Page:Popular Science Monthly Volume 63.djvu/221

Rh. By comparing ethylene, C2H4, with acetylene, C2H2 (where for equal consumption the same number of carbon atoms were present), and also with coal-gas, it was seen that the luminous portion of the acetylene, flame is not as hot as that of either ethylene or coal-gas, while the illuminating powers of the flames were: acetylene, 240.0 candle power, ethylene, 65.5 c.p. and coal-gas, 16.8 c.p. Evidently the heat of combustion does not account for the incandescence of the carbon; for if it did the cooler acetylene flame would give less light, while, as a matter of fact, it gives twice as much as the ethylene and about fourteen times as much light as the very much hotter coal-gas flame. It was evident that our temperature measuring instruments do not detect the heat of the carbon particles themselves.

To see if luminosity be even partly due to the latent heat of acetylene, Lewes exploded that gas in a closed tube. This was done by wrapping a bit of fulminate of mercury in tissue paper and suspending it by copper wires joined by platinum in contact with the fulminate, and passing an electric current. There followed a brilliant flash of light and a complete decomposition of the gas, and of the eudiometer as

well. Pieces of glass were coated with carbon, and the tissue paper was not scorched except in a small hole where the explosion of the fulminate had burst through. This experiment showed the formation of carbon, the emission of a brilliant light and the localization of the heat liberated. But as the decomposition in a flame can hardly be as rapid as in this experiment, and as hydrogen and oxygen also give a feeble light when exploded, he sought to detect the rise in temperature at the moment of decomposition when this is caused by heat. He arranged a thermo-couple in a small tube so that only the turn of wires was exposed, and after sweeping out the air passed a slow current of acetylene through the tube, the arrangement being as shown in Fig. 3. The heat was raised throughout the tube at a rate of about 10° C. per minute, and almost as soon as the temperature of area a passed 800° C. it took a sudden leap to 1000° C, the gas burst into a lurid flame and streams of carbon passed on through the tube. Although the temperature of area h was made considerably higher than a the carbon