warm in the meantime by the fire of his enthusiasm. Many interesting 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in ordinary flames led him into error, and, although he found adherents at the time, his views have long since been replaced by those based upon more careful observation. 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 Frankland'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 probably 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 elaborate 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 illuminating 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 destroyed 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.
