LUMINOSITY OF FLAMES 273 



experiments," he says, " convinced me that this was the true 

 solution of the problem." Nearly half a century later, however, 

 Frankland drew attention to the fact that flames may be pro- 

 duced which are brilliantly luminous but contain no solid 

 matter, and further that a gas like hydrogen, which burns under 

 ordinary conditions without emission of light, may be made to 

 give out light if burned under increased pressure. It appeared, 

 therefore, that the light of a flame might be due only to the 

 presence in it of dense gases or vapours. 



Notwithstanding these results it appears certain for reasons 

 which cannot be discussed that ordinary hydrocarbon flames, 

 such as those of coal-gas, do contain solid particles, and a reason 

 has to be sought for the deposition of carbon from the burning 

 gas. The hypothesis brought forward a few years ago by 

 the late Professor Vivian Lewes, which involved the production 

 and immediate decomposition of acetylene within such flames, 

 was at one time much discussed, but seems to be no longer 

 tenable, and we are still waiting for " the true solution of the 

 problem " which Davy thought he had got hold of a hundred 

 years ago. 



That the introduction of solid matter into a non-luminous 

 flame causes the emission of light is a matter of common know- 

 ledge, and Davy himself showed that it is of no consequence 

 whether the solid is combustible or not, for he showed that not 

 only did dust of charcoal but fine powder of silica or magnesia 

 thrown into a flame produces light. From the time when coal- 

 gas in the earliest years of last century became a common source 

 of light, efforts have continuously been made to increase its 

 illuminating power, first by the improvements in the jets or 

 burners at which the gas was burned, later by the introduction 

 of gases or vapours rich in heavy carbonaceous compounds. 



The last is a method still employed in our own day (see crack- 

 ing of Petroleum, Chap. XIX). But it has long been known that 

 the introduction into a flame of different solid substances is 

 attended by the emission of very different amounts of light. 



Berzelius in 1829 noticed that thoria, zirconia, and other of 

 the rare earths in a non-luminous flame give out a very brilliant 

 light, and similar observations were made later by Bunsen and 

 other chemists. Lime is one of the substances which when 

 strongly heated gives a bright light, but the temperature re- 

 quired in this case to give a satisfactory effect is higher than 



