The alternative mode of escape from the perplexity consists in assuming that the vapour in motion is rendered luminous under conditions which reduce its spectrum to a few rays, the unaffected lines being derived from a totally distinct mass of the same substance shining with its ordinary emissions.[662] Thus, calcium can be rendered virtually monochromatic by attenuation, and analogous cases are not rare.
Sir Norman Lockyer only asks us to believe that effects which follow certain causes on the earth are carried a stage further in the sun, where the same causes must be vastly intensified. We find that the bodies we call "compound" split asunder at fixed degrees of heat within the range of our resources. Why should we hesitate to admit that the bodies we call "simple" do likewise at degrees of heat without the range of our resources? The term "element" simply expresses terrestrial incapability of reduction. That, in celestial laboratories, the means and their effect here absent should be present, would be an inference challenging, in itself, no expression of incredulity.
There are indeed theoretical objections to it which, though probably not insuperable, are unquestionably grave. Our seventy chemical "elements," for instance, are placed by the law of specific heats on a separate footing from their known compounds. We are not, it is true, compelled by it to believe their atoms to be really and absolutely such—to contain, that is, the "irreducible minimum" of material substance; but we do certainly gather from it that they are composed on a different principle from the salts and oxides made and unmade at pleasure by chemists. Then the multiplication of the species of matter with which Lockyer's results menace us, is at first sight startling. They may lead, we are told, to eventual unification, but the prospect appears remote. Their only obvious outcome is the disruption into several constituents of each terrestrial "element." The components of iron alone should be counted by the dozen. And there are other metals, such as cerium, which, giving a still more complex spectrum, would doubtless be still more numerously resolved. Sir Norman Lockyer interprets the observed phenomena as indicating the successive combinations, in varying proportions, of a very few original ingredients;[663] but no definite sign of their existence is perceptible; "protyle" seems likely long to evade recognition; and the only intelligible underlying principle for the reasonings employed—that of "one line, one element"—implies a throng beyond counting of formative material units.
Thus, added complexity is substituted for that fundamental unity of matter which has long formed the dream of speculators. And it is extremely remarkable that Sir William Crookes, working along totally different lines, has been led to analogous conclusions. To take only one example. As the outcome of extremely delicate operations of sifting and testing carried on for years, he finds that the metal yttrium splits up into five, if not eight constituents.[664] Evidently, old notions are doomed, nor are any preconceived ones likely to take their place. It would seem, on the contrary, as if their complete reconstruction were at hand. Subversive facts are steadily accumulating; the revolutionary ideas springing from them tend, if we interpret them aright, towards the substitution of electrical for chemical theories of matter. Dissociation by the brute force of heat is already nearly superseded, in the thoughts of physicists, by the more delicate process of "ionisation." Precisely what this implies and involves we do not know; but the symptoms of its occurrence are probably altogether different from those gathered by Sir Norman Lockyer from the collation of celestial spectra.
A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate founder, so to speak, of solar spectroscopy. His great map of the "normal" solar spectrum[665] was published in 1868, two years before he died. Robert Thalèn was his coadjutor in its execution, and the immense labour which it cost was amply repaid by its eminent and lasting usefulness. For more than a score of years it held its ground as the universal standard of reference in all spectroscopic inquiries within the range of the visible emanations. Those that are invisible by reason of the quickness of their vibrations were mapped by Dr. Henry Draper, of New York, in 1873, and with superior accuracy by M. Cornu in 1881. The infra-red part of the spectrum, investigated by Langley, Abney, and Knut Ångström, reaches perhaps no definite end. The radiations oscillating too slowly to affect the eye as light may pass by insensible gradations into the long Hertzian waves of electricity.[666]
Professor Rowland's photographic map of the solar spectrum, published in 1886, and in a second enlarged edition in 1889, opened fresh possibilities for its study, from far down in the red to high up in the ultra-violet, and the accompanying scale of absolute wave-lengths[667] has been, with trifling modifications, universally adopted. His new table of standard solar lines was published in 1893.[668] Through his work, indeed, knowledge of the solar spectrum so far outstripped knowledge of terrestrial spectra, that the recognition of their common constituents was hampered by intolerable uncertainties. Thousands of the solar lines charted with minute precision remained unidentified for want of a corresponding precision in the registration of metallic lines. Rowland himself, however, undertook to provide a remedy. Aided by Lewis E. Jewell, he redetermined, at the Johns Hopkins University, the wave-lengths of about 16,000 solar lines,[669] photographing for comparison with them the spectra of all the known chemical elements except gallium, of which he could procure no specimen. The labour of collation was well advanced when he died at the age of fifty-two, April 16, 1901. Investigations of metallic arc-spectra have also been carried out with signal success by Hasselberg,[670] Kayser and Runge, O. Lohse,[671] and others.
Another condition sine quâ non of progress in this department is the separation of true solar lines from those produced by absorption in our own atmosphere. And here little remains to be done. Thollon's great Atlas[672] was designed for this purpose of discrimination. Each of its thirty-three maps exhibits in quadruplicate a subdivision of the solar spectrum under varied conditions of weather and zenith-distance. Telluric effects are thus made easily legible, and they account wholly for 866, partly for 246, out of a total of 3,200 lines. But the death of the artist, April 8, 1887, unfortunately interrupted the half-finished task of the last seven years of his life. A most satisfactory record, meanwhile, of selective atmospheric action has been supplied by the experiments and determinations of Janssen, Cornu and Egoroff, by Dr. Becker's drawings,[673] and Mr. McClean's photographs of the analysed light of the sun at high, low, and medium altitudes; and the autographic pictures obtained by Mr. George Higgs, of Liverpool, of certain rhythmical groups in the red, emerging with surprising strength near sunset, excite general and well-deserved admiration.[674] The main interest, however, of all these documents resides in the information afforded by them regarding the chemistry of the sun.
The discovery that hydrogen exists in the atmosphere of the sun was made by Ångström in 1862. His list of solar elements published in that year,[675] the result of an investigation separate from, though conducted on the same principle as Kirchhoff's, included the substance which we now know to be predominant among them. Dr. Plücker of Bonn had identified in 1859 the Fraunhofer line F with the green ray of hydrogen, but drew no inference from his observation. The agreement was verified by Ångström; two further coincidences were established; and in 1866 a fourth hydrogen line in the extreme violet (named h) was detected in the solar spectrum. With Thalèn, he besides added manganese, titanium, and cobalt to the constituents of the sun enumerated by Kirchhoff, and raised the number of identical rays in the solar and terrestrial spectra of iron to no less than 460.[676]
Thus, when Sir Norman Lockyer entered on that branch of inquiry in 1872, fourteen substances were recognised as common to the earth and sun. Early in 1878 he was able to increase the list provisionally to thirty-three,[677] all except hydrogen metals. This rapid success was due to his adoption of the test of length in lieu of that of strength in the comparison of lines. He measured their relative significance, in other words, rather by their persistence through a wide range of temperature, than by their brilliancy at any one temperature. The distinction was easily drawn. Photographs of the electric arc, in which any given metal had been volatilised, showed some of the rays emitted by it stretching across the axis of the light to a considerable distance on either side, while many others clung more or less closely to its central hottest core. The former "long lines," regarded as certainly representative, were those primarily sought in the solar spectrum; while the attendant "short lines," often, in point of fact, due to foreign admixtures, were set aside as likely to be misleading.[678] The criterion is a valuable one, and its employment has greatly helped to quicken the progress of solar chemistry.
Carbon was the first non-metallic element discovered in the sun. Messrs. Trowbridge and Hutchins of Harvard College concluded in 1887,[679] on the ground of certain spectral coincidences, that this protean substance is vaporised in the solar atmosphere at a temperature approximately that of the voltaic arc. Partial evidence to the same effect had earlier been alleged by Lockyer, as well as by Liveing and Dewar; and the case was rendered tolerably complete by photographs taken by Kayser and Runge in 1889.[680] It was by Professor Rowland shown to be irresistible. Two hundred carbon-lines were, through his comparisons, sifted out from sunlight, and it contains others significant of the presence of silicon—a related substance, and one as important to rock-building on the earth, as carbon is to the maintenance of life. The general result of Rowland's labours was the establishment among solar materials, not only of these two out of the fourteen metalloids, or non-metallic substances, but of thirty-three metals, including silver and tin. Gold, mercury, bismuth, antimony, and arsenic were discarded from the catalogue; platinum and uranium, with six other metals, remained doubtful; while iron was recorded as crowding the spectrum with over two thousand obscure rays.[681] Gallium-absorption was detected in it by Hartley and Ramage in 1889.[682]