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38 THE ADVANCE OF PHOTOGRAPHY
directions. The direction of the propagation of the undulation we call a ray of light. We perceive it as soon as it reaches our eye, because the vibrating ether affects our retina.
Now, we know that the undulations of sound are able to set other bodies in motion. If the A string of a violin is struck, the A string of a piano standing near sounds distinctly with it. Nay, even if the damper of a piano is raised and any note be sung, instantly the string of the piano which has the same tone sounds. The same thing happens with a glass bell of the same tone. There are people even who can break a glass by a shrill tone of their voice. The glass is so shaken by the violent undulations communicated to it by the air, that it falls to pieces. Under such circumstances, it need not surprise us that the undulations of light agitate bodies so forcibly that they fall to pieces.
Realgar offers the most remarkable example of this kind. This is a beautiful mineral of a ruby red colour, in the form of splendid crystals composed of sulphur and arsenic. If a crystal of this kind be exposed for months to the light, it falls into powder ; and in this way many very fine pieces of this beautiful mineral have been lost in the mineralogical museum of Berlin.
This is only a mechanical, and not a chemical, operation of light ; but it gives an insight into its chemical action. Heat occasions chemical decomposition by expanding bodies, and thereby removing their atoms so far apart that the chemical power which unites them loses effect, and the component parts separate. Thus oxide of mercury is by heat resolved into its constituent parts, mercury and oxygen.
Decomposition is effected by light when the atoms of a body are agitated by its undulations — that is to say, are made to vibrate ; and if these vibrations cause a separation of the parts to take place, the body falls to pieces.