VTHY A FLAME EMITS LIGHT 93 



tions (under the impulse of more or less heat) dispose the 

 particles (of bodies) in such a manner that their faces, 

 acting like so many little mirrors, reflect upon our eyes the 

 rays of light which are in the air by night as well as by 

 day; for we are involved in darkness during the night for 

 no other reason but because they are not then so directed 

 as to face our organs of sight." 



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 litera- 

 ture. 



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 toward the interior of the flame, where the air 

 is in smallest quantity, and the deposition of solid charcoal, 

 which, first by its ignition and afterward by its combus- 

 tion, 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 car- 



