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 uniform green.
Hence the greenness of shoal water. You go to bed with the black water
of the Atlantic around you. You rise in the morning, find it a vivid
green, and correctly infer that you are crossing the Bank of