colour. The same remarks apply to flowers. The rose is red, in virtue,
not of the light reflected from its surface, but of light which has
entered its substance, which has been reflected from surfaces within,
and which, in returning _through_ the substance, has had its green
extinguished. A similar process in the case of hard green leaves
extinguishes the red, and sends green light from the body of the
leaves to the eye.

All bodies, even the most transparent, are more or less absorbent of
light. Take the case of water. A glass cell of clear water interposed
in the track of our beam does not perceptibly change any one of the
colours of the spectrum. Still absorption, though insensible, has
here occurred, and to render it sensible we have only to increase the
depth of the water through which the light passes. Instead of a cell
an inch thick, let us take a layer, ten or fifteen feet thick: the
colour of the water is then very evident. By augmenting the thickness
we absorb more of the light, and by making the thickness very great we
absorb the light altogether. Lampblack or pitch can do no more, and
the only difference in this respect between them and water is that a
very small depth in their case suffices to extinguish all the light.
The difference between the highest known transparency and the highest
known opacity is one of degree merely.

If, then, we render water sufficiently deep to quench all the light;
and if from the interior of the water no light reaches the eye, we
have the condition necessary to produce blackness. Looked properly
down upon, there are portions of the Atlantic Ocean to which one would
hardly ascribe a trace of colour: at the most a tint of dark indigo
reaches the eye. The water, in fact, is practically _black_, and this
is an indication both of its depth and purity. But the case is
entirely changed when the ocean contains solid particles in a state of
mechanical suspension, capable of sending the light impinging on them
back to the eye.

Throw, for example, a white pebble, or a white dinner plate, into the
blackest Atlantic water; as it sinks it becomes greener and greener,
and, before it disappears, it reaches a vivid blue green. Break such a
pebble, or plate, into fragments, these will behave like the unbroken
mass: grind the pebble to powder, every particle will yield its
modicum of green; and if the particles be so fine as to remain
suspended in the water, the scattered light will be a un