212 POPULAR SCIENCE MONTHLY. 



At a single step we pass from the rather crude ideas of the older 

 thinkers to those ideas which obtain at the present day, and the transi- 

 tion finds little expression in the literature. 



About the year 1816 Sir Humphry Davy advanced what has been 

 known ever since as the '^olid particle' theory of luminosity; a theory 

 which went unchallenged for forty-five years and was accepted by prac- 

 tically every one. 



He was experimenting upon the combustion taking place in his 

 famous safety lamp and said, "I was led to imagine that the cause of 

 the superiority of the light of a stream of coal gas might be owing to 

 the decomposition of a part of the gas towards the interior of the 

 flame, where the air is in smallest quantity, and the deposition of solid 

 charcoal, which, first by its ignition and afterwards by its combustion, 

 increased to a liigh degree the intensity of the light; and a few experi- 

 ments soon convinced me that this was the true solution of the prob- 

 lem. " "Whenever a flame is remarkably brilliant and dense, it may 

 always be concluded that some solid matter is produced in it; on the 

 contrary, whenever a flame is extremely feeble and transparent it may 

 be inferred that no solid matter is formed. ' ' The idea that solid carbon 

 in the flame is the source of its light was not original with Davy — he 

 says it was suggested by a Mr. Hare — but it was Davy's investigations 

 which put it on a firm basis and he formulated the theory. 



Davy showed the relation between the heat and light of flames, the 

 effects of rarefaction and compression of the surrounding air and the 

 influence of cooling and heating. He pointed out also that a luminous 

 flame will deposit carbon on a cold surface, and if rendered non-lumin- 

 ous no carbon can be obtained. These conclusions were immediately 

 accepted and were not seriously disputed until the appearance in 1861 

 of a communication to the Eoyal Society from E. Frankland. 



In this article Frankland advanced what has come to be known as 

 the ' dense vapor ' theory. He and his adherents claimed that, although 

 solid particles in a flame do cause it to emit light, the light from our 

 ordinary illuminating flames is dependent to a great extent upon the 

 presence of dense, transparent, hydrocarbon vapors from which it is 

 radiated, and is not due to the presence of incandescent solid carbon 

 particles. They further claimed that the soot deposited is not carbon, 

 but a mixture of dense hydrocarbons of remarkably high boiling points. 



Frankland was led to take up his investigations by seeing a report 

 that candles burned at the same rate on the top of ]\It. Blanc as in the 

 valley at its foot; and a second report regarding the retardation of the 

 bursting of shells with time fuses at high elevations in India. 



Besides carrying on investigations in artficially rarefied air in his 

 laboratory, he climbed to the top of Mt. Blanc mth a goodly supply of 

 standard candles and timed their slow wasting away; probably keeping 



