2i6 POPULAR SCIENCE MONTHLY. 



tion 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 break- 

 ing 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 tem- 

 peratures 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, CgH^, 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 hydro- 

 gen. 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 rela- 

 tively 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 accept- 

 ance of the theory that acetylene is the immediate source of the car- 

 bon. 



But a yet harder problem presented itself. What gives rise to heat 

 sufScient 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 suffi- 

 cient to render carbon faintly luminous, Lewes compared the temper- 

 atures 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 de- 

 veloped when acetylene is exploded and the localization of the heat set 

 free by its decomposition. His experiments were ingenious and con- 



