Page:Popular Science Monthly Volume 63.djvu/220

216 and about twenty-four per cent, is lost in the lower half of the luminous part. In the dark part there occurs a transformation of saturated into unsaturated hydrocarbons, along with a general breaking down of all to yield products less rich in hydrogen and the oxides of carbon. At the point where luminosity just begins, seventy to eighty per cent, of the unsaturated compounds is acetylene, although less than one per cent, was originally present. No acetylene could be found in the flame when it was made non-luminous.

By causing pure gases to pass through tubes heated to known temperatures and analyzing the products formed, Lewes studied the effects of heat upon both saturated and unsaturated hydrocarbons. At 800° C. an unsaturated compound, like ethylene, C2H4, breaks down into hydrogen and the still more unsaturated acetylene, C2H2. At 1200° C. the very stable, saturated hydrocarbons decompose into acetylene and hydrogen, and the acetylene in turn decomposes into carbon and hydrogen. Even very dense hydrocarbons decompose at 1200° C. These results strengthened Lewes 's conviction that under the baking action of the flame-walls in the lower portions acetylene is produced in relatively large quantities and that this is the source of the carbon.

The question which immediately presented itself was. Does there exist in an ordinary flame such conditions of temperature as may bring about the formation of acetylene from the very stable constituents of the illuminants? On measuring the temperatures at various places the necessary temperatures were found to exist.

The work was complete and conclusive and forced a general acceptance of the theory that acetylene is the immediate source of the carbon.

But a yet harder problem presented itself. What gives rise to heat sufficient to make the carbon become incandescent?; a burning question certainly and one not easy to answer.

From the time of Davy to the year 1892 the only opinion was that the burning hydrogen, carbon monoxide and hydrocarbons furnished the heat necessary to raise carbon to incandescence. In that year Lewes advanced his 'latent heat' theory. This theory declared that the latent heat set free when acetylene is decomposed instantly heats the carbon particles thus set free to incandescence.

After showing that the heat of combustion of a flame is only sufficient to render carbon faintly luminous, Lewes compared the temperatures of flames burning coal-gas, the unsaturated hydrocarbon gas, ethylene, and the still less saturated acetylene, and also the amount of light given by each when burning equal volumes of gas per hour from burners best suited to each. He likewise studied the temperatures developed when acetylene is exploded and the localization of the heat set free by its decomposition. His experiments were ingenious and