over white-hot platinum has had the dust particles destroyed, thus
showing that they were almost wholly of organic origin, which is also
indicated by their extreme lightness, causing them to float permanently
in the atmosphere. The dust being thus got rid of, and pure air being
entirely transparent, there is nothing in the cylinder to reflect the
light, which is sent through its centre in a beam of parallel rays so
that none of it strikes against the sides; hence the inside of the
cylinder appears absolutely dark. But when the larger dust particles are
wholly or partially burnt, so that only the very smallest fragments
remain, a blue light appears, because these are so minute as to reflect
chiefly the more refrangible rays, which are of shorter
wave-length--those at the blue end of the spectrum--and which are thus
scattered in all directions, while the red and yellow rays pass straight
on as before.

We have seen that the air near the earth's surface is full of rather
coarse particles which reflect all the rays, and which therefore produce
no one colour. But higher up the particles necessarily become smaller
and smaller, since the comparatively rare atmosphere will support only
the very smallest and lightest. These exist throughout a great thickness
of air, perhaps from one mile to ten miles high or, even more, and blue
or violet rays being reflected from the innumerable particles in this
great mass of air, which is nearly uniform in all parts of the world as
regards the presence of minute dust particles, produces the constant and
nearly uniform tint we call sky-blue. A certain amount of white or
yellow light is no doubt reflected from the coarser dust in the lower
atmosphere, and slightly dilutes the blue and renders it not quite so
deep and pure as it otherwise would be. This is shown by the increasing
depth of the sky-colour when seen from the tops of lofty mountains,
while from the still greater heights attained in balloons the sky