ite light is withdrawn. The hue of a purple liquid, for example, is
immediately accounted for by its action on a spectrum. It cuts out
the yellow and green, and allows the red and blue to pass through. The
blending of these two colours produces the purple. But while such a
liquid attacks with special energy the yellow and green, it enfeebles
the whole spectrum. By increasing the thickness of the stratum we may
absorb the whole of the light. The colour of a blue liquid is
similarly accounted for. It first extinguishes the red; then, as the
thickness augments, it attacks the orange, yellow, and green in
succession; the blue alone finally remaining. But even it might be
extinguished by a sufficient depth of 'the liquid.

And now we are prepared for a brief, but tolerably complete, statement
of that action of sea-water upon light, to which it owes its darkness.
The spectrum embraces three classes of rays--the thermal, the visual,
and the chemical. These divisions overlap each other; the thermal
rays are in part visual, the visual rays in part chemical, and vice
versa. The vast body of thermal rays lie beyond the red, being
invisible. These rays are attacked with exceeding energy by water.
They are absorbed close to the surface of the sea, and are the great
agents in evaporation. At the same time the whole spectrum suffers
enfeeblement; water attacks all its rays, but with different degrees
of energy. Of the visual rays, the red are first extinguished. As
the solar beam plunges deeper into the sea, orange follows red, yellow
follows orange, green follows yellow, and the various shades of blue,
where the water is deep enough, follow green. Absolute extinction of
the solar beam would be the consequence if the water were deep and
uniform. If it contained no suspended matter, such water would be as
black as ink. A reflected glimmer of ordinary light would reach us
from its surface, as it would from the surface of actual ink; but no
light, hence no colour, would reach us from the body of the water.

In very clear and deep sea-water this condition is approximately
fulfilled, and hence the extraordinary darkness of such water. The
indigo, already referred to, is, I believe, to be ascribed in part to
the suspended matter, which is never absent, even in the purest
natural water; and in part to the slight reflection of the light from
the limiting surfaces of strata of different densities. A modicum of
light is thus