Popular Science Monthly/Volume 7/October 1875/The Cause of the Light of Flames

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THE correctness of the old and well-founded conception that the light of flame is caused by incandescent carbon-molecules, has been disputed by Dr. Frankland, who contends and tries to prove that it is derived from hydrocarbon-vapors. It is evident that the old theory would have to give place to the new doctrine as soon as the untenability of the former and the correctness of the latter are proved. But neither the one nor the other has, I think, yet been done. Prof. Frankland can, therefore, only be pleased if the present paper subjects the pros and cons of the new and old theory to an impartial examination.

As proof of his ideas he mentions that the soot deposited on a cool surface, when introduced into a flame, does not consist of pure carbon, but that it contains also hydrogen; that, in fact, it seems nothing else than a collection of the densest light-giving hydrocarbons, whose vapors condense on the cold surface.

Against this we may mention that not only do the heavy hydro-carbons, but even marsh-gas, split up at high temperatures on exclusion of atmospheric air; and as the hydrocarbons, whose vapors are supposed to cause the luminosity of the flame, are precisely under such conditions before they come into contact with the air, it cannot be doubted that they suffer decomposition into carbon and hydrogen in the luminous portion of the flame. It is of little importance whether the eliminated carbon is chemically pure, or whether it contains still a hydrogen compound; the important question is this, Is the soot held by the flame in the shape of vapor or in the solid form? If the soot was nothing but a conglomeration of the densest light-giving hydro-carbons, whose vapors condense on a cool body, then, when sufficiently highly heated by exclusion of air, it ought to reassume vapor-form. This is, however, not the case, as every one will find who tries the experiment.

Its chemical composition is just as little favorable to Frankland's view. It ought, presumedly, to vary according to the lighting material from which it was derived—nay, even according to the place of the flame wherefrom it was deposited. It is well known that the temperature of the flame varies in various places, and Magnus's experiments have proved that from heavy hydrocarbons at a less high temperature a hydrogenous tarry product besides hydrocarbon is also eliminated. The soot, whose analysis I give, was obtained from a bat's-wing burner by allowing a small silver basin, filled with water, to dip for about two or three minutes in the flame. Benzine removed traces of a solid yellow body, but the small amount of it prevented its being further investigated. Alcohol, and alcoholic solution of caustic potash, and dilute sulphuric acid, dissolved nothing.

After being carefully and repeatedly washed with boiling water and dried at 130°, 0.206 yielded: Carbonic acid, 0.6985; water, 0.0195; ash, 0.0020—which amounts in 100 parts to.

Containing Ash. Free from Ash.
Carbon............ 96.446 97.390
Hydrogen........... 1.051 1.061
Ash........... 0.970
Oxygen............ 1.533 1.533

I attribute the presence of oxygen to a small amount of water, which, even at 130°, was still retained, and this, when deducted, gives the composition of 100 parts of soot free from water and ash as consisting of carbon, 99.095; hydrogen, 0.905.

This analysis is in accordance with the chemical composition of the soot of the flame, and with the well-known behavior of heated hydrocarbons.

2. "How could the light of a flame be as transparent as in reality it is, if it was filled with solid carbon-particles?" asks Dr. Frankland.

In reply to this, it must be admitted that one is able to read the writing held behind the flame of a bat's-wing burner. It is, however, easily observable that the flame is more transparent in the lower non-luminous portion. The reading becomes also more difficult through a flame of greater thickness, and impossible through the flame of a candle or petroleum-burner. If, as is proved hereby, the transparency of a flame is only very limited, it may also be remembered that one can also read the same writing through media which are known to be filled with solid particles. The fact that solid bodies are by preference apt to become light-radiating is not at all changed by this, and thus far it is demonstrated only that there can be but one solid body to which the luminosity of flame can be attributed. If we consider, therefore, all the facts, we can draw only one conclusion, namely, that the light of our illuminating flame comes from incandescent carbon-molecules, and that the old view is still to be retained.

Experience teaches that, for the artificial production of light, a high temperature is requisite before all things. Temperature is, however, that part of the total heat of a body which influences the surrounding parts, or the surplus of atomic movement which is not consumed by its inner work. A high temperature means, therefore, a great excess of such movement, which again is identical with a greater number of momentary vibrations. In fact, the movement of light and the movement of heat differ essentially by regularity (rhythmen), and greater movement of heat passes, therefore, presumedly into movement of light, if it has reached the lowest number of vibrations for light, namely, those of red light. If, after a greater and greater rising of temperature up to its highest possible degree, the rapidity of movement increases more and more, we observe, besides the red light, first, yellow light, forming orange with the former; later, we meet also blue light, which, however, in most cases, only serves to form white light with the red and yellow, and which is only predominant in very rare cases, as observed by Deville. Under ordinary circumstances, we only get a yellow or red light containing more or less white. The more white it contains the greater is, naturally, its effect of light; and, as white only appears at the highest temperatures, it becomes evident that the temperature of a flame does not exert a secondary influence on its luminosity, but is its principal factor. The second factor is the eliminated carbon, the molecules of which radiate the light. The luminosity of two flames of the same temperature corresponds, therefore, to the number of its carbon-molecules, and "luminosity in general equal to the product of the radiating molecules and their temperatures" for illuminating purposes, it may be presumed that the latter should amount to at least 1,000°.

The above-mentioned phenomena of light may easily be observed on solid bodies if heated. They are not observable on gases as long as they expand unhindered. It would, however, be wrong to attribute this negative behavior to the circumstance alone that, by the unhindered expansion, the amount of the added or produced heat was changed into power. This is contradicted by the high temperature which, among others, the non-luminous explosive gas-flame (Knall-gas) possesses. Besides, it is also observed that platinum wire becomes incandescent in every possible non-luminous flame, even in a flame produced by nitrogen on coal-gas, if the requisite temperature to change heat into light is present.

If we may conclude from this that the atoms of gases may be brought into light-vibrations without becoming luminous, then we possess bodies which conduct the light (the gases), and others which radiate light (the solid bodies), just as we have conductors of electricity and idio-electrical bodies.

An explanation of this difference is offered when light is considered as atomic movement. Its effect to the eye is then the product of quantity and velocity.

In a given space we find a much larger number of vibrating atoms if filled with solid matter than if filled with gas. The waves of light of solid bodies must, therefore, be much denser than those of gases, and exert also a more intense effect on the nerves of our eyes, "Light-conductors" differ, therefore, from "light-radiators" by the lesser density of their waves of light; for which reason they cannot, under ordinary circumstances, form "optical molecules," as I expressed on another occasion. How powerfully the condensation of the waves of light affects the eye is shown by the effect of collecting lenses.

The minimum of density which a body must possess to become light-radiating—that is, to become self-luminous to the eye, or to appear a source of light—is just now not known; but one sees, if this view is correct, the possibility of even vapors or dense gases becoming luminous, as Frankland tried to prove. The results of his experiments might even serve as foundation for the lowest limit of density, if it were not so very difficult, nay, even just now impossible, to make such an experiment in a manner so as to exclude every doubt about the assisting influence of solid bodies.—English Mechanic.