1897.] on the Source of Light in Flames. 369 



In 1867 Dr. Frankland, lecturing before the Eoyal Institution,* 

 gave strong reasons for dissenting from Davy's views, both as to the 

 cause of the luminosity of flames in general and of the flames of 

 hydrocarbons in particular. Dr. Frankland's conclusions may be 

 summarised as follows : — 



(i.) Bright flames exist which do not contain solid particles. 



(ii.) The luminosity of flames depends mainly on the density of 

 the substances contained in them. 



(iii.) Feebly luminous flames may be made bright by compressing 

 the burning gases. 



(iv.) The luminosity of ordinary hydrocarbon flames, such as 

 that of coal gas, is not due in any important degree to solid particles 

 of carbon, but almost entirely to the glow of dense hydrocarbon 

 vapours. 



Of these conclusions, two are beyond doubt. The flame of 

 phosphorus, or of carbon-di sulphide burning in oxygen, are examples 

 of bright flames in which no solid matter can be supposed reasonably 

 to exist. The explosion of electrolytic gas in a eudiometer resting 

 on an india-rubber pad produces a bright light, the gas is hindered 

 from expanding, and hence the flame travels through the mixture 

 under increasing pressure. 



A table, in Dr. Frankland's paper, shows the kind of evidence 

 from which he concluded that the brightness of flames depends on 

 the density of the substances they contain, and the general agreement 

 of fact with theory is very striking. It is important to know whether 

 the rule holds without exception, and whether it is in harmony with 

 other general laws. There are flames containing dense substances 

 which are not bright, and flames which are bright though they do 

 not contain dense substances; but these apparent exceptions are 

 explained by supposing that the temperature in one case is very low 

 and in the other very high. If this kind of accommodation is per- 

 missible. Dr. Frankland's principle can hardly be submitted to a 

 rigorous test. 



The fact that the light of compressed flames is so intense can 

 hardly be held to support the general doctrine in any rational sense, 

 for it cannot be said either physically or chemically that two gases 

 are in a like state when they have the same density. As a fact the 

 increased luminosity here accompanying increased density is unde- 

 niable, and Dr. Frankland has contended for no more than this ; 

 but the matter must be looked at in the light of the molecular theory. 

 This theory would lead us to expect increased light from a flame 

 containing dense matter if the density were a result of molecular 

 crowding, whilst it can at present tell us nothing about the effect 

 likely to ensue from an increase of density arising from the greater 



* Proc. Roy. Inst. 5, p. 419. The best account of Dr. Frankland's views is 

 contained in six lectures delivered at the Royal Institution, and admirably 

 reported in the ' Journal of Gas Lighting.* 



