Chapter II: The Chemistry of the Sun

Knowledge of solar chemistry is based exclusively upon the analysis of solar light. It advances _pari passu_ with the interpretation of the Fraunhofer lines. And by their interpretation is signified the process of identifying them, one by one, with the rays of known substances, made to glow artificially in the laboratory. They are the characters of a script in the main decipherable, and already, to a satisfactory extent, deciphered. Their reversal from bright to dark simply implies that the prismatic background upon which they are projected represents a hotter source of radiation than theirs. In other words, the temperature of the photosphere is above that of the ignited vapours through which its light is sifted, and by which it is selectively absorbed.

Fraunhofer’s survey of the solar spectrum was necessarily confined to its visible section, and was executed with very imperfect appliances. Yet the lines laid down in his map had the importance of permanent landmarks. The following is a list of the chief among them, their wave-lengths on Rowland’s scale, and the chemical origins ascertained for them, being added:—

Designation. Wave-length in ten-millionths of a Origin. millimetre. A 7594·059 (upper edge of a band) Terrestrial oxygen B 6867·461 „ „ „ „ C 6563·054 Hydrogen D_{1} 5896·156 Sodium D_{2} 5890·182 „ E_{1} 5270·495 Iron _b__{1} 5183·792 Magnesium F 4861·496 Hydrogen G 4308·034 Iron H 3968·620 Calcium K 3933·809 „

The first and last lines in this table approximately define the range of dispersed sunlight visible to ordinary eyes. It extends over nearly an octave; and a higher half-octave in the ultra-violet is disclosed photographically. The infra-red, however, offers a far vaster scope for exploration. Using a “bolographic” method, in which the camera registers what the bolometer[4] _feels_, Professor Langley has surveyed a stretch of dark radiations eight times longer than the bright strip mapped by Fraunhofer; nor was his advance downward in the spectrum checked by any insurmountable barrier. There is, indeed, much probability that long heat-waves and short “Hertzian” waves are really indistinguishable, and that the luminous spectrum passes without a break through the thermal into the electric spectrum.

In the ultra-violet region, on the other hand, a peremptory stop is put to research by the interposition of the air. It excludes by absorbing ether-waves shorter than about λ 2950. Cornu found sunlight to be arrested just at this point. Sir William Huggins fixed the limit for the photographic spectrum of Vega (α Lyræ) at λ 2970.[5] It does not fluctuate with meteorological conditions. Dampness and drought are equally ineffective in shifting the atmospheric barrier against the entry of quick vibrations. Cornu ascertained in 1881[6] that it is affected only by the height of the barometer. Nor is the reduction of impermeability through ascent above the earth’s surface nearly as great as it would be if aqueous vapour were the producing agent. That oxygen is chiefly concerned is rendered certain by converging proofs. Nitrogen seems to be, in this respect as in others, nearly inert.

PLATE II.

Fraunhofer’s A-Band in the Solar Spectrum. (_Photographed by Frank
McClean, F.R.S._)
]

The general enfeeblement, by transmission through our atmosphere, of the violet and blue sections of sunlight becomes obvious in the redness of the sinking sun. On the lower radiations telluric absorption acts more specifically. They are interrupted by a multitude of dark bands and lines certainly referable to it. The question as to the terrestrial or solar origin of such effects is evidently of fundamental importance to solar chemistry. It can be answered in two distinct ways. The earlier and simpler method is by comparing the spectra of the high and low sun. The groovings that gain strength with approach to the horizon stand self-declared as atmospheric, while lines unaffected by altitude tell plainly of exotic conditions. The latter class are much the more numerous. Of 3200 lines mapped by Thollon, 2090 are purely solar, 866 telluric, and 246 of compound production.[7] And the proportion is not very different in Dr. Becker’s catalogue of 3637 spectral lines, published in 1890.[8] Among them 928 came out blackened in “low-sun” observations, and proved in the main due to the selective absorption of water-vapour. A considerable proportion, indeed, belonged to the “rain-band,” and varied hygroscopically. Dry-air absorption is almost exclusively an oxygen product. It takes effect chiefly in three wide bands, Fraunhofer’s “A” and “B” and Ångström’s “_a_,” all relieved against a crimson background. They are characteristic of cool oxygen. The molecules, whose vibrations they in a manner reflect, are broken up at high temperatures. They survive, however, the liquefaction of the gas at −181° C. Professors Liveing and Dewar observed the atmospheric A and B in light that had been transmitted through three inches of this frigid fluid.[9] The rhythmical flutings composing the former are shown in Plate II., from a photograph by Mr. McClean. The work, of which it is a specimen, portrays the solar spectrum in seven sections, from D to below A (λ 5800 to λ 7700), the dispersion having been effected by means of a Rutherfurd grating of 17,296 lines to the inch.

Fig. 1 gives a general view of the atmospheric spectrum, so far as it can be seen, but it is largely invisible. Langley found the immense tract of the heat spectrum, down to wave-lengths of nearly six “microns,”[10] thronged with “cold” rays, 652 of which[11] were accurately determined from “bolographs,” but remain, with few exceptions, chemically unidentified.

FIG. 1.—General View of Atmospheric Spectrum (Schemer’s _Astronomical
Spectroscopy_, translated by Frost).
]

FIG. 2.—The Infra-Red Spectrum (Langley).
]

Fig. 2 reproduces Langley’s drawing of part of the infra-red spectrum. The blank strip to the left shows the comparative brevity of the visible part of the scroll. The invisible part includes the distinctive signature of one other atmospheric constituent besides oxygen and water-vapour. Two strong bands in the infra-red are assigned by Knut Ångström to the absorption of carbon dioxide,[12] a substance of which four volumes are present in ten thousand of air at sea-level. The huge nitrogen envelope of our globe, together with its argon-ingredient, appears to be perfectly transparent to rays of all refrangibilities. The unexplained fact of its spectral nullity emphasises the inadmissibility of negative conclusions regarding the chemistry of the heavenly bodies.

The second peculiarity which distinguishes telluric lines is a negative one. They do not shift as the sun rotates. But lines genuinely emanating from the equatorial edges of the sun are displaced towards the blue by the advancing movement of the left or eastern limb, towards the red by the recession of the western limb. The juxtaposition, accordingly, of spectra from these two opposite sources serves as an unfailing test of the origin of their constituent markings, those claimed by the sun being perceptibly notched at the points of junction, while their telluric associates run on continuously.

Little has been added to knowledge of the sun’s constitution by researches in the infra-red part of the spectrum. They are as yet crippled by the lack of metallic comparison-lines. Sir William Abney obtained in 1879 a modification of bromide of silver sensitive to slow heat-vibrations, and thus succeeded in directly photographing the solar spectrum between the wave-lengths λ 7600 and λ 10,750. Of 590 absorption-lines measured by him in this region in 1886,[13] only an insignificant fraction have been identified. All these belong to metals with low melting-points.[14] Further, certain bands which Becquerel succeeded in rendering visible by phosphorescence proved assignable to magnesium, calcium, sodium, and potassium.[15] This confirmation of the presence in the sun of potassium was far from superfluous, as only one line due to it can ordinarily be seen.

The recent era in solar chemistry may be said to date from Rowland’s production of a perfect screw in 1882. This minor feat of ingenuity opened the way for vital improvements. Through its means, gratings ruled with almost ideal regularity became widely available, and the difficulties impeding the diffractive mode of light-analysis were removed or diminished. Now observations are mutually comparable only when the _absolute_ wave-lengths of the observed rays are known; and they are derivable immediately from the diffraction spectrum, while in the refraction spectrum several complicating circumstances come into play. Hence the supreme value of gratings. For in the spectra afforded by them the positions of rays depend simply and solely upon the distance from crest to crest of the minute ethereal undulations they represent.

Rowland’s photographic map of the solar spectrum[16] was a document in advance of the time. The amount of detail shown in it may be exemplified by the statement that 150 lines of absorption could be separately reckoned between H and K, the great calcium pair in the violet. No comparable delineations of terrestrial spectra (apart from that of iron) were, however, then extant; coincidences between the rays in them and Fraunhofer lines might, accordingly, be often apparent only, and devoid of chemical significance. Dr. Scheiner gave expression to a general sense of discouragement when he wrote in 1890: “It is unfortunately the case that less is known to-day as to the meaning of the Fraunhofer lines than was supposed to be known ten years ago.”[17]

The need for fresh efforts was, however, promptly met. Photographic investigations of metallic spectra, fully coming up to the new standard of accuracy, were set on foot, among others, by Kayser and Runge at Hanover, by Hasselberg at Stockholm, above all, by Rowland and his coadjutors at the Johns Hopkins University. Here the spectra of nearly all the chemical elements have been photographed with high dispersion for the purpose of solar comparisons. And the end of the process is well within view. Measurements have already been carried far enough to give a multitude of identifications. Between 1895 and 1897 Professor Rowland published in the _Astrophysical Journal_ a “Preliminary Table of Solar Wave-Lengths,” extending from λ 7331 to λ 2976—that is, from dusky crimson up to the highest ultra-violet ray capable of penetrating the aerial barrier. He unhappily did not live to make the list definitive; but it comprises, as he left it, nearly 20,000 lines, about a third of which, by a rough estimate, may be confidently referred to absorption by various terrestrial substances. These are enumerated below, according to the number of lines associated with them in the sun. The corresponding atomic weights are given in a second column.

ROWLAND’S TABLE OF SOLAR ELEMENTS.
Element. Atomic Weight.
Iron (about 2750 line-coincidences) 56
Nickel 58
Titanium 48
Manganese 55
Chromium 52
Cobalt 59
Carbon (about 240) 12
Vanadium 51
Zirconium 65
Cerium 140
Calcium (over 75) 40
Scandium 44
Neodymium 140
Lanthanum 139
Yttrium 89
Niobium 94
Molybdenum 96
Palladium 106
Magnesium (about 24 coincident lines) 24
Sodium (13) 23
Silicon 32
Hydrogen 1
Strontium 87
Barium 137
Aluminium 27
Cadmium 112
Rhodium 103
Erbium 166
Zinc 65
Copper (2) 63
Silver (2) 108
Glucinum (2) 9
Germanium 72
Tin 117
Lead (1) 207
Potassium (1) 39

Of the following substances no traces could be found the solar spectrum:—

Element. Atomic Weight.
Antimony 120
Arsenic 75
Bismuth 208
Boron 11
Cæsium 133
Gold 197
Indium 113
Lithium 7
Mercury 200
Nitrogen (vacuum tube) 14
Phosphorus 31
Praseodymium 144
Rubidium 85
Selenium 79
Sulphur 32
Thallium 232

Argon may now be included among the non-apparent elements, and the presence in the sun of platinum and the metals cognate with it is still an open question. A metal belonging to a very different class was added to the number of solar ingredients by Messrs. Hartley and Ramage in 1897.[18] They convincingly identified two blue rays of gallium with Fraunhofer lines, pointing out that the proportion to iron of the new metal indicated as existing in the reversing stratum was by weight only one to thirty thousand. This accords well with its terrestrial relations. Gallium, discovered by Boisbaudrin in 1875, seems to be widely, but very minutely, diffused throughout the earth’s crust. It occurs also in meteorites. It has an atomic weight of 70, is singularly volatile, and melts almost as readily as butter.

No substance has been more eagerly looked for in the sun than oxygen. But the search was long in vain. Henry Draper’s recognition, in 1877, of _bright_ lines of oxygen in the solar spectrum created a sensation, but proved illusory. J. C. Draper’s _dark_ lines were a still less plausible personation. Eisig ascertained in 1894 that none of the eighty-one emission-lines measured by himself occur in the solar spectrum.[19] Janssen demonstrated, by observations from the summit of Mont Blanc, that the _cool_ oxygen-absorption conspicuous in it is of purely telluric origin;[20] and his conclusion was ratified by Dunér’s application of the motion-displacement test.[21] Oxygen, however, is a substance of most complex, and perhaps unstable molecular structure. No less than six distinct spectra characterise it, two of them produced at low temperatures, and known through their absorptive effects alone; four derived from vacuum tubes, under varying degrees of electrical excitement. Moreover, one of these forms of emission is a series spectrum of the most intricate kind, comprising six different sets of harmonic vibrations, three made up of triple, three of single lines.[22] And here at last a significant coincidence was found. A triplet in the red part of the Fraunhofer spectrum, photographed by Higgs and McClean, was in 1897 clearly identified by Runge and Paschen as a fundamental oxygen group[23] (see Fig. 3). The representation of the element, although certain and authentic, is reduced to a minimum.

FIG. 3.—Oxygen-Triplet in the Solar Spectrum (_Astroph. Journ._ vol.
iv. p. 318).
]

The spectrum of helium, which is analogous to the “series spectrum” of oxygen, makes no show in analysed sunlight, but appears bright above the limb. In the case of this substance, moreover, the usual order of detection was reversed. Its recognition as a chromospheric material preceded by nearly a quarter of a century the expulsion in Professor Ramsay’s laboratory of an identical gas from clevite. But about helium more will be said presently.

So far, then, thirty-nine of the chemical elements are known to be common to the earth and sun, and the remaining forty may very well be so likewise. The absence from among the solar ingredients of any single terrestrial species of matter is unproved, and perhaps unprovable. The Fraunhofer spectrum sums up the combined absorption of a heterogeneous mixture of vapours. But the aggregate is widely different from what would be obtained by simply adding together the separate effects. For it is the outcome, so to speak, of struggle and survival. In a medley of ignited substances, the rays of certain among them predominate, while those of others are effaced. Thus non-metals, as a rule, make a poor figure in the spectral competition with metals, and this is doubtless one reason for their inconspicuousness in the sun. Apart from hydrogen, the properties of which are exceptional, only three metalloids, silicon, carbon, and oxygen, contribute to produce the Fraunhofer lines, and their contributions are feeble and fragmentary. That other similar substances—selenium, sulphur, nitrogen, argon, and the rest—may be there, yet exercise no perceptible absorption, is amply possible.

The metals themselves, too, differ widely as regards conditions of visibility. Some are rich in strong lines, favourably situated for observation. Iron is an example. It emits thousands of rays, widely distributed over the spectrum, although most crowded in its higher sections; and they hold their own vigorously against the adverse influences of dilution. Again, the rare metal cerium is extraordinarily prolific of blue rays. No less than 400 were measured by O. Lohse in 1897 in the comparatively narrow region between λ 4000 and λ 4600.[24] Most of them, however, are quite feeble, and only twenty-nine have yet been identified as Fraunhofer lines. Those of bismuth, on the other hand, being all compound, are too diffuse (as Rowland observes) to be detected in sunlight. And most of the radiations of lithium are so highly refrangible as to fall under the ban of atmospheric exclusion. Their reversal in the solar spectrum can thus only be a matter of inference. That the inference should be negative is suggested by the absence of a strongly characteristic line in the carmine red. It _ought_ to be readily seen, if lithium be a solar constituent. Its visibility should be promoted by the small atomic weight—only seven times that of hydrogen—and low fusing-point of the metal; and the persistence of the carmine beam is actually shown by its emergence in the spectrum of the Bessemer flame. Yet it is unlikely that lithium is, in fact, missing from the sun. The case deserves particular attention.

Heavy substances are obviously at a disadvantage as regards the production of absorptive effects. Their vapours must tend to lie low, like carbonic acid in the earth’s atmosphere. Hence the average lightness of the solar elements is only what we should expect. The mean atomic weight of the thirty-five metals represented in the Fraunhofer spectrum is, in fact, just 72, while that of the non-apparent metals is 159. Atomic weight, however, is only one of many conditions affecting this result. The inclusion of lead-absorption in the scroll, and the exclusion from it of lithium, sufficiently prove that vapour-density is far from being alone concerned.

The detection of carbon in the sun was difficult and noteworthy. Originating with Sir Norman Lockyer in 1878,[25] it was ratified by Trowbridge and Hutchins in 1887,[26] and still more decisively ten years later by Rowland. It was, and could only have been made, photographically. The visible carbon bands are barely discernible in the Fraunhofer spectrum. One, however, in the ultra-violet (beginning at λ 3883) comes out unmistakably on sensitive plates. It is due, according to the best authorities,[27] not to elementary carbon, but to cyanogen—that is, to carbon in its combination with nitrogen. The fact is not easy to explain. No other compound body is known to exist in the sun; and it might have been judged _à priori_ impossible that any could prove capable of resisting the enormous temperature reigning near the photosphere. Sir Norman Lockyer attempted to get rid of the anomaly by locating the absorbing cyanogen in coronal regions, where relative coolness must prevail; but recent observations point rather to its presence as a shallow, deeplying stratum. It is certain, moreover, that the line-spectrum derived from free carbon through the exciting influence of a powerful electric spark has no counterpart in the sun. We are then bound to admit, at least provisionally, that the ultra-violet solar band genuinely indicates absorption by cyanogen. There is a further complication. The green fluting, a few _shreds_ of which were measured by Rowland among the Fraunhofer lines, makes part of the typical hydro-carbon spectrum given primarily by acetylene. The whole subject is indeed thick-set with embarrassing considerations; they need careful sifting out. Carbon molecules are remarkably sensitive in their modes of vibration, four of which have been separately distinguished.[28] The conditions, however, prescribing the replacement of one by another are still in large measure obscure. Temperature is concerned, but it is not alone concerned; density, admixture with foreign substances, perhaps variations of electrical state, come into play. Yet the broad certainty has been gained that carbon, in one, if not in several of its many forms, exists in the photospheric neighbourhood. And this has an important bearing, not only upon theories of the solar constitution, but also upon questions of great interest regarding solar relationships with the stars.

Professor Rowland disbelieves in any fundamental difference between solar and terrestrial chemistry. The earth, heated to the solar pitch, would give, he affirms, a spectrum virtually identical with that of the sun. Yet we cannot well ignore evidences, apparently valid, of some real diversity. Even if all our “elements,” without exception, are found in the sun, they are unlikely to occur in the same proportions there as here; _quantitative_, if not _qualitative_ dissimilarity must be recognised. Thus certain metals, so scarce that their ores rank as mineralogical curiosities, produce marked effects of absorption in the sun. Zirconium, yttrium, cerium, lanthanum may be instanced. Titanium and vanadium are multitudinously represented in the solar spectrum. Hasselberg ascertained for the former substance in 1896 no less than 562 coincidences with Fraunhofer lines out of a total of 718 photographed by him from the metal.[29] On the exclusion of the feeblest rays on his plates as being of quite uncertain origin, the percentage of agreement rose to 88 per cent. The Swedish spectroscopist might well claim that “the presence of titanium in the solar atmosphere is confirmed, with even superfluous evidence, by these investigations.”

Vanadium—first registered as a solar constituent by Sir Norman Lockyer—might be called the satellite of titanium. Where one is, the other is sure to be not far off. Hasselberg’s recent discovery of vanadium in the Scandinavian mineral rutile[30]—a form of titanic acid—accentuates the relationship. Both occur, too, although very scantily, in lead and iron ores, and just traceably in trap and basalt. This close association may be accounted for by inherent resemblance. The atomic weight of titanium is 48, that of vanadium 51. Both are eminently infusible. They share the unusual peculiarity of exhibiting a strong high-temperature affinity for nitrogen.[31] They exemplify, moreover, a transition-stage from metals to metalloids, titanium approximating to silicon, vanadium to phosphorus. Both, it may be added, have been detected in meteorites. A large proportion of the numerous rays emitted by vanadium are reversed in the sun, but somewhat faintly reversed, except in cases of special disturbance, and with these we are not at present concerned. It is worth remarking that titanium and vanadium, notwithstanding their near kinship, physical and chemical, show no coincident spectral lines. In these twin elements, if in any, a common material substratum might be looked for. There is not the slightest sign, however, that it exists.

The new metal germanium is an obvious solar constituent; yet it only _lurks_ in one uncommon terrestrial product. Winkler recognised it in the mineral argyrodite in 1886. And the sun appears to be much richer than the earth in Sir William Crookes’s “meta-elements.”[32] The number of these is almost indefinite; their individualisation, resting upon the dubious principle, “one band, one element,” is often imperfect or misleading; and many of the evasive substances, ranked for a time as separate entities, have failed to make good their footing, and relapsed into the condition of “sub-aggregates of atoms.” The chemistry of “rare earths” has of late assumed a kind of departmental importance. It began in 1794 with the extraction by the Finnish chemist Gadolin of “yttria” from a jet-black material picked up at Ytterby, near Stockholm. “Ceria,” detected in 1803 in the “heavy stone” from Bastnäs, was named after the first asteroid; “lanthana,” obscurely associated with it, came to light in 1839; “didymia,” “terbia,” “erbia,” successively followed.[33] The opening, in 1878, of a fresh and fairly abundant source of supply in the American mineral samarskite started vigorous inquiries into the nature of these remarkable bodies; and Cleve enumerated in 1895 nine fully characterised metallic bases, most of them emitting, under electrical excitation, a brilliant array of spectral beams. The nine “rare” metals are scandium, yttrium, lanthanum, cerium, erbium, praseodymium, samarium, gadolinium, and ytterbium. The first four absorb strongly in the sun, where the presence of erbium and of neodymium, a constituent of the original didymium, is also evident. Twenty-two additional meta-elements swell Sir William Crookes’s “suspense account,” but only a minority are at all likely to obtain ultimate recognition as substantive forms of matter. Perhaps the surest test of their quality will be found in the appearance among the Fraunhofer lines of their characteristic emissions. Those, at any rate, of terbium, holmium, and thulium should be carefully looked for.

The light of glowing metallic vapours tends, as we have seen, to suppress or efface the rays of non-metals. Professor Trowbridge made some experiments in 1896 with a view to determining the conditions of obliteration. Photographing on a single plate the spectra of pure carbon and of an electric arc between carbons containing 28 per cent of iron, he found that the iron ingredient sufficed very nearly to wipe out the carbon bands in the arc.[34] “This proportion, therefore, of iron,” he remarked,[35] “in the atmosphere of the sun, were there no other vapours of metals present, would be sufficient to prevent our seeing the full spectrum of carbon.” An interesting illustration was thus afforded of a fundamental principle in solar interpretations. The principle, indeed, has scarcely yet begun to be applied. Hitherto the absorptive effects of each of the forty substances vaporised above the photosphere have been considered apart. But they are not independently produced. They are often profoundly modified by extraneous action. A systematic investigation of the various modes in which it comes into play is desirable, although likely to prove arduous. “There is at the present time,” Dr. Ames wrote in 1895,[36] “no more fruitful field open to research than that of the study of the influence of the presence of one substance upon the spectrum of another.” And Mr. Percival Lewis’s recent treatment of the subject has had the preliminary result of showing that “very small traces of an impurity in a gas may cause considerable changes in its spectrum, whether this impurity be chemically active or not.”[37] The changes, too, are in many ways perplexing. They are governed by no traceable rules, depending apparently, in each case, upon intimate, and to us unknown, molecular relations with electricity.[38] The explanation of their anomalies is evidently needed for the satisfactory future progress of solar chemistry.