
another out of its place in virtue of a superior force of at-
traction. But though the amount of force exerted varies
thus from atom to atom, it is still an attraction of the same
mechanical quality, if I may use the term, as that of grav-
ity itself. Its intensity might be measured in the same
way, namely, by the amount of motion which it can impart
in a certain time. Thus the attraction of gravity at the
earth’s surface is expressed by the number thirty-two, be-
cause, when acting freely on a body for a second of time,
it imparts to the body a Velocity of thirty-two feet a second.
In like manner the mutual attraction of oxygen and hydro-
gen might be measured by the velocity imparted to the
atoms in their rushing together. Of course such a unit of
time as a second is not here to be thought of, the whole
interval required by the atoms to cross the minute spaces
which separate them not amounting probably to more than
an inconceivably small fraction of a second.

It has been stated that when a body falls to the earth
it is warmed by the shock. Here we have what we may
call a mechanical combination of the earth and the body.
Suffer the falling body and the earth to dwindle in imagi-
nation to the size of atoms, and for the attraction of grav-
ity substitute that of chemical affinity, which is the name
given to the molecular attraction, we have then what is
called a chemical combination. The effect of the union in
this case also is the development of heat, and from the
amount of heat generated we can infer the intensity of the
atomic pull. Measured by ordinary mechanical standards,
this is enormous. Mix eight pounds of oxygen with one
of hydrogen, and pass a spark through the mixture; the
gases instantly combine, their atoms rushing over the little
distances between them. Take a weight of forty-seven

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