The explanation of these phenomena is that the number of dust particles
in ordinary air is so great that they reflect abundance of light of all
wave-lengths, and thus cause the interior of the vessel containing them
to appear illuminated with white light. The air which is passed slowly
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.
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