2 14 POPULAR SCIENCE MONTHLY. 



A porcelain rod introduced into the lower part of a flame cooled it and 

 decreased its light, but collected no carbon, while, if introduced into 

 the upper part, its under side became coated with soot. Heumann 

 argued that if Frankland was right and the light is reflected from dense 

 hydrocarbon vapors, these should be condensed on all sides of the rod 

 at once in a quiet flame, while, as a matter of fact, soot was deposited 

 only on the under side ; and furthermore, soot can also be collected upon 

 a surface too hot to condense hydrocarbons at all. He therefore con- 

 cluded that the surface merely stops carbon which is formed lower down 

 in the flame. If one luminous flame is allowed to play aga,inst an- 

 other, the carbon is rolled up and can be seen as glowing particles in 

 the outer non-luminous sheath. 



Frankland had said that flames can not contain solid particles be- 

 cause they are transparent. Heumann pointed out that tliick flames 

 are opaque and that tliin ones are no more transparent than is an equal 

 layer of soot rising from burning turpentine; the rapidity of the mo- 

 tion of the particles preventing any obstruction to the view, just as is 

 the case with a rapidly revolving, spoked wheel. 



Heumann next took up the phenomena of shadows and showed that 

 the luminous portion casts a definite shadow when interposed between 

 sunlight and a screen, and that the shadow is continuous for a lumi- 

 nous turpentine flame and the column of soot above it. And further, 

 that a hydrogen flame which ordinarily casts no shadow and gives no 

 light will cast a sharp shadow and emit a fairly bright light if passed 

 through suspended lampblack or if it sweeps any solid matter into the 

 flame. Luminous vapors do not cast shadows, absorption bands being 

 very different from true shadows. 



C. J. Burch found that when sunlight is reflected from a luminous 

 flame it is polarized, while if reflected by glowing vapors, however 

 dense, it does not exhibit this phenomenon. Sunlight which was re- 

 flected and refracted by luminous flames was found to exhibit phenom- 

 ena identical with that reflected and refracted by non-luminous flames 

 rendered luminous by the introduction of solid matter, and also with 

 light reflected, and refracted by very finely divided solid matter held in 

 suspension in a liquid. The phenomena presented by like experiments 

 with glowing vapors were totally different. All of Burch 's work was 

 confirmed by Stokes some years later. 



There was now left no shadow of doubt about carbon being the 

 source of the light rays, and the next question that concerned investi- 

 gators was the chemical changes which give rise to carbon particles. 



Sir Humphry Davy thought the separation of carbon to be due to 

 a decomposition of the hydrocarbon compounds (of which all illumi- 

 nants are composed) within the flame where the air is in smallest quan- 

 tity, and no other cause was assigned by other investigators. Prior to 



