Popular Science Monthly/Volume 33/June 1888/The Flame of a Candle

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THE little yellow candle-flame, which, is gradually disappearing from our households to give place to brilliant gas and electric lights, still plays a considerable part in the labors and researches of physicists, chemists, and astronomers. The former find in it a source of heat capable of melting and oxidizing or reducing the most refractory metals. The last employ it as a photometric unit, both to measure the most considerable lights and to determine the luminosity of stars so faint that they can hardly be seen in the great telescopes. But the most curious and interesting thing about this little flame is the fact that the optical study of it has contributed very largely to our knowledge of the elementary composition of the celestial bodies.

Carefully examined with the naked eye, the flame of a candle is composed of three distinct layers or envelopes, viz., a dark central part, the dark cone around the wick, formed of gaseous products of low temperature, and holding in suspension carbon in a state of fine division, but not yet incandescent; a luminous part, surrounding the dark part, and composed of carbon raised to a bright incandescence; and a thin external envelope, only faintly luminous and faintly colored, yellow toward the top, where the carbon is completely burned, and bluish toward the base, where the primary products of the decomposition of the matter of the candle are burning in contact with the air.

Analyzed by the aid of the spectroscope, the luminous cone gives a brilliant and continuous spectrum—that is, one having the appearance of a ribbon exhibiting all the colors of the rainbow, while the exterior, faintly luminous envelope gives a discontinuous spectrum formed of three bright bands—one yellow, one green, and one blue.

As only solid incandescent bodies are capable of giving a continuous spectrum, we conclude that carbon in the solid state is incandescent in the luminous envelope of the flame. But, the spectrum of the exterior envelope being discontinuous, we conclude that it is composed entirely of gaseous products.

The flame of illuminating gas presents, both to the naked eye and in the spectroscope, the same aspect as the flame of the candle, whence it is concluded that its lighting and heating powers are derived from the same cause—the more or less complete combustion of carbon.

By blowing air or injecting oxygen through the blow-pipe into a candle-flame or a gas-light, its aspect is greatly changed. The bright envelope nearly all disappears, while the inner dark cone is considerably developed, reaches a very high temperature, and exhibits a spectrum identical with that of the outer cone of the original flame. The brightness of the spectral bands is augmented by the rising of the temperature, and two new luminous bands, a red and a violet one, become visible in the spectroscope. At this moment we recognize that these bands are composed of a series of rays or bright lines, separated from one another by dark spaces.

While performing the prismatic analysis of the inner cone of a gas-light flame fed with pure oxygen, M. Stas observed, with the same spectroscope, a spectrum sensibly different as to the number of rays constituting the bands, according as the observation was made upon the top of the inner cone, where the temperature is highest and sufficient to keep iridium in fusion, or on the front or the side of this inner cone. The physiognomies of these three spectra vary according to the spectroscope employed. If we use a spectroscope with direct vision and weak dispersion, we observe a spectrum resembling that of the candle flame; but,-with an instrument of more considerable dispersive power, the bands define themselves into brilliant rays, some fine, and others broad, having extremely clear edges. These facts, M. Stas remarks, inseparably connect the facies of the spectrum of the flame with its greater or less elevation of temperature, and with the analyzing instruments employed.

Although the luminous intensity of the inner cone of the oxyhydrogen-flame is quite weak, Mr. Piazzi Smyth has discovered more than 400 bright rays in the spectral bands of this cone; viz., 97 rays in the red, 94 in the yellow, 97 in the green, 107 in the blue, and 71 in the violet bands.

But it is the analysis of the electric arc, the light of which does not differ essentially from that of the candle—for it is also the result of the ignition of carbon—that shows us these spectral bands in all their splendor, and initiates us into the grand complexity of their constitution. Like a luminous ribbon passing insensibly from one shade to another with diminishing brilliancy, each band is composed of a considerable number of bright rays of different breadths, disposed with a wonderful symmetry, increasing with the power of the analyzing instrument and the luminous intensity of the electric arc; the broader bright rays doubling into finer rays, and new luminous rays appearing in the dark spaces that separate the bright rays. While these bright lines are not arranged rigorously in the same manner in each band, they nevertheless show a great resemblance in their grouping and spacing.

In order to show how far the resolution into bright lines of the spectral bands of the electric arc (which are identical with those of the candle and the oxyhydrogen blow-pipe) may be carried, I have published a specimen of the facies of the yellow, green, and blue bands, indicating the intensity and normal distance of the rays composing them. It appears, from this work, that, for a fifth part alone of their total length, these bands show, respectively, 163, 160, and 120 lines; this would bring to about 800 the number of lines constituting each band, and to at least 4,000 the number of the lines forming the five bands of the spectrum of the electric arc; for the more intense bright lines are doubled again when they are observed under conditions favorable to their brilliancy and dispersion. In comparing, with the same spectroscope, the spectrum of the electric arc and the solar spectrum, we observe that the former spectrum displays a more considerable number of bright rays than the solar spectrum of dark rays. Since it is nearly certain that the spectral bands belong to the spectrum of carbon—for they are observed when the electric arc shines in a vacuum, that is, when carbon alone is in ignition—it follows that the spectrum of this element contains more rays than the entire solar spectrum.

Some physicists doubted for a long time the identity of the spectra of carbon and the candle-flame, because there existed a spectrum of carbon entirely different from the banded spectrum. But as I have succeeded in demonstrating, on the one hand, that this spectrum does not belong to carbon, and on the other hand that the spectrum of the candle-flame was brightly visible in the ignited filament of the incandescent lamp when the vacuum is as perfect as it is possible to make it, I think there should now be little doubt respecting the identity of the two spectra. Carbon, being found in various combinations everywhere on the surface of the globe, should of necessity reveal its presence in most of the bodies subjected to spectrum analysis. Eminent chemists have even found traces of it in the nearly perfect vacuum of our pneumatic machines.

The absorption spectrum of carbon, or that which should be composed of the dark lines detaching themselves upon a continuously bright spectrum, has not yet been obtained. In the comparative study that I have made of the solar spectrum and the spectrum of carbon, I have shown that most of the bright rays forming the carbon bands do not coincide with the dark rays of the solar spectrum. I have been inclined to believe from this that the absorption spectrum of carbon does not exist in the solar spectrum, but I have not been able to declare the same conclusion respecting the emission spectrum—that is, the spectrum with bright bands—because the discovery of the bright bands in the solar spectrum offers a real difficulty, resulting from the fact that the bright rays can be recognized on the bright part of the solar spectrum only by the difference of their brilliancy. On the other hand, we have not recognized the presence of the emission spectrum of carbon among the numerous bright rays observed in the spectrum of the solar atmosphere; and this fact goes to indicate the absence of carbon among the constituent elements of the sun. But such an assertion can not be made until our acquaintance with the subject becomes more complete and clear.

Although it is hardly possible, in the actual state of our knowledge, to establish the presence of carbon in the sun, it is extremely easy to recognize it in the spectra of comets. In 1868, when the comets of Winnecke and Brorsen appeared, Secchi in Italy, Huggins in England, and Wolf in France, studying the spectra of those stars in respect to their composition, discerned that the three bright bands, yellow, green, and blue, of which they were formed, could be regarded as analogous with the spectrum of carbon. It is possible, in fact, in a gas-light re-enforced by oxygen, if the brightness of the flame is reduced and a spectroscope of feeble dispersion is used, to obtain a spectrum exactly like that of a comet. After these observations, Mr. Christie and myself recognized the violet band in the spectrum of the comet h of 1881, and Dr. Young has found that the green band, or the most brilliant one in the spectrum of this comet, is formed of rays like the corresponding green band of the spectrum of flame. The identity of the two spectra is therefore demonstrated.

The presence of carbon is also suspected, if not definitely recognized, in the spectra of certain stars, the orange or red color of which indicates a temperature of relatively inferior elevation. The spectra of these stars consist of several dark bands, superposed upon a continuous bright spectrum, which present a great similarity of aspect and position with the luminous bands of the spectrum of carbon in comets, illuminating gas, and the candle flame. We have then, here, the absorption spectrum of carbon. The spectral analysis of the candle-flame thus permits us to discover by optical methods the presence of one of the most important elements of our globe in luminous bodies, whether celestial or terrestrial, whatever their distance, even though it be so great that light occupies thousands of years in coming to us.—Translated for the Popular Science Monthly from Ciel et Terre.

While generally accepting Mr. James Murray's views regarding the formation of barrier reefs and atolls, Mr. J. L. Wheaton would regard as the principal agent in forming the interior lagoons, not the solution and washing out of dead coral by sea-water, but privation of the inner part of the reef of food, all nourishment having been absorbed by the corals of the outer reef from the water before it reaches the interior.