Page:Popular Science Monthly Volume 23.djvu/98

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HE phenomena of refraction and dispersion teach us that a body in a state of intensest heat emits not alone powerful thermal rays, but also all possible sorts of light (luminous colors). Diffraction convinces us that radiation is a wave-motion of an extremely fine, elastic, fluid medium, ether, and at the same time it enables us to compute the wave-length of the single rays. As is known, our apparatus suffices for taking cognizance of from one hundred and sixty to seven hundred and ninety billion undulations of heat or light per second, while all the vibrations of the ether lying either below or above are withdrawn from our direct observation. It is the purpose of this article to show what ways and means have been found for rendering the latter rays at least partly visible to our eye.

We throw a spectrum upon a white screen by means of a prism. The rays of the inferior number of undulations (ultra-red) will lie beyond the red, the superior (ultra-violet) beyond the violet end of the spectrum. We will begin with the latter.

We replace the screen, generally covered with ordinary white paper, by another one, the covering of which is impregnated with silver chloride, a combination of the two elements, chlorine and silver. When the light has for some time operated upon this preparation, we interrupt it, and examine the screen by the light of a candle. We find that the coating has become blackened; that the blackening is insignificant at the place where we formerly had red light, but that it increases the nearer we approach toward the violet end; that it finally attains its maximum beyond this place, and gradually grows weaker until, at a certain distance, it disappears from the violet end. Whence this blackening? By the operation of the ether-waves, the combination of chlorine and silver was dissolved, the chlorine passed into the air in the form of vapor, while the silver was precipitated in microscopically fine pearls upon the paper. The coating became black, because silver is not metallically lustrous in such minute division, but simply constitutes a black powder.

This experiment convinces us that rays will still be found beyond the violet end of the spectrum, which, on account of their high number of undulations, are shrouded from our sight, and yet betray their presence by the decomposition of silver preparations. These rays have been called actinic or chemical rays, and their spectrum the chemical spectrum.