of Physical Chemistry WHEN opaque substances such as carbon, platinum or earthenware are heated sufficiently they emit light, the quality and intensity of which depends on the tempera ture and not on the nature of the substance heated. Radiation of this sort is called temperature radiation. An opaque gas would also emit light if heated to a suit able temperature. Iodine vapor, for instance, glowsl when heated to above 500 deg. Cent. While this may not be entirely a temperature radiation, it is

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... sually so considered. The law of temperature radiation holds only for opaque substance, which are sometimes called perfect radiators. An absolutely transparent substance would give no temperature radiation. At the end of the eighteenth century Wedgwoodt showed that heated air is not luminous. Subsequent experiments have con firmed this conclusion of Wedgwood's. Most artificial lighting is due to temperature radiation from solid particles. In the kerosene lamp the light is due to glowing particles of carbon. The difference be tween the kerosene lamp and the gas jet is that the tem perature of the latter is higher. If all the solid particles are burned, as in the Bunsen burner, a so-called non luminous flame is obtained, even though the temperature is much higher than in the burner with a lumnious flame. The brilliancy of the lime light is due to temperature radiation from intensely heated lime. In the Welsbach mantle and in the Nernst lamp there are suitable mix tures of rare earth oxides instead of the calcium oxide used in the lime light. There is some question whether the light from the Welsbach mantle is exclusively due to temperature radiation, but it is unnecessary to go into that matter now. At first one would s-oppose that the incandescent lamp would give the most efficient temperature radiation known because graphite melts at a higher temperature than any other known substance. The carbon lamp can be made to give an extraordinary light efficiency, but its life is extremely short under these conditions. The graphite vaporizes or disintegrates and the filament breaks.3 There has, therefore, been a systematic search for substances with high melting points and low vapor pressures. As a result, there have been produced suc cessively the osmium, the tantalum, and the tungsten lamps. In the nitrogen-filled tungsten lamp the thermal radiation has been cut down and consequently less power is needed to heat the filament to a given temperature. While it would be foolish to claim that the limit of efficiency has been reached, it must be remembered that a large number of very able men have been attacking this problem of temperature radiation systematically and that consequently the limit of efficiency is probably being approached. That brings up the question whether light may not be produced in other ways than by tem perature radiation and, if so, whether it is possible to produce cold light. The possibility of cold light cannot be disputed because the firefly produces it. Langley's studies of the firefly have shown that the insect gives about 95 per cent efficiency, meaning thereby that 95 per cent of the radiations are in the portion of the spec trum visible to the human eye while only about 5 per cent of the radiations are in the ultra-red portion of the spectrum and what are popularly called heat rays. The light of the firefly cannot be due to a temperature radi ation because the firefly does not burn up instantane ously. It is not a question involving life because the abdomiRal portion of the firefly can be dried, pulverized in a mortar, and kept for two years. At the end of that time the powder will glow if moistened and exposed to oxygen. It is simply an oxidation process. The firefly has the power of secreting a substance which burns with a luminous, cold flame. If one were to make in the laboratory the unknown substance which the fire fly makes, it would behave in exactly the same way as the natural product. It would be amusing to do this; but that is all that it would be, because the product would be too expensive to use as a source of light. No body claims for the firefly a low cost of production. In fact, it is not known how one would estimate the firefly's cost of production. Under certain circumstances cold light can be pro duced in the laboratory. Angstrom4 has calculated that *A paper read at a mooting of the Pittsburgh Academy of ScI ences and the Illuminating Engineering Society.