WHY A FLAME EMITS LIGHT. 209 



WHY A FLAME EMITS LIGHT— THE DEVELOPMENT OF 



THE THEORY. 



BY Professor ROBERT MONTGOMERY BIRD, Ph.D., 



UNIVERSITY OF THE STATE OF MISSOURI. 



AS one would naturally suppose, the theory now generally held re- 

 garding the nature of an ordinary flame and its power to emit 

 light is not altogether the result of modern research, but one which has 

 been evolved from very ancient and hazy notions. Naught else is to 

 be expected when we consider the important place fire has held through- 

 out the development of mankind. It is the first recorded object 

 of his worship, and we have reason to believe that all architecture had 

 its beginning in rude structures erected to protect the sacred fire. It 

 is not the nature of man to see phenomena so striking as those which 

 attend the consumption of matter by fire and not speculate upon them. 

 But the centuries had multiplied and modern times had been reached 

 before man's ideas regarding fire, flame and light became distinct, and 

 the use of these terms differentiated. The best text-books and works 

 on natural philosophy published near the end of the eighteenth century 

 still used the terms with great looseness, and the conceptions of the 

 material nature of flame and light were yet in their death struggles. 



After the corpuscular theory of light had given place to the wave 

 theory, conflicting ideas arose as to why and how a flame emits light 

 waves. When it was agreed that the waves were sent out by solid par- 

 ticles of carbon heated to incandescence, the question of the origin of 

 the carbon, or the chemical changes taking place in the flame, was dis- 

 cussed, and along with this the source of heat which renders it incan- 

 descent. The last and most generally accepted answer to these two 

 questions — ^the origin of carbon particles and the source of heat — is 

 given in the 'acetylene theory,' first advanced in 1892 by Professor 

 Vivian B. Lewes, of England. 



This theory expressed briefly is that a portion of the hydrocarbon 

 gas, by the heat of combustion of another portion, is converted into 

 acetylene, and that this on being decomposed by heat furnishes the car- 

 bon particles, which particles are rendered incandescent mainly by the 

 heat liberated when the gas is decomposed; acetylene being a substance 

 which absorbs heat during its formation and hence liberates heat when 

 it breaks down. Whatever is burned, whether a solid candle or liquid 

 oil, must pass through the gaseous state, and hence this applies to all 

 flames used for lighting purposes. 



VOL. LXTTT. — 14. 



