_multitude_ come into play in absorption and
radiation--atomic _complexity_ must also be taken into account. I would
recommend to the particular attention of chemists the molecule of
water; the deportment of this substance towards radiant heat being
perfectly anomalous, if the chemical formula at present ascribed to it
be correct.

Sir William Herschel made the important discovery that, beyond the
limits of the red end of the solar spectrum, rays of high heating
power exist which are incompetent to excite vision. The discovery is
capable of extension. Dissolving iodine in the bisulphide of carbon,
a solution is obtained which entirely intercepts the light of the most
brilliant flames, while to the ultra-red rays of such flames the same
iodine is found to be perfectly diathermic. The transparent
bisulphide, which is highly pervious to invisible heat, exercises on
it the same absorption as the perfectly opaque solution. A hollow
prism filled with the opaque liquid being placed in the path of the
beam from an electric lamp, the light-spectrum is completely
intercepted, but the heat spectrum may be received upon a screen and
there examined. Falling upon a thermo-electric pile, its invisible
presence is shown by the prompt deflection of even a coarse
galvanometer.

What, then, is the physical meaning of opacity and transparency as
regards light and radiant heat? The visible rays of the spectrum
differ from the invisible ones simply in period. The sensation of
light is excited by waves of aether shorter and more quickly recurrent
than the non-visual waves which fall beyond 'the extreme red. But why
should iodine stop the former and allow the latter to pass? The
answer to this question no doubt is, that the intercepted waves are
those whose periods of recurrence coincide with the periods of
oscillation possible to the atoms of the dissolved iodine. The
elastic forces which keep these atoms apart compel them to vibrate in
definite periods, and, when these periods synchronise with those of
the aethereal waves, the latter are absorbed. Briefly defined, then,
transparency in liquids, as well as in gases, is synonymous with
discord, while opacity is synonymous with accord, between the periods
of the waves of aether and those of the molecules on which they
impinge.

According to this view transparent and colourless substances owe their
transparency to the dissonance existing between the oscillating
periods of their