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THE CHEMICAL ACTION OF LIGHT 47
to make ultra-violet rays visible. If a piece of white card which has been painted thickly with a paste made from sulphate of quinine moistened with a little dilute sulphuric acid is held in the spectrum of an incandescent solid immediately beyond the violet end, the originally invisible ultra-violet part is seen to shine with a bluish green light.
Other substances produce this effect, such as glass coloured with uranium, and fluor-spar from Devonshire, and therefore this property has received the name of fluorescence.
The aniline derivative fluorescene is a very brilliantly fluorescing substance. If a piece of paper is dipped in a solution of this, and then placed on the surface of water contained in a large beaker, a greenish tree-like fluorescence will grow in a downward direction from the point of contact of the paper with the water.
Chemical Action of Light from Various Sources. — From the facts just previously explained it follows that chemical effects are chiefly produced by the ultra-violet, violet, and blue rays. It is therefore evident that the chemical action of any light will be proportional to the amount of these rays it contains.
Lamplight (gas or petroleum) is very poor in such rays, and therefore acts but feebly on the photographic plate ; photographers were thus enabled to prepare their less sensitive wet plates in a subdued lamplight.
This was also frequently done in the day by allowing the light to pass through yellow glass.
The white Bengal light, the flames of the blue Bengal light, and those of burning sulphur, produce a much more powerful chemical effect. The latter possesses only a small illuminating power, because it contains few yellow and red rays ; but, on the other hand, it is rich in blue and violet. Photographs have been actually taken by help of these flames.
But the chemical action of the foregoing lights is greatly