Purple eBooks

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.


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