368 Dr. Karl Heumann's Contributions to 



combine with the hydrogen present, and so produce a hydro- 

 carbon rich in carbon, or set free pure carbon itself. 



The substitution-products of marsh-gas, CH 3 CI and CHC1 3 , 

 are gases fulfilling these conditions. While marsh-gas burns 

 with a feebly luminous flame, the flame of methyl chloride 

 (CH 3 CI) is smoky and strongly luminous. Chloroform 

 (CH Cl 3 ) also burns at the surface of a wick with a luminous 

 flame. Hydrogen containing a little chloroform vapour burns 

 with a brilliant although non-smoky flame ; if a large quan- 

 tity of chloroform be present, the flame becomes somewhat 

 opaque and deposits much soot. In each of these cases hy- 

 drochloric acid is produced. 



The flame of a hydrocarbon may become feebly luminous, or 

 even non-luminous when (2) the temperature is not sufficiently 

 high to cause separation of solid carbon. Two cases here pre- 

 sent themselves. A low temperature may be occasioned by 

 withdrawal of heat by extraneous causes, as when a luminous 

 flame is brought into contact with cold substances, or by the 

 action of admixed gases which absorb heat, and so reduce the 

 original temperature of the flame. These flames may be so 

 hot as to cause to glow a piece of platinum wire held within 

 them, and yet not hot enough to bring about a deposition of 

 carbon from the hydrocarbonaceous material of the gas. On 

 the supposition already put forward, chlorine ought in either 

 case to render the non-luminous flame luminous. 



A small luminous flame was rendered non-luminous by 

 causing it to play upon a porcelain basin. (I have already 

 shown that withdrawal of heat is here the cause of non-lumi- 

 nosity.) So soon as chlorine or bromine vapour was brought 

 into the blue flame, it became luminous and deposited soot 

 upon the basin. This experiment proves that the flame be- 

 came non-luminous because the temperature was not attained 

 at which carbon is deposited, and that so soon as carbon was 

 separated, even by other means than increase of temperature, 

 luminosity returned. 



The blue flame of mixed coal-gas and carbon dioxide, coal- 

 gas and air, or coal-gas and carbon monoxide, when mixed 

 with a little chlorine became very luminous ; this was espe- 

 cially noticeable with the flame of mixed coal-gas and air, 

 because this flame is possessed of a higher temperature than 

 the others ; and I have already shown that the temperature 

 at which deposition of carbon takes place in flames admixed 

 with indifferent gases is higher than that at which the same 

 phenomenon occurs in the case of flames not so admixed. 



The luminosity of a hydrocarbon flame may be diminished 

 (3) by the temperature not being sufficiently high to maintain 



