volcanic glass. Much of this was of course ground to impalpable dust by
the violence of the discharge, and was carried up to a height of many
miles. Here it was caught by the return currents of air continually
flowing northward and southward above the equatorial zone; and since,
when these currents reach the temperate zone, where the surface rotation
of the earth is less rapid, they continually flow eastward, the fine
dust was thus carried at a great altitude completely around the earth.
Its effects were traced some months after the eruption in the appearance
of brilliant sunset glows of an exceptional character, often flushing
with crimson the whole western half of the visible sky. These glows
continued in diminishing splendour for about three years; they were seen
all over the temperate zone; and it was calculated that, before they
finally disappeared, some of this fine dust must have travelled three
times round the globe.

The same principle is thought to explain the exquisite blue colour of
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,