The explanation was found through experiments on the visibility or
non-visibility of air, which were made by the late Professor Tyndall
about the year 1868. Everyone: has seen the floating dust in a sunbeam
when sunshine enters a partially darkened room; but it is not generally
known that if there was absolutely no dust in the air the path of the
sunbeam would be totally black and invisible, while if only very little
dust was present in very minute particles the air would be as blue as
the summer sky.

This was proved by passing a ray of electric light lengthways through a
long glass cylinder filled with air of varying degrees of purity as
regards dust. In the air of an ordinary room, however clean and well
ventilated, the interior of the, cylinder appears brilliantly
illuminated. But if the cylinder is exhausted and then filled with air
which is passed slowly through a fine gauze of intensely heated platinum
wire, so as to burn up all the floating dust particles, which are mainly
organic, the light will pass through the cylinder without illuminating
the interior, which, viewed laterally, will appear as if filled with a
dense black cloud. If, now, more air is passed into the cylinder
through the heated gauze, but so rapidly that the dust particles are not
wholly consumed, a slight blue haze will begin to appear, which will
gradually become a pure blue, equal to that of a summer sky. If more and
more dust particles are allowed to enter, the blue becomes paler, and
gradually changes to the colourless illumination of the ordinary air.

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