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HISTORICAL SURVEY 3
into a colourless or differently coloured body ; and that this is the effect of light is evident from the fact that those parts of the material in question which are covered up from the light — beneath the folds, for example — remain unchanged. This effect of light upon colour has been long turned to practical use in the bleaching of linen. The unbleached fabric is spread out in the sunlight, and repeatedly moistened with water ; and thus, through the combined effect of light and moisture, the dark colouring substance becomes gradually soluble, and can then be removed from the linen by boiling it in alkaline lye.
It was formerly believed that the changes we have just described were caused by the heat which is produced in bodies by the sun's rays. That this is an erroneous view is evident from the fact that fabrics dyed in colours which are not fast can be exposed for months together in the temperature of a hot oven without any bleaching effect ; and further, that wax, which the sunlight likewise bleaches, becomes darker, rather than paler, through heat.
As we remarked before, the bleaching effect of sunlight is a slow process, and this circumstance renders the phenomenon less striking. A sudden and rapid occurrence surprises us, and stirs us up to inquire and to reflect.
Silver Chloride and Silver Nitrate. — In the mines of Freiberg is occasionally found a vitreous dull-shining silver ore, which, on account of its appearance, is called horn silver (chloride of silver).
This horn silver consists of silver and chlorine in chemical combination, and this compound can be artificially produced by passing chlorine gas over metallic silver, or by adding a solution of nitrate of silver to a solution of common salt (sodium chloride). This horn silver in its original position in the earth's crust is completely colourless, but when it is exposed to the daylight it assumes, in a few minutes, a violet tint.
In another substance containing silver this phenomenon