the difference between gold and silver, copper and tin, or oxygen
and hydrogen.

"In all chemical compounds, such as water and alcohol, the
molecules at the base of the two or more substances break up into
their original atoms and form a new molecule composed of all the
atoms in the two or more things combined. To make this chemical
combination we must change the rate of vibration of one or the
other or both until they strike a common chord. As we saw last
term, oxygen and hydrogen have different specific heats, and no
two other elements have the same specific heat, while heat raises
the rate of vibration. Any given amount of heat raises the
vibration of one more than another. Apply heat, and the rate of
one will rise faster than that of the other until they reach a
common chord. Then they fall apart and recombine.

"If we pass a current of electricity through this sealed jar
containing oxygen and hydrogen in mechanical union, the spark
that leaps across the points furnishes the heat, and a drop of
water appears and falls to the bottom. A large portion of the
gases has disappeared. It has been converted into water. What
is left of the gases will expand and fill the bottle.

"The drop of water but for local causes, but for a certain
attraction of the earth, would float in the centre of the jar at
the centre of gravity, as the earth does in space. But the
centre of gravity of the two bodies is far within the earth, and
the drop gets as close to it as it can. The earth's 'pull' takes
it to the bottom. If the jar were far enough away in space the
drop would float, as the earth floats, at a point where all pulls