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 débris 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 it in
order to escape its less agreeable effects. It has, however, been
recently discovered that dust has another part to play in nature; a part
so important that it is doubtful whether we could even live without it.
To the presence of dust in the higher atmosphere we owe the formation of
mists, clouds, and gentle beneficial rains, instead of water spouts and
destructive torrents.