| 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 regarding 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 throughout 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 particles of carbon heated to incandescence, the question of the origin of the carbon, or the chemical changes taking place in the flame, was discussed, and along with this the source of heat which renders it incandescent. 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 carbon 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.
