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extended continuous spectrum, which contains only two characteristic lines, namely, one line, K. a, in the outermost red approaching the ultra-red rays, exactly coinciding in position with the dark line A of the solar spectrum ; and a second line, K. ß, situated far in the violet rays towards the other end of the spectrum, and also corresponding with a particular line observed by Fraunhofer. These lines, being situated almost at the extremities of the visible solar spectrum, are not so easy to observe; still the 60,000th part of a grain of potassium cannot escape a careful observer.
Lithium, an element discovered by Arfwedson in 1818, was always thought to be very scarce in nature, and was indeed only known to exist in four varieties of mica, coming from one or two localities on the face of the globe. Bunsen, on examining its spectrum, found it to be most beautiful and characteristic, consisting of a single intensely brilliant crimson line, Li, a, and one less distinct orange line, Li. B. By help of these simple marks, the presence of the 70,000,000th part of a grain of lithium compound may be observed. Now mark the triumphant result of the process : if Bunsen took any substance, and, testing it in the flame, observed a ray of crimson light in the right place, he unhesitatingly declared that lithium was there.
“In this way," say the professors, “we arrive at the unexpected conclusion that lithium is most widely distributed throughout nature, occurring in almost all bodies. Lithium was easily detected in forty cubic centimetres of the water of the Atlantic Ocean, collected in 41° 41' N. latitude, and 39° 14' W. longitude. Ashes of marine plants (kelp), driven by the Gulf-stream on the Scotch coasts, contain evident traces of this metal. All the orthoclase and quartz from the granite of the Odenwald which we have examined contain lithium. A very pure spring water from the granite in Schlierbach, on the west side of the valley of the Neckar, was found to contain lithium ; whereas the water from the red sandstone which supplies the Heidelberg laboratory was shown to contain none of this metal. Mineral waters, in a litre of which lithium could hardly be detected according to the ordinary methods of analysis, gave plainly the line Li, a, even if only a drop of the water on a platinum-wire was brought into the flame. All the ashes of plants growing in the Odenwald on a granite soil, as well as Russian and other pot-ashes, contain lithium. Even in the ashes of tobacco, vine-leaves, of the wood of the vine, and of grapes, as well as in the ashes of the crops grown . . . on a non-granitic soil, was lithium found. The milk of the animals fed upon these crops also contains this widely diffused metal.” We may also mention that its presence in human blood and tissue has been since proved.
A beautiful carmine-coloured flame was long since known to be characteristic of lithium; but so long' as the prism was not employed, it is obvious that the greater masses of coloured light,
produced by other more abundant elements usually present, en· tirely masked and overwhelmed the fainter red due to small traces of lithium. Here the beauty of this method of analysis is surely strongly shown. Let no one think that Bunsen is rash in trusting to the single crimson ray of light as an infallible guide. The process of reasoning can almost always be verified by the ordinary processes of chemical analysis, if it be worth while to take the trouble; but the infallibility of the process was established to the utmost when Bunsen, as will shortly be described, predicted the existence of a minute quantity of a new element, and then procured it!
We may here mention that the process of spectrum-analysis has already been applied by A. and F. Dupré* to the examination of many specimens of water from the Thames, and from the principal London wells. They detected the general presence of strontium and lithium.
Sodium, however, is altogether the Instantia Ostensiva, as Bacon would say, or the striking and predominant Instance in this subject. It has been long known, indeed, that sodium gives to any flame a strong yellow colour, called homogeneous, because it is not decomposable into several kinds by the prism. Hence its spectrum consists of a powerful yellow line, capable of resolution, however, by a high magnifying power, into two very close lines. It is also an important fact that this sodium line is exactly coincident with the prominent dark line in the solar spectrum named D by Fraunhofer; that is to say, if both the sodium and solar spectra were allowed to fall over one another, the yellow sodium line would exactly cover the dark solar line D. Now for a long time back, as will appear in our subsequent historical remarks, this yellow line was met by experimenters on coloured flames, whatever substances they tried. Bunsen drew the right conclusion, that the test for sodium is inconceivably delicate, and that sodium is present in almost every place and thing. Diffusing a minute quantity of soda in the air at one end of his laboratory, he found a flame at the other end coloured yellow in consequence; thus he was enabled to prove that the presence of less than the 180,000,000th part of a grain of soda in the testing flame may be surely detected! The professors truly remark, “ that the chemist possesses no reaction which in the slightest degree will bear comparison, as regards delicacy, with this spectrum-analytical determination of sodium.”
It is easy also to understand, that as two-thirds of the earth's surface is covered with the saline solution, sea-water, the atmo
• London and Edinburgh Phil. Mag., 4th Series (1860), vol. xx. p. 373.
sphere must be impregnated with minute particles of salt; and it is a fact, that every thing exposed to the air for a few hours will become slightly saline.
“That sodium is always contained in the air can be easily shown, by allowing a fine platinum-wire, which has been cleaned by ignition in the flame, to remain exposed to the air for a few hours, when, on placing it again in the colourless flame, a bright flash of the soda line is seen. In the same way, the dust which settles from the air in a room shows the soda re-action ; we only need to knock a dusty book near the flame, in order to obtain the wonderfully brilliant yellow soda line.”
The yellow colour in the last experiment will be quite apparent without prismatic analysis. The wide diffusion of common salt in the air was, however, a fact long known, and proved by the saltness of rain-water. Thus the writer of the present article has, by the common chemical tests, found appreciable and sometimes large quantities of salt present in almost every sam. ple of rain which fell throughout a year. The yellow homogeneous light of sodium is that produced in the familiar Christmas amusement called Snap-dragon, when, in the midst of the game, salt is thrown into the dish of burning brandy, and gives to every friendly face, and every object around, such a ghastly yellow hue as few are likely to see and afterwards forget.
As an example of the application of these tests in actual qualitative analysis, we quote the following from Kirchhoff and Bunsen's paper:
“A mixture of the chlorides of potassium, sodium, lithium, calcium, strontium, and barium, containing at the most of a milligramme* of each of these salts, was brought into the flame, and the spectra produced were observed. At first, the bright yellow sodium line Na, a appeared, with a background formed by a nearly continuous pale spectrum. As soon as this line began to fade, the exactly defined bright red line of lithium, Li. a, was seen, and still further removed from the sodium line the faint red potassium line, Ka. a, was noticed, whilst the two barium lines, Ba. a, Ba. B, with their peculiar form, became visible in the proper position. As the potassium, sodium, lithium, and barium salts volatilised, their spectra became fainter and fainter, and their peculiar bands one after the other vanished, until, after the lapse of a few minutes, the lines Ca. a, Ca. B, Sr. a, Sr. B, Sr. Y, and Sr. , became gradually visible, and, like a dissolving view, at last attained their characteristic distinctions, colouring, and position, and then, after some time, became pale, and disappeared entirely.
“The absence of any one or of several of these bodies is at once indicated by the non-appearance of the corresponding bright lines.”
• A milligramme is equal to 176 of a grain.
The facility and certainty of the process are strikingly manifest in the following statement :
“Those who become acquainted with the various spectra by repeated observation, do not need to have before them an exact measurement of the single lines in order to be able to detect the presence of the various constituents; the colour, relative position, peculiar form, variety of shade, and brightness of the bands are quite characteristic enough to insure exact results, even in the hands of persons unaccustomed to such work. These outward distinctions may be compared with the differences of outward appearance presented by the various precipitates which we employ for detecting substances in the wet way. Just as it holds good as a character of a precipitate that it is gelatinous, pulverulent, flocculent, granular, or crystalline, so the lines of the spectrum exhibit their peculiar aspects, some appearing sharply defined at their edges, others blending off either at one or both sides, either similarly or dissimilarly, or some again appearing broader, others narrower.”
Those who have ever been engaged in practical chemistry' will remember the long, weary, and often fruitless labour of qualitative analysis according to the long-established methods of dissolving, filtering, precipitating, washing, drying, fusing, boiling, digesting, re-precipitating, re-dissolving, and so on, in endless succession. How charming to them will seem even the possibility of a process in which all this is dispensed with, and the slight trouble of reducing a substance to a volatile form, and then observing it through a telescope, is alone required. It is as yet uncertain, however, to what degree of perfection and general application the process can be brought. That this same method of investigation can be extended to all the metallic elements is more than probable, for Kirchhoff writes to Professor Roscoe: “I have assured myself that even the metals of the rarest earths, as yttrium, erbium, and terbium, can be most quickly and certainly determined by help of the spectrum-analytical method.”
It yet remains shortly to notice the most remarkable triumph which the method has achieved. In examining the spectra of the alkalies obtained from certain mineral waters, Bunsen observed the occurrence of two bright blue lines, which he had not seen before when he examined alkalies from other sources. Hence he concluded that these bright lines must be produced by a new, hitherto undetected, alkaline metal. Subsequent search proved the validity of the reasoning. The new metal was found, and isolated in sufficient quantity to demonstrate its individuality. Bunsen appropriately chose its name, Cæsium (ccesius, the primitive of cæruleus=bluish-gray), with regard to the colour of its spectrum.
Again, more lately, Bunsen writes to Professor Roscoe :
“The substance which I sent you as impure Tartrate of Cæsium contains a second new alkaline metal. I am at present engaged in preparing its compounds. I hope soon to be able to give you more detailed information concerning it. The spectrum of the new metal consists of two splendid red lines, situated beyond the red line K. a, in the ultra-red portion of the solar spectrum. Hence I propose to call the new metal Rubidium.""*
· Thus another instance is added to the list of those admirable triumphs of intellect, verified predictions. The memory runs back to Halley's Comet, to the discovery of Uranus, and afterwards the Asteroids, in accordance with Bodes' law of the planetary distances, to the determination of Neptune's position before he had ever been seen, and to the discovery of conical refraction by Sir W. R. Hamilton on à priori grounds. Nor would we omit from the list the very curious deduction of Young, Poisson, and Arago, that mercury has a refractive index of 5.829 out of air, though this fact never has, and never can be observed, owing to the opacity of mercury. In the science of chemistry the verified predictions have not previously been so remarkable, although analogy has often been confidently and safely trusted. The determination of the vapour density of carbon, from the law of combining volumes, is a prediction resembling the last mentioned, and probably incapable of direct verification.
By the new method of discovery now established, we almost fear that a list of new elements, diminishing in importance in a sliding scale, will soon be found. The severe dignity of a chemical element will then suffer in the same way that the solitary grandeur of the principal planets of our system has suffered from the asteroids, to which an addition, under the name of a new planet, is now made nearly every month. This is as bad as swamping the House of Lords.
Considering the extreme beauty of the process thus described, we have thought it a work of considerable interest to inquire how far the simple principle of spectrum-analysis had been previously entertained in theory. Thus, we propose to trace the history, not of the spectrum in its general bearings, but chiefly as regards the restricted truth, that a given definite series of rays proceed only from a certain element, of which they may be used as a sure indication. In doing this, we utterly disavow any intention of in the least disparaging the labours of the professors whose discoveries we are considering. If they were not chronologically the first to conceive the idea, they were the
* Mr. Crookes, of London, has very recently announced his discovery of a further new element. It seems as if he would be Mr. Hind's representative in the chemical world.