Page:Popular Science Monthly Volume 14.djvu/588

570 about 95 per cent, of the total radiation from the white-hot platinum wire. They make up nearly 90 per cent, of the emission from a brilliant electric light. You can by no means have the light of the carbons without this invisible emission as an accompaniment. The visible radiation is, as it were, built upon the invisible as its necessary foundation.

It is easy to illustrate the growth in intensity of these invisible rays as the visible ones enter the radiation and augment in power. The transparency of the simple gases and metalloids—of oxygen, hydrogen, nitrogen, chlorine, iodine, bromine, sulphur, phosphorus, and even of carbon—for the invisible heat-rays is extraordinary. Dissolved in a proper vehicle iodine cuts the visible radiation sharply off, but allows the invisible free transmission. By dissolving iodine in sulphur. Professor Dewar has recently added to the number of our effectual ray filters. The mixture may be made as black as pitch for the visible, while remaining transparent for the invisible rays. By such filters it is possible to detach the invisible rays from the total radiation, and to watch their augmentation as the light increases. Expressing the radiation from a platinum wire when it first feels warm to the touch—when, therefore, all its rays are invisible—by the number one, the invisible radiation from the same wire raised to a white heat may be five hundred or more. It is not, then, by the diminution or transformation of the non-luminous emission that we obtain the luminous; the heat-rays maintain their ground as the necessary antecedents and companions of the light-rays. When detached and concentrated these powerful heat-rays can produce all the effects ascribed to the mirrors of Archimedes at the siege of Syracuse. While incompetent to produce the faintest glimmer of light, or to effect the most delicate air-thermometer, they will inflame paper, burn up wood, and even ignite combustible metals. When they impinge upon a metal refractory enough to bear their shock without fusion, they can raise it to a heat so white and luminous as to yield, when analyzed, all the colors of the spectrum. In this way the dark rays emitted by the incandescent carbons are converted into light rays of all colors. Still, so powerless are these invisible rays to excite vision that the eye has been placed at a focus competent to raise platinum-foil to bright redness without experiencing any visual impression. Light for light, no doubt, the amount of heat imparted by the incandescent carbons to the air is far less than that imparted by gas-flames. It is less because of the smaller size of the carbons, and of the comparative smallness of the quantity of fuel consumed in a given time. It is also less because the air can not penetrate the carbons as it penetrates a flame. The temperature of the flame is lowered by the admixture of a gas which constitutes four fifths of our atmosphere, and which, while it appropriates and diffuses the heat, does not aid in the combustion; and this lowering of the temperature by the inert atmospheric nitrogen renders necessary the combustion of a greater amount of gas to produce the