WHY A FLAME EMITS LIGHT. 213 



warm in the meantime by the fire of his enthusiasm. Many interest- 

 ing facts were brought to light by these investigations, but his use of 

 them in interpreting the causes of luminosity in ordinary flames led 

 him into error, and, although he found adherents at the time, his views 

 have long since been replaced by those based upon more careful obser- 

 vation. The importance of the work of Frankland lay not so much in 

 what he did as in what he led others to do; and since the publication 

 of his views a great deal has been done by Heumann, Stein, Smithells, 

 Burch, Lewes and others. 



Stein disproved Franldand's assertion that soot is a mixture of 

 dense hydrocarbons by showing that it can not be volatilized even by 

 great heat, and that it contains only about nine tenths of one per cent, 

 of hydrogen, which can be separated from it only at high temperatures 

 in an atmosphere of chlorine. 



Nor did Frankland 's view that glowing, dense vapors cause the light 

 appeal to Heumann, who thought it unlikely that such dense vapors 

 exist in a flame or that there is a sufficiently high temperature to 

 cause them to glow. He knew, of course, that at a temperature like 

 that of an electric arc many gases do glow and give continuous spectra, 

 and that a highly heated gas under pressure acts likewise; but he 

 argued that if carbon really does exist as such in a flame, it most prob- 

 ably is the source of luminosity. To prove its presence or absence he 

 studied the effects upon a flame of heating and cooling it, of diluting 

 and varying the temperature of the gases supplied to it, its transparency 

 and the shadows cast by it, as well as other phenomena ; and the results 

 of his experiments led him to give unqualified support to the theory 

 of Davy. 



Some account of the salient features at least of Heumann 's elab- 

 orate investigation must be given in order to convey any idea of his part 

 in firmly fixing the 'solid particle' theory. By allowing a luminous 

 flame to play upon a surface which rapidly conducted heat away from 

 it, like a platinum dish, its luminosity was destroyed. Heating the 

 upper surface of the dish restored the luminosity, and hence Heumann 

 concluded that cooling a flame diminishes its light-giving properties, 

 while heating increases them. He varied the temperature of illumi- 

 nating gas before it reached the burner and found that the same effects 

 were produced. The heating in some cases increased the normal light- 

 giving power as much as a hundred and twenty-five per cent. Further 

 investigation showed that luminosity can also be diminished or de- 

 stroyed by rapid oxidation of the hydrocarbons, as well as by diluting 

 them with a neutral gas like nitrogen or carbon dioxide; the effect of 

 dilution being to necessitate a higher temperature for luminosity. He 

 next rendered a flame non-luminous by cooling, introduced chlorine 

 into it to break down the hydrocarbons, and obtained a brilliant light. 



