Chapter V: Hydrogen, Helium, and Coronium

Three tenuous gases—hydrogen, helium, and coronium—are of essential importance in solar physics. The first plays also a leading part in terrestrial and vital economy. The second exists on the earth merely as a chemical curiosity. The third must for the present be classed as an exclusively solar product.

Solar hydrogen was discovered by Ångström in 1862. He recognised it by the identity of its three least refrangible rays with the Fraunhofer lines C, F, and G[67] (now designated Ηα, Hβ, and Hγ), to which, in 1865, he associated the indigo line _h_ (Ηδ). A fifth line (Hε), photographed by H. W. Vogel in 1879, is situated quite close to the calcium H—so close that, like Teucer behind the shield of Ajax, it lies concealed, in the sun, under covert of its neighbour’s broad shadow, if indeed it be present in the Fraunhofer spectrum at all; for it is so effectually hidden that the point remains uncertain. Shortly afterwards, Sir William Huggins’s spectrographic investigations of Sirian stars gave the key to the true character of the hydrogen emissions. Nine ultra-violet lines came out on his plates, and their rhythmical arrangement at intervals continually lessening upward left no doubt of their forming a connected series. That this included the visible lines was manifest at sight. Its law was stated by Balmer in 1885.[68] The relations expressed by his formula are not those of wave-_lengths_, but of their reciprocals, wave-_frequencies_. These quantities obviously bear to each other an inverted proportion. Deep crimson light, for instance, consists of undulations about twice as long as those of violet light; only half as many of them, accordingly, enter the eye in a given time. A doubled length corresponds to a halved frequency, a tripled length to a frequency of one-third, and so on. Now oscillation-frequencies are, for several reasons, more important natural constants than wave-lengths; hence until they were made the basis of investigation, no real progress was effected in the detection of spectral series.

Balmer’s law has the following form: N = N_{0} − (4N_{0})/(_m_^2), where N is the wave-number ((1)/(λ)), N_{0} is a constant to be determined by trial, and _m_ is any integer greater than 2. By assigning to N_{0} the empirical value 27418·75, the places in the spectrum of each individual ray emitted by hydrogen may be calculated with approximate accuracy. That of C (Hα) corresponds to _m_ = 3, and the series has been photographed up to _m_ = 34, its constituent lines growing fainter and more crowded as the scale is ascended. They approach, in fact, with the increase of _m_, indefinitely near to a definite limit, marked by the constant N_{0} minus 0 (the second term having disappeared). This limit, known as the “convergence frequency,” is a distinctive feature of spectral series.

Many have contributed to their elucidation. Johnstone Stoney,[69] Alexander Herschel,[70] Hartley,[71] and Cornu[72] prepared the ground, and the subject was treated, in its larger bearings, and with more definite results, by Liveing and Dewar,[73] Schuster,[74] Rydberg of Lund,[75] Runge and Paschen[76] of Hannover, Kayser[77] of Bonn, and Ames[78] of Baltimore. Their labours have been unexpectedly successful in educing partial order out of all but total emissive confusion. Harmonic series of identical type were marshalled from promiscuous throngs of rays, and their association into sets of three, or even into double sets of six, simultaneously given forth by a single element, proves the extraordinary complexity of the molecular systems through the movements of which they originate. That some of these movements are of an orbital nature is strongly indicated, and they not improbably show perturbative effects analogous to those manifested in lunar and planetary revolutions. “The final impression,” M. Balmer writes,[79] “which our mind involuntarily receives in contemplating these fundamental relations is that of a wonderful mechanism of nature, the functions of which are performed with never-failing certainty, though the mind can follow them only with difficulty, and with a humiliating sense of the incompleteness of its perception.”

Until 1897 the spectrum of hydrogen was thought to be of unique simplicity. It apparently consisted of one individual series resembling that formed by a musical note and its overtones. No outstanding lines interrupted the perfect regularity of the progression. All this, however, was changed by Professor Pickering’s discovery, in a few peculiar stars, of a second hydrogen series.[80] It is associated with the first in such a manner as to indicate that both are subordinate to a principal series, the three together forming a triple group on the normal pattern. Of the principal series, one member has been probably identified as a blue band in certain “bright-line” stars,[81] the rest being placed inaccessibly high up in the ultra-violet. They would be cut off by atmospheric absorption. None of the new hydrogen rays occur in the sun, and none have, so far, been rendered visible in the laboratory, possibly because the temperatures available are inadequate for their production. This indeed is a matter of conjecture; what is certain is that hydrogen affords the only known example of a spectral series capable of isolation from its fellows. Here evidently we have a clue to some specialty of intimate structure, the guidance of which may lead to surprising disclosures.

Hydrogen has other singularities. In some respects it is solitary among the elements. The “periodic law,” by which their properties are connected with their atomic weights, does not apply to it. Chemically and electrically it behaves as a metal; reduced to the liquid state, however, it definitely ranges itself with non-metals. Its condensation is effected with the utmost difficulty, physical and mechanical agencies being only just competent to vanquish the elasticity of this lightest of terrestrial substances. But what force can barely compel, affinity readily obtains. United to oxygen under the form of water, it can exist as a liquid up to a temperature of 100° C., and it is of all gases the most readily “occluded.” Imprisoned thus in metallic masses, it remains inert for unlimited periods, but recovers freedom and activity by heat. Meteoric irons bring to the earth no inconsiderable supply of occluded hydrogen, and palladium can take it up to the extent of six hundred times its own volume. In this quasi-combination it is, by a curious anomaly, six times denser than when liquefied by sheer cold.[82]

The volatility of hydrogen perhaps transcends the earth’s power of control. By a necessary consequence of the kinetic theory, adverted to by Dr. Johnstone Stoney in 1870,[83] light gases in a free state can be permanently retained only by massive globes. For atmospheric particles no sooner attain a speed just overbalancing the holding power of gravity than they irrevocably fly off into space, and the process being continued unintermittently, eventuates in the total dissipation of the envelope they once constituted. It is, however, a matter of some delicacy to discriminate between the gases that may escape from any individual planet and those that must remain. According to a recent calculation,[84] the earth could now maintain a hydrogen atmosphere virtually without waste; but in former ages, when the agility of the gaseous molecules was quickened by heat, the strength of its grasp upon them must have been insufficient for their lasting retention. This was nevertheless effected by their reduction to the liquid state in the form of water. The presence of an excess of oxygen hence saved terrestrial hydrogen.

Only the four lowest members of the hydrogen series show dark in the sun.[85] The absence of the higher rays is enigmatical. All are ablaze in the chromosphere; but the chromospheric gases emit sensibly as much light as they stop. In the reversing layer it would then seem that hydrogen glows so imperfectly as to emit vibrations of no more than four or five qualities, the upper “notes” being somehow quenched. It might be supposed that the temperature there is too low for their production, were it not that they have been photographed from vacuum tubes held, on good grounds, to be cool relatively to the electric arc. The true explanation is probably to be found in the heterogeneous composition of the stratum in question. Intermixed particles of different kinds of matter mutually check each other’s oscillations, and those of shortest periods are the most susceptible to this adverse influence. Its nature and the laws of its action remain obscure, but much may be learned about them by careful experimental inquiry.

A similar anomaly is more markedly visible in the case of helium. This gas exists near the sun in scarcely less profusion than hydrogen, yet the Fraunhofer spectrum includes no trace of its action. Absorptive nullity is not a quality inherent in the substance, as we shall see presently; hence it probably depends, like the partial inertness of hydrogen, upon conditions present in the reversing layer.

Until March 1895 helium was known only as a chromospheric element. A bright yellow ray at λ 5876, distinguished as “D_{3},” because it forms a trio with the sodium pair D_{1} and D_{2}, was noticed in the prominences uncovered during the eclipse of 18th August 1868, and can always be observed spectroscopically at the edge of the sun. But the substance emitting the yellow ray lay outside the range of our acquaintanceship, and seemed unlikely to be brought within it. That contingency, nevertheless, came to pass. In the course of a search for compounds of argon, Professor Ramsay, at the suggestion of Professor Miers, fortunately examined the reputed nitrogen occluded by the Scandinavian mineral “clevite.”[86] This velvety-black stone, remarked as peculiar by Nordenskiöld and analysed by Cleve, is a kind of pitch-blende, composed of uranate of lead mixed with rare earths. The gas evolved from it at University College gave a brilliant spectrum, in which the prominence-line D_{3} shone conspicuous. Helium was indeed captured! A beautiful confirmation of the identity was soon afterwards afforded. The golden line seen in the laboratory was perceived by Runge to have a faint close companion, and he declared that, unless the solar D_{3} were also double, clevite-gas should be regarded as different from helium.[87] The challenge was taken up on both sides of the Atlantic. Professor Hale on 20th June, and Sir William Huggins independently on 10th July, succeeded in resolving the prominence-ray into a delicate, unequal pair, and our possession of helium as a truly indigenous element was rendered incontrovertible.

FIG. 5.—Diagram of the Helium Spectrum.
]

Meantime sundry other leading chromospheric rays—four especially, coloured deep red, green, blue-green, and intense violet[88]—had been recognised in the complex spectrum of clevite gas.[89] The task, however, of reducing its tangled rays to harmonic order seemed desperate until it was performed. Without exception, they ranged themselves, at the bidding of MM. Runge and Paschen, into six related series (see Fig. 5). These form two sets, each consisting of a subordinate pair drawing together towards a common limit in the ultra-violet, with a principal series “leaping over the other two in large bounds,” and ending in the more refrangible part of the spectrum.[90]

Not only did Runge and Paschen’s formula (which may be regarded as a modification of Balmer’s law for hydrogen) include all the perceptible emissions of helium, but it intimated the presence of others beyond the reach of ordinary observation. Each of the principal series, it was inferred, should own a “leader line” far down among the heat rays, and with the aid of the bolometer the prediction was strictly verified. Thus “the actual spectra” (as Mr. Maunder remarked) “corresponded to the theoretical, and were complete from their rise far in the obscure regions of the infra-red till they died away in the darkness which lies on the other side of the visible spectrum.”

Their number, however, suggested a twofold origin, since there was then no precedent for assigning more than three series to a single substance. Clevite gas was accordingly regarded as a mixture of two solar elements, distinguished as “helium” and “parhelium,” the rays of the former, like D_{3}, being all double, those of the latter single. Each set of three series was, in fact, “analogous to the complete spectrum of a distinct element.” Yet parhelium has failed to make good its footing in either cosmical or terrestrial chemistry. Attempts to isolate it have entirely failed, and the spectroscopic argument for its existence collapsed with the discovery that oxygen, no less than clevite gas, claims six series, which are certainly inseparable, and represent in combination the vibrations of perfectly similar, highly intricate molecular systems. “Parhelium” may then safely be treated as fictitious. Clevite gas, or mineral helium, is the identical undiluted material of prominences. The hypothetical companion-stuff exists neither in the sun nor upon the earth.

The qualities of helium are most unusual. Like argon, it is monatomic; its ultimate chemical units are the same as its ultimate mass-units. This inference is based upon the heat-relations of the substance. Its vapour-density is hence only half its atomic weight; for the molecule of helium possesses nearly double the mass of the hydrogen molecule; and it is, by hypothesis, indivisible, while that of hydrogen includes two combining atoms. But helium has no “atoms,” or rather its molecules _are_ its atoms. Its chemical equivalent is accordingly 3·96 on the hydrogen scale. A value indeed rather nominal than real, since helium is devoid of sensible affinities. It enters into no combinations. It again resembles argon in being a “rogue” element. To both equally, one of the ordinary properties of matter is wanting. They form with three other inert gases a class apart as “non-valent” substances.[91] In choosing its mineral cloisters, helium showed, nevertheless, some original preferences. The heavy metal uranium has a special attraction for it, and it is constantly associated with rare earths. Once released, however, it can scarcely be re-incarcerated. Mr. Tilden’s experiments led him to conclude that helium-yielding rocks must have been primitively charged under a pressure of several hundred atmospheres.[92] The earth may in those early days have possessed a vast helium-envelope, since dissipated in space. Opinions differ on the subject,[93] and data for precise calculations are wanting. But the probability is strong that the helium now sparsely lurking on our globe is a mere remnant of a far ampler store, which terrestrial potencies, whether gravitational or chemical, were incompetent to hold.

Helium scarcely retards the passage of light. Its refractive index, which is the smallest known, is expressed by the fraction 0·146, those of air and hydrogen being respectively 1·0 and 0·5. Still more unexpected than its low refractivity, is its high conductive power for electricity. Professor Ramsay ascertained that the “sparking distance” through helium at atmospheric pressure is nearly 300 millimetres, while the same current is stopped in hydrogen by a gap of 40, in oxygen by a gap of just 24 millimetres.[94] Another surprising property of this gas is its abnormal faculty of diffusion. It has a rate of self-dispersion ten times that of hydrogen, or fifteen times what, by Graham’s law, it ought to be. On the other hand, it has the lowest solubility on record;[95] water absorbs it in evanescent quantities. This led to the anticipation, amply justified by experience, that helium would prove to be one of the most obstinately gaseous bodies in existence. It has not indeed yet (October 1902) surrendered to the compulsion brought to bear by Professor Dewar in his memorable researches at low temperatures. Still, the cooling efficacy of liquid hydrogen evaporating under exhaustion may be expected finally to overcome its all but invincible recalcitrance, and the “salamander gas” of the chromosphere will assume the guise of a frigid fluid boiling five or six degrees above absolute zero.

The reversing layer—properly so-called—emits apparently no helium rays. A reason for their absence has been already suggested, and is tolerably obvious. They are extremely sensitive to damping influences. Foreign admixtures readily occasion their suppression. Thus 10 per cent of helium just shows spectroscopically in hydrogen, and that only if the pressure in the tube is very low; while one part of hydrogen in 100,000 of helium glows manifestly when the current is made to pass.[96] Nitrogen has a similar adverse effect upon helium-radiation, which would, however, gain relatively in strength with diminution of pressure in ascending through the reversing-layer into the chromosphere.

Of the chief coronal ingredient no terrestrial trace has yet been found. A bright green ray observed during total eclipses is its only assured badge, for eight or nine other more refrangible associated rays may quite possibly emanate from different substances. As the leading gaseous constituent of a structure on the borderland of nothingness, coronium must be an unimaginably subtle form of matter. It exists in prodigious volumes near the sun, rising to heights altogether inaccessible to hydrogen or helium, yet under conditions differing from those of an ordinary atmosphere. Successive coronal strata are not mutually superincumbent. There is no sign that their density increases downwards. The characteristic green line is no less fine and sharp given out by the inner than by the outer corona. No reversal of it has ever been detected. There is no corresponding Fraunhofer line. Thus the substance most plentifully present about the sun is, by a strange anomaly, absolutely passive as regards its light. This is most probably a result, not of any specific incapacity, but of the peculiar state in which it subsists. The real qualities of coronium, however, and its entire spectrum can only be ascertained by laboratory investigations. That these will ever become practicable it would be rash to assume, but it is permissible to hope. There seems, at any rate, no valid reason why coronium should not be added to the number of unearthed or frozen-out recondite gases. The former possession by our planet of a coronium-atmosphere may be plausibly surmised. But it most likely vanished still earlier than that of helium. Had its material been endowed with chemical affinities, some compound or compounds should have preserved it more or less abundantly. It would have been detained, as hydrogen was detained in water, and kept available for our late acquaintanceship. Since no compound of the kind appears to exist, coronium presumably resembles helium in being “non-valent.”