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length of the tube surrounding the flame. The shorter the
tube the higher is the pitch. The flames are now silent.
within their respective tubes, but each of them can be
caused to respond to a proper note sounded anywhere in
this room. Here is an instrument called a siren, by which
a powerful musical note can be produced. Beginning with
apnote of low pitch, and ascending gradually to a higher
one, I finally reach the note of the flame in the longest tube.
The moment it is reached, the flame bursts into song. But
the other flames are still silent within their tubes. I urge
the instrument on to higher notes; the second flame has
now started, and the third alone remains. But a still higher
note starts it also. Thus, as the sound of the siren rises
gradually in pitch, it awakens every flame in passing, by
striking it with a series of waves whose periods of recur-
rence are similar to its own.

Now the wave-motion from the siren is in part taken up
by the flame which synchronizes with the waves ; and had
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 tuning-fork, while this cold
carbonic acid resembles the silent one. What is the con-
sequence ? Through the synchronism of the hot and cold
gas transmission of motion through the gas is prevented;
it is all transferred. 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,

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