the deep seas and oceans and of many lakes and springs. Absolutely pure
water, like pure air, is colourless, but all seas and lakes, however
clear and translucent, contain abundance of very finely divided matter,
organic or inorganic, which, as in the atmosphere, reflects the blue
rays in such quantity as to overpower the white or coloured
light-reflected from the fewer and more rapidly sinking particles of
larger size. The oceanic dust is derived from many sources. Minute
organisms are constantly dying near the surface, and their skeletons, or
fragments of them, fall slowly to the bottom. The mud brought down by
rivers, though it cannot be traced on the ocean floor more than about
150 miles from land, yet no doubt furnishes many particles of organic
matter which are carried by surface currents to enormous distances and
are ultimately dissolved before they reach the bottom. A more important
source of finely divided matter is to be found in volcanic dust which,
as in the case of Krakatoa, may remain for years in the atmosphere, but
which must ultimately fall upon the surface of the earth and ocean. This
can be traced in all the deep-sea oozes. Finally there is meteoric dust,
which is continually falling to the surface of the earth, but in such
minute quantities and in such a finely-divided state that it can be
detected only in the oozes of the deepest oceans, where both inorganic
and organic debris is almost absent.

The blue of the ocean varies in different parts from a pure blue
somewhat lighter than that of the sky, as seen about the northern tropic
in the Atlantic, to a deep indigo tint, as seen in the north temperate
portions of the same ocean: owing, probably, to differences in the
nature, quantity, and distribution of the solid matter which causes the
colour. The Mediterranean, and the deeper Swiss lakes, are also a blue
of various tints, due also to the presence of suspended matter, which
Professor Tyndall thought might be so fine that it would require ages of
quiet subsidence to reach the bottom. All the evidence goes to show,
therefore, that the exquisite blue tints of sky and ocean, as well as
all the sunset hues of sky and cloud, of mountain peak and Alpine snows,
are due to the finer particles of that very dust which, in its coarser
forms, we find so annoying and even dangerous.

But if this production of colour and beauty were the only useful
function of dust, some persons might be disposed to dispense with