104 Dr. Karl Heumann's Contributions to 



thus communicated to the gas was neutralized by the deposi- 

 tion of carbon within the platinum tube. No carbon, how- 

 ever, could be detected within the heated tube, even when the 

 experiment had proceeded for a very considerable time. 

 Special experiments also showed that when a stream of gas 

 was very slowly passed for a considerable time through a 

 tube of hard glass heated to redness, scarcely a trace of carbon 

 was deposited. The deposition of large quantities of carbon 

 within gas-retorts is to be attributed much more to the de- 

 composition of condensed hydrocarbons than of marsh-gas and 

 ethylene. 



The gas used in the foregoing experiments, however, was 

 almost perfectly free from condensed hydrocarbons and tarry 

 matter ; hence the non-deposition of carbon is only what might 

 have been expected. 



A small quantity of water containing tarry matter of a 

 yellowish brown colour, collected in the cold tube through 

 Avhich the gas was passed after leaving the strongly heated 

 platinum tube ; inasmuch as this liquid was not formed unless 

 the gas had been passed through the hot tube previously to 

 being cooled, its formation must be traced to the decompo- 

 sition brought about in the heated platinum tube among the 

 constituents of the gas. 



The quantity of this liquid, however, was too small to lead 

 one to suppose that its production could exert any influence 

 upon the luminosity of the flame. I passed pure hydrogen 

 through the cold tube ; the flame produced by igniting this 

 hydrogen was non-luminous, even when the few drops of 

 liquid in the tube were heated, showing the absence of liquid 

 hydrocarbons such as benzene &c. I must therefore regard 

 the results of this experiment as attributable to the composi- 

 tion of the special sample of coal-gas which was made use of. 

 The fact observed in previous experiments, that heating 

 the burner caused an increase in the luminosity of the flame, 

 must not, therefore, be referred to any decomposition brought 

 about in the gas itself by the heat ; for were it so the gas 

 would have retained its increased illuminating power after 

 being cooled to the ordinary temperature. 



The direct addition of heat must therefore be regarded as 

 the only, or at any rate the most important, cause of the in- 

 creased luminosity. 



Burners in which the temperature of the gas is raised 

 before combustion are supposed to effect a very considerable 

 increase in the luminosity of the gas-flame. By the use of 

 such burners 60 per cent, more light may, it is said, be ob- 

 tained than by using an ordinary fishtail burner. 



