silent within their tubes. But by urging the
instrument on to higher notes, the second flame is started, and the
third alone remains. A still higher note starts it also. Thus, as
the sound of the syren rises gradually in pitch, it awakens every
flame in passing, by striking it with a series of waves whose periods
of recurrence are similar to its own.

Now the wave-motion from the syren is in part taken up by the flame
which synchronises with the waves; and were these waves to impinge
upon a multitude of flames, instead of upon one flame only, the
transference might be so great as to absorb the whole of the original
wave motion. Let us apply these facts to radiant heat. This blue
flame is the flame of carbonic oxide; this transparent gas is carbonic
acid gas. In the blue flame we have carbonic acid intensely heated,
or, in other words, in a state of intense vibration. It thus
resembles the sounding fork, while this cold carbonic acid resembles
the silent one. What is the consequence? Through the synchronism of
the hot and cold gas, the waves emitted by the former are intercepted
by the latter, the transmission of the radiant heat being thus
prevented. The cold gas is intensely opaque to the radiation from
this particular flame, though highly transparent to heat of every
other kind. We are here manifestly dealing with that great principle
which lies at the basis of spectrum analysis, and which has enabled
scientific men to determine the substances of which the sun, the
stars, and even the nebulae are composed; the principle, namely, that
a body which is competent to emit any ray, whether of heat or light,
is competent in the same degree to absorb that ray. The absorption
depends on the synchronism existing between the vibrations of the toms
from which the rays, or more correctly the waves, sue, and those of
the atoms on which they impinge.

To its almost total incompetence to emit white light, aqueous vapour
adds a similar incompetence to absorb bite light. It cannot, for
example, absorb the luminous rays of the sun, though it can absorb the
non-luminous rays of the earth. This incompetence of the vapour to
absorb luminous rays is shared by water and ice--in fact, by all
really transparent substances. Their transparency is due to their
inability to absorb luminous rays. The molecules of such substances
are in dissonance with luminous waves; and hence such waves pass
through transparent bodies without di