376 Dr. Karl Heumann's Contributions to 



nothing else than carbon. The shadow test therefore supplies 

 ns with a means of detecting the presence of solid carbon in 

 such flames. 



Some time after writing the foregoing pages I noticed a 

 memoir by G. A. Him entitled " Sur les proprietes optiques 

 de la flamme des corps en combustion, et sur la temperature 

 du soleil"*. In this paper Hirn describes the behaviour of 

 various flames when viewed in sunlight ; his conclusions, 

 however, are directly opposed to those which I have deduced. 



Hirn starts with the assumption that luminous hydrocarbon- 

 flames contain solid carbon, luminous phosphorus-flame con- 

 tains solid pentoxide of phosphorus &c. — that the transparency 

 of these flames is due to a change, brought about by the high 

 temperature, in the optical properties of the solid particles con- 

 tained in the flames, whereby these particles become transpa- 

 rent and incapable of reflecting light. 



I shall endeavour to show that HirnV conclusions are in- 

 valid. In his preliminary assumption Hirn takes no notice of 

 Knapp's experiments, which show that the decrease in lumino- 

 sity of hydrocarbon-flames, brought about by admitting air, is 

 not to be traced to oxidation of the carbon, inasmuch as pure 

 nitrogen causes the same result |. Frankland's supposition 

 that the luminosity of hydrocarbon-flames is not due to the 

 presence of solid carbon had been disputed by no one when 

 Hirn's paper appeared. Frankland had shown, five years 

 previous to the appearance of Hirn's paper, that phosphorus 

 pentoxide is volatilized at temperatures lower than that of 

 the flame of phosphorus burning in oxygen, and that there- 

 fore the luminosity of this flame cannot be due to the presence 

 of solid phosphorus pentoxide. 



Of Hirn's assumptions — that the luminosity of hydrocarbon- 

 flames is due to the presence of solid carbon, and that the lumi- 

 nosity of phosphorus burning in oxygen is due to the pre- 

 sence of solid phosphorus pentoxide — the first was unproved 

 at the time he wrote, and the second is untrue. 



Light reflected from a solid body is known to be polarized. 

 Hirn found the light from ordinary hydrocarbon-flames, as 

 also the light from burning phosphorus, to be non-polarized ; 

 he also failed to detect evidence of polarization in the light 

 coining from the flame of the blast-furnace. He found, how- 

 ever, that the white smoke rising from the phosphorus-flame 

 emitted polarized light, as did also the smoke coming from the 

 blast-furnace when the furnace-doors were opened. He con- 

 cluded that the flame of the furnace owes its brilliancy to the 



* Ann. Chim. Phys. [4] vol. xxx. p. 319. 

 t Journ. Tract. Chem. [2] vol. i. p. 425. 



