Popular Science Monthly/Volume 48/January 1896/Helium, its Identification and Properties
By Prof. C. A. YOUNG.
THE famous "D3," so called because it is very near the D lines of sodium, is a bright yellow line in the spectrum of the solar chromosphere, in which it is more conspicuous than anything except the C and F lines of hydrogen. Unlike them, however, it has no corresponding dark line in the ordinary solar spectrum, a rather perplexing fact which has caused much discussion, and has not even yet found an explanation in which all authorities agree.
It was discovered in 1868, when the spectroscope was for the first time directed upon a solar eclipse. Most of the observers supposed it to be the sodium line, but Janssen noted its non-coincidence; and very soon, when Lockyer and Frankland took up the study of the chromosphere spectrum, they found that the line could not be ascribed to hydrogen or to any other known terrestrial element. As a matter of convenient reference Frankland proposed for the unknown substance the provisional name of "helium" (from the Greek "helios," the sun), and this ultimately, though rather slowly, gained universal acceptance.
Within a year, two other lines (λ 7,065 and λ 4,472) were discovered in the chromosphere spectrum by Rayet and Respighi respectively, which like D3 are always present in the prominences, but have no corresponding dark lines. It was of course early suggested, but without proof, that these lines might also be due to helium. Since then some eight or ten other lines have been found, frequently, but not always, presenting themselves in the chromosphere spectrum, and, like the first three, also without dark analogues. Moreover, still more recently, D3 and its congeners have been detected in the stellar spectra—dark. in the spectra of the "Orion stars," bright in the spectra of certain variables and of the so-called Wolf-Rayet stars; and both bright and dark in β Lyræ and the "new star" of Auriga which appeared in 1892.
Naturally there has been much earnest searching after the hypothetical element, but until very recently wholly without success; though it should be mentioned that in 1881, Palmieri, the director of the earthquake observatory upon Vesuvius, announced that he had found D3 in the spectrum of one of the lava minerals with which he was dealing. But he did not follow up the announcement with any evidence, nor has it ever received any confirmation, and from what we now know as to the conditions necessary to bring out the helium spectrum, there is every reason to suppose that he was mistaken.
The matter remained a mystery until April, 1895, when Dr. Ramsay, who was Lord Rayleigh's chemical collaborator in the discovery of argon, in examining the gas liberated by heating a specimen of Norwegian clèveite, found in its spectrum the D3 line, conspicuous and indubitable. The mineral was obtained from Dr. Hillebrand, one of our American chemists, who had previously studied it, and ascertained that it could be made to give off a gas which he identified with nitrogen. It really was nitrogen in part, but Ramsay suspected that he should also find argon, as he did and helium besides, which was unexpected.
Clèveite is a species of uraninite or pitchblende, and it soon appeared that helium could be obtained from nearly all the uranium minerals, and from many others; from many, mingled with argon; from others, nearly pure. In fact, it turns out to be very widely distributed, though only in extremely small quantities, and generally "occluded," or else in combination—seldom, if ever, free. It has been detected in meteoric iron, in the waters of certain mineral springs in the Black Forest and Pyrenees, and Kayser even reports traces of it in the atmosphere at Bonn.
It is generally obtained by heating the substance that contains it in a close vessel connected with an air pump of some kind by which the liberated gases are drawn off and collected. They are then laboriously treated to remove as far as possible all the foreign elements (nitrogen, etc.), since the presence of no more than five or ten per cent of other gases prevents the new elements from giving any spectroscopic evidence of their presence; they are too shy and modest to obtrude themselves. In many cases, as has been said, argon and helium come off together, and certain lines in their spectrum are nearly coincident, so that for a time there was supposed to be some close bond of connection between them. The latest observations, however, make it certain that this is not so: as Mr, Lockyer puts it, "argon is of the earth, earthy, but helium is distinctly celestial."
Its spectrum has been thoroughly studied by Crookes, Lockyer, and Runge, who agree as to all its leading characteristics.
Runge, whose work is the most complete and authoritative, finds that its lines have a remarkably regular arrangement, falling into two distinct "sets," each set consisting of a principal series and two subordinate ones, the lines in each series corresponding very accurately to a formula quite similar to that discovered by Balmer as governing the hydrogen spectrum.
In the whole spectrum he finds (by photography mainly, though two of the most important were detected by the bolometer) sixty-seven lines, twenty of which only are in the visible part of the spectrum. Of the sixty-seven, twenty-nine belong to the first "set" and thirty-eight to the second. Of the twenty "visual" lines, thirteen have been observed in the spectrum of the chromosphere; the missing lines all belong to the second subordinate series of the first "set," and are so faint in the artificial spectrum of the gas that their failure to be found in the chromosphere needs no explanation.
The fact that the lines thus divide into two mathematically independent "sets" has led Runge to believe that the helium obtained from the minerals is really a mixture of two distinct gases, and he has found it possible to partially separate the two by a process of diffusion. The true helium, the element that gives D3 and the other lines that are always present in the chromosphere spectrum, he considers to be the denser of the two; the spectrum of the other contains most of the lines that appear only occasionally in prominences. The lighter component has as yet received no name. Lockyer calls it simply X.
The lines of the series to which D3 belongs are all double, having a very faint companion on the lower (i. e., red-ward) side, extremely close to the principal line. When Runge announced this discovery early in June it at first produced something like consternation among spectroscopists, for at that time there still remained more or less doubt as to the validity of Ramsay's identification, and the solar D3 had never been observed to have such a companion. Very soon, however, Hale, Huggins, Lockyer, Reed of Princeton, and other observers who had sufficiently powerful instruments, detected the little attendant of D3 in the spectrum of prominences, so that the momentary distrust was replaced by absolute confidence.
As to the physical and chemical properties of the new gas, our knowledge is still limited and our conclusions are embarrassed by the uncertainty whether we are dealing with a single element or a mixture—whether Dr. Ramsay has introduced to the world one infant or a pair of twins.
The gas liberated from clèveite, and purified as far as possible, shows a density just a little more than double that of hydrogen, and is therefore much lighter than any other known gas except hydrogen itself. If it is a mixture, the lighter gas must have a density less than two, and may even prove to be lighter than hydrogen; while the true D3 helium may have a density anywhere between two and four, depending on the proportions of the mixture and the density of the lighter compound. In any case both the true helium and X are lighter than anything else but hydrogen.
It would be very fine, we may remark in passing, if the lighter component could have been identified with "coronium," but this seems impossible since the characteristic 1,474 line (λ 5,316) does not appear at all in the spectrum of terrestrial "helium" derived from any source whatever.
Ramsay's acoustic experiments tend to show that helium, like argon, is monatomic, but can hardly be considered conclusive. If he is right, the atomic weight of helium regarded as a single element would be not far from four; but thus far all attempts to make it enter into chemical combination have failed, though it seems rather probable that in the uraninite minerals it is held by stronger bonds than those of mere occlusion.
Olszewski has tried his best to liquefy the gas, but thus far unsuccessfully; the methods that have conquered every other gas, hydrogen itself included, have failed with helium a circumstance very remarkable, since generally a.denser gas liquefies more easily than a lighter one, and hitherto hydrogen has stood pre-eminent in its refractoriness. The fact that the gas is probably a mixture may explain his failure: air is more difficult to liquefy than either oxygen or nitrogen.
Probably the question has suggested itself to every reader how it happens that helium, so conspicuous in the atmosphere of the sun and many stars, should be so nearly absent from our own atmosphere and so scantily present in any form upon the earth. The answer seems to depend upon two facts—the chemical inertness of the substance and its low density.
According to Johnstone Stoney's deductions from the accepted theory of gases, no free gas of low density can remain permanently upon a heavenly body of small mass and habitable temperature, but the molecules will fly off into space. A particle leaving the earth with a velocity of about seven miles a second would never return to it, this "limiting velocity" depending upon the mass of the earth and its diameter. Now, according to the dynamic theory of gases, the molecules of our atmosphere are flying swiftly about with velocities (at ordinary temperatures) of from fifteen hundred to fifteen thousand feet per second; the heavier molecules, like those of oxygen and nitrogen, move comparatively slowly, but if any lighter gas, like free hydrogen or helium, is present, its molecules take up velocities several times more swift, and any that may happen to be near the upper limits of the atmosphere would be likely to be thrown off into space. In the case of the moon even the oxygen and the nitrogen would go, since she is so small that a velocity not much exceeding a mile a second would carry them off. If this is correct, it is easy to see why we now have no appreciable quantity of free hydrogen or other light gas in our atmosphere.
But while we have no atmospheric hydrogen to speak of, hydrogen in combination is extremely abundant; one eighth part by weight of all the water in the sea is hydrogen; and hydrogen combines freely with many other elements besides oxygen, so that we continually liberate it in all sorts of chemical decompositions. Helium, on the other hand, enters into combination most sparingly, is therefore scarce, and even when present is, as we have said before, not easy to detect.