372 Professor Arthur Smitliells [Marcli 12, 



substance in a hydrocarbon flame, which may be collected in the 

 form of soot, contains a smaller quantity of hydrogen than could 

 reasonably be expected if soot were a hydrocarbon or a mixture of 

 hydrocarbons. He also remarked upon the non-volatile character 

 of soot. A recent analysis of soot from an acetylene flame showed 

 1*4 parts of hydrogen to 98*6 parts of carbon, after the soot had 

 been extracted with ether and dried. Now the hydrocarbon richest 

 in carbon recognised in organic chemistry (chrysogene) contains 

 about 5 per cent, of hydrogen. The soot, therefore, could not con- 

 tain more than about 30 j)er cent, of it, leaving a surplus of 70 per 

 cent, of uncombined carbon. To maintain Frankland's doctrine 

 that the light is essentially due to dense hydrocarbons in the gaseous 

 state, would compel us, in fact, to recognise soot as a hydrocarbon 

 of quite exceptional composition and properties. The doctrine was, 

 in its inception, an inference from experiments on other flames in 

 which high luminosity was found to be associated with high density 

 of the substances contained in the flames ; but it is to be remarked 

 that in most, if not all of these flames, the glow was ascribed to the 

 product of oxidation, and not merely to something separated and 

 subjected to a purely roasting process. 



But even if we regard the glowing substance soot of a flame as a 

 hydrocarbon or a mixture of hydrocarbons, and to this extent accept 

 Frankland's view, there remains the question whether the glowing 

 substance in the flame is solid or gaseous. The optical test, first 

 used by Soret, shows indisputably that a finely divided solid pervades 

 the whole of the luminous region of a hydrocarbon flame, and there 

 seems no reason to doubt that the glow of this solid matter would be 

 adequate to produce the light of the flame. 



According to the views of Lewes, the luminosity of a hydrocarbon 

 flame is determined essentially by the formation and subsequent 

 decomposition of acetylene. This theory, which is certainly in- 

 genious, need not be discussed on the present occasion. 



The development of bright light in a hydrocarbon flame, what- 

 ever be the full explanation, is certainly a secondary process, 

 demanding a particular mode of burning the gas for its production. 

 When the hydrocarbon meets the air in other ways, as when it is 

 burnt in a very small flame or at a very high pressure, or when air 

 is added to the gas before it leaves the burner, the bright light 

 disappears, and we then have the primary light of combustion which 

 is of feeble intensity and blue colour. The changes which a hydro- 

 carbon flame undergoes with varying air supply are well seen when 

 benzene vapoar is burned with a gradually increasing quantity of 

 admixed air. The flame is at first very bright; the next phase, 

 reached when the bright yellowish light has just disappeared, shows 

 two cones of bluish light, corresponding to those of a Bunsen burner ; 

 the last phase is reached when, by adding more air, the outer cone 

 is quenched, and the flame presents the appearance of a thin conical 

 shell of blue light. [Experiment shown.] The two-coned phase 



