456 Prof. V. B. Lewes. The Cause of Luminosity [Mar. 21 r 



the thermo- junction in such a position that it registers the tempera- 

 tures of the gas in the tube, not that of the wail of the tube. Ta 

 each end of the platinum tube glass T'pi eces are fitted, down the 

 steins of which the wires pass to mercury seals ; from the metal seals 

 conducting wires lead to the resistance coils, the key, and a reflecting 

 galvanometer. 



A steady flow of acetylene was allowed to pass through the tube y 

 and was led into water at the other end. The tube was slowly and 

 carefully heated for about 4 in. of its length, and, as the tempera- 

 ture reached 700 C., white vapours began to flow from the tube,, 

 and these, as the temperature rose, increased in quantity. The 

 source of heat had been so regulated that the temperature had risen 

 about 10 per minute, but, almost immediately 800 C. was passed, the 

 galvanometer registered a sudden leap up in temperature to about 

 1000 C., whilst finely-divided carbon poured from the tube. This 

 seemed to indicate that 800 was about the temperature at which the 

 pure acetylene broke up into its constituents, and an experiment was 

 now made to see if this developed incandescence in the liberated 

 carbon. 



A small glass combustion tube was well supported, and heated to 

 the highest temperature attainable with one of Fletcher's big blow- 

 pipes, whilst pure acetylene was slowly flowing through it, the 

 heating not being commenced until the tube was filled with the pure 

 gas, all air being thoroughly rinsed out. As the temperature reached 

 the softening point of the glass, the acetylene apparently burst into a 

 lurid flame at the point where it entered the zone of heat, and clouds 

 of carbon swept forwards through the tube ; but, although the carbon 

 particles had to traverse an inch or more of tube more highly heated 

 than the point of entering the hot zone, it was only at this latter 

 point that the luminosity was developed, proving beyond doubt that 

 it was the heat evolved by the decomposition, and not the external 

 heating, which caused the carbon particles to emit light. 



If it is the decomposition of the molecule of acetylene which 

 develops the heat which is the cause of the incandescence of the 

 carbon particles, then, if acetylene could be burnt without decomposi- 

 tion, a non-luminous flame should be produced. It is conceivable 

 that this might be done by so diluting the acetylene that it would 

 require a much higher temperature to break it up. 



It was Heumann who showed* that hydrocarbon gases may burn 

 with luminous flames, i.e., with separation of carbon in the flame, or 

 with non-luminous flames, i.e., without any separation of carbon, and 

 that the maintenance of a high temperature is an essential condition 

 of luminosity : a flame, the temperature of which has been lowered 

 by any means, being no longer able to bring about the required 

 * ' Liebig's Annalen,' vol. 183, Part I, pp. 102131. 



