atoms and those of the waves of the whole visible
spectrum. From the prevalence of transparency in compound bodies, the
general discord of the vibrating periods of their atoms with the
light-giving waves of the spectrum, may be inferred; while their
synchronism with the ultra-red periods is to be inferred from their
opacity to the ultra-red rays. Water illustrates this in a most
striking manner. It is highly transparent to the luminous rays, which
proves that its atoms do not readily oscillate in the periods which
excite vision. It is highly opaque to the ultra-red undulations,
which proves the synchronism of its vibrating periods with those of
the longer waves.

If, then, to the radiation from any source water shows itself
eminently or perfectly opaque, we may infer that the atoms whence the
radiation emanates oscillate in ultra-red periods. Let us apply this
test to the radiation from a flame of hydrogen. This flame consists
mainly of incandescent aqueous vapour, the temperature of which, as
calculated by Bunsen, is 3259 deg.C, so that, if the penetrative power of
radiant heat, as generally supposed, augment with the temperature of
its source, we may expect the radiation from this flame to be
copiously transmitted by water. While, however, a layer of the
bisulphide of carbon 0.07 of an inch in thickness transmits 72 per
cent. of the incident radiation, and while every other liquid
examined transmits more or less of the heat, a layer of water of the
above thickness is entirely opaque to the radiation from the hydrogen
flame. Thus we establish accord between the periods of the atoms of
cold water and those of aqueous vapour at a temperature of 3259 deg.C. But
the periods of water have already been proved to be ultra red--hence
those of the hydrogen flame must be sensibly ultra-red also. The
absorption by dry air of the heat emitted by a platinum spiral raised
to incandescence by electricity is insensible, while that by the
ordinary undried air is 6 per cent. Substituting for the platinum
spiral a hydrogen flame, the absorption by dry air still remains
insensible, while that of the undried air rises to 20 per cent. of
the entire radiation. The temperature of the hydrogen flame is, as
stated, 3259 deg.C; that of the aqueous vapour of the air 20 deg.C. Suppose,
then, the temperature of aqueous vapour to rise from 20 deg.C. to 3259 deg.C,
we must conclude that the augmentation of temperature is applied to an