on still
continues; the motion of the mass is converted into a motion of the
atoms of the mass; and these small motions, communicated to the
nerves, produce the sensation we call heat.

We know the amount of heat which a given amount of mechanical force
can develope. Our lead ball, for example, in falling to the earth
generated a quantity of heat sufficient to raise its own temperature
three-fifths of a Fahrenheit degree. It reached the earth with a
velocity of 32 feet a second, and forty times this velocity would be
small for a rifle bullet; multiplying 0.6 by the square of 40, we find
that the amount of heat developed by collision with the target would,
if wholly concentrated in the lead, raise its temperature 960 degrees.
This would be more than sufficient to fuse the lead. In reality,
however, the heat developed is divided between the lead and the body
against which it strikes; nevertheless, it would be worth while to pay
attention to this point, and to ascertain whether rifle bullets do
not, under some circumstances, show signs of fusion. [Footnote: Eight
years subsequently this surmise was proved correct. In the
Franco-German War signs of fusion were observed in the case of bullets
impinging on bones.]

From the motion of sensible masses, by gravity and other means, we now
pass to the motion of atoms towards each other by chemical affinity. A
collodion balloon filled with a mixture of chlorine and hydrogen being
hung in the focus of a parabolic mirror, in the focus of a second
mirror 20 feet distant a strong electric light was suddenly generated;
the instant the concentrated light fell upon the balloon, the gases
within it exploded, hydrochloric acid being the result. Here the atoms
virtually fell together, the amount of heat produced showing the
enormous force of the collision. The burning of charcoal in oxygen is
an old experiment, but it has now a significance beyond what it used
to have; we now regard the act of combination on the part of the atoms
of oxygen and coal as we regard the clashing of a falling weight
against the earth. The heat produced in both cases is referable to a
common cause. A diamond, which burns in oxygen as a star of white
light, glows and burns in consequence of the falling of the atoms of
oxygen against it. And could we measure the velocity of the atoms
when they clash, and could we find their number and weights,
multiplying the weight of each atom by the square of its velocit