the Theory of Luminous Flames. 381 



changed at the high temperature of the flame, is, I think, 

 based upon no solid foundation of fact. 



Him describes experiments with coloured flames (Bengal 

 fire, &c). His conclusion, that these flames contain much 

 solid matter, although they cast no shadow, is evidently in- 

 correct. The colour is, of course, due to vapour, not to solid 

 matter. 



Lastly, Hirn regards the petroleum-flame as diathermanous ; 

 but even admitting that this flame allows more or less the pas- 

 sage through it of heat-rays (a problem which I am not in a 

 condition to examine experimentally), this would not contra- 

 dict the demonstrated fact of the presence of solid carbon in 

 ordinary luminous flames. 



I think I have now shown that Hirn's hypothesis, viz. 

 that solid bodies when raised to a white heat lose their power 

 of reflecting light and become transparent, is without experi- 

 mental support, and therefore cannot be maintained. 



In the attempt to discover the causes of the luminosity of 

 flame, it is necessary that the observer should be acquainted 

 with the physical as well as with the chemical data which he 

 will be required to examine. A want of this twofold know- 

 ledge has already led many to view the phenomena of lumi- 

 nous flames either from the purely physical, or from the 

 purely chemical standpoint ; in either case the result has been 

 disastrous. 



In conclusion, I would once more draw attention to the 

 observations of W. Stein. If soot be present in the form of 

 vapour in luminous flames, the application of a high tempera- 

 ture, after condensation, should cause it to again assume the 

 gaseous form ; but this is not the case. Further, the soot 

 deposited from a coal-gas flame does not contain more than 

 0*9 per cent, of hydrogen. 



I would now gather together the 



Proofs of the Presence of Solid Carbon in Luminous Hydro- 

 carbon-Flames. 



(1) Chlorine causes an increase in the luminosity of feebly- 

 luminous, or non-luminous, hydrocarbon-flames. Inasmuch as 

 chlorine decomposes hydrocarbons at a red heat, with separa- 

 tion of carbon, it follows that the increased luminosity is due 

 to the production of solid carbon particles. 



(2) A small rod held in the luminous flame becomes rapidly 

 covered on its lower surface (the surface opposed to the issuing 

 gas) with a deposit of soot. The solid soot is evidently driven 

 against the rod. If the soot were present as vapour in the 

 luminous flame, its deposition would be due to a lowering of 



