rtue of a superior force of attraction. 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 gravity itself. Its intensity might be measured in the same
way, namely by the amount of motion which it can generate in a certain
time. Thus the attraction of gravity at the earth's surface is
expressed by the number 32; because, when acting freely on a body for
a second of time, gravity imparts to the body a velocity of thirty-two
feet a second. In like manner the mutual attraction of oxygen and
hydrogen 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 amounting only to 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, to use the terminology of Mayer, we have a
_mechanical_ combination of the earth and the body. Let us suffer the
falling body and the earth to dwindle in imagination to the size of
atoms, and for the attraction of gravity let us substitute that of
chemical affinity; 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 which separate them. Take a weight
of 47,000 pounds to an elevation of 1,000 feet above the earth's
surface, and let it fall; the energy with which it will strike the
earth will 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 gravity is distinguished from all other
forces by the fact of its resisting conversion into other forms of
force. 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; being a force maintaining itself under all
circumstances, and not capable of disappearing to give place to
another. The statement arises from vaguenes