vibrations, and consequently its colour, which wholly
depends upon that rapidity, remain unchanged.

The vapour of water, as well as the vapour of silver, has its definite
periods of vibration, and these are such as to disqualify the vapour,
when acting freely as such, from being raised to a white heat. The
oxyhydrogen flame, for example, consists of hot aqueous vapour. It is
scarcely visible in the air of this room, and it would be still less
visible if we could burn the gas in a clean atmosphere. But the
atmosphere, even at the summit of Mont Blanc, is dirty; in London it
is more than dirty; and the burning dirt gives to this flame the
greater portion of its present light. But the heat of the flame is
enormous. Cast iron fuses at a temperature of 2,000 deg. Fahr; while the
temperature of the oxyhydrogen flame is 6,000 deg. Fahr. A piece of
platinum is heated to vivid redness, at a distance of two inches
beyond the visible termination of the flame. The vapour which
produces incandescence is here absolutely dark. In the flame itself
the platinum is raised to dazzling whiteness, and is even pierced by
the flame. When this flame impinges on a piece of lime, we have the
dazzling Drummond light. But the light is here due to the fact that
when it impinges upon the solid body, the vibrations excited in that
body by the flame are of periods different from its own.

Thus far we have fixed our attention on atoms and molecules in a state
of vibration, and surrounded by a medium which accepts their
vibrations, and transmits them through space. But suppose the waves
generated by one system of molecules to impinge upon another system,
how will the waves be affected? Will they be stopped, or will they be
permitted to pass? Will they transfer their motion to the molecules
on which they impinge, or will they glide round the molecules, through
the intermolecular spaces, and thus escape?

The answer to this question depends upon a condition which may be
beautifully exemplified by an experiment on sound. These two
tuning-forks are tuned absolutely alike. They vibrate with the same
rapidity, and, mounted thus upon their resonant cases, you hear them
loudly sounding the same musical note. Stopping one of the forks, I
throw the other into strong vibration, and bring that other near the
silent fork, but not into contact with it. Allowing them to continue
in this position for four or five seconds, and then stopping the
vibr