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 transition finds little expression in the literature.
About the year 1816 Sir Humphry Davy advanced what has been known ever since as the 'solid particle' theory of luminosity; a theory which went unchallenged for forty-five years and was accepted by practically 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 high degree the intensity of the light; and a few experiments soon convinced me that this was the true solution of the problem. "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-luminous 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 Royal 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 Mt. 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 with a goodly supply of standard candles and timed their slow wasting away; probably keeping
