2i8 POPULAR SCIENCE MONTHLY. 



passing through it was not luminous. This experiment would seem to 

 leave no doubt that the incandescence is caused by latent heat, yet fur- 

 ther evidence was produced. In another experiment in which diluted 

 acetylene was used it required a higher heat to cause the decomposition 

 and luminosity. This latter is the condition existing in a flame, and 

 the temperature there found is above that required. In other experi- 

 ments it was found that if the flame temperature were high enough 

 the luminosity was directly proportional to the amount of acetylene in 

 the flame at the point where luminosity generally begins. Acetylene 

 was introduced at the corresponding place in a non-luminous flame 

 through very fine holes in a small capillary platinum tube, and the rate 

 of its flow, as well as that of the illuminating gas, was measured and 

 controlled so as to have present the amount of acetylene, which analy- 

 sis showed to exist in a similar luminous flame. At the holes there was 

 an intense light, and dull red streams of carbon passed upward in the 

 flame. 



Lewes sums up his conclusions, drawn from all his work, about as 

 follows: When the hydrocarbon gas leaves the jet at which it is 

 burned, those portions which come in contact with the air are consumed 

 and form a wall of flame, which surrounds the issuing gases. The 

 unburnt gas in its passage through the lower heated area undergoes 

 a number of chemical changes, brought about by the heat radiated from 

 the flame walls ; the principal change being the conversion of hydrocar- 

 bons into acetylene, hydrogen and methane. The temperature of the 

 flame rapidly increases with the distance from the jet and reaches a 

 point at which it is high enough to decompose acetylene into carbon 

 and hydrogen with a rapidity almost that of an explosion. The latent 

 heat so suddenly set free is localized by the proximity of carbon par- 

 ticles, which by absorbing it become incandescent and emit the larger 

 part of the light given out by the flame ; although the heat of combus- 

 tion causes them to glow somewhat until they come into contact with 

 oxygen and are consumed. This external heating gives rise to little 

 of the light. 



There have been opponents to this theory of the cause of luminosity 

 — as there are, fortunately, of all theories — but the evidence is so strong 

 and covers so many points, and so many investigators have confirmed 

 one part or another of the work, that it has been generally accepted as 

 a true statement of the facts with which it deals. 



