Page images
PDF
EPUB

upon the nature of the electric current, and we do not observe any allusion to the principle of spectrum-analysis. As Herschel was absorbed by the optical interest of coloured chemical flames, so is Masson absorbed by their electrical interest.

In the year 1852, Professor Stokes presented to the Royal Society his celebrated paper on Fluorescence, or “ The Change of Refrangibility of Light,” in which he devotes a section to “ Optical Tests of Uranium in Blowpipe Experiments."*

Speaking of the phenomenon of internal dispersion in a bead of microcosmic salt fused with oxide of uranium, by which a curious green light is produced, he says:

“When properly examined by means of sunlight, its sensibility is evident at once, and when the dispersed light is viewed through a prism, it is resolved into bright bands. .... So delicate is this test when applied to uranium, that on one occasion, when engaged in examining a bead coloured green by chromium, which had been fused in the exterior flame, I observed the appearance given by uranium. This turned out to be actually due to uranium, of which a mere trace was accidentally present without my knowledge." He continues :

“The green communicated to microcosmic salt by uranium, after exposure to the reducing flame, has a very peculiar composition, by means of which the presence of uranium may be instantly detected. For this purpose it is sufficient to view through a prism the inverted image of the flame of a candle formed by the bead, the latter being so held as to be seen projected on a dark object. The observation is perfectly simple, and occupies only a few seoonds. The spectrum exhibits an isolated band at the red extremity, followed by a very intense dark band of absorption.”

This refers only to light derived from internal dispersion, a peculiar source; but the generalisation to light from other sources is now of course obvious.

Next in the order of time, we find that in April 1856 Professor William Swan communicated to the Royal Society of Edinburgh some researchest “ On the Prismatic Spectra of the Flames of Carbon and Hydrogen.” He shows, as Fraunhofer indeed had originally shown, that several bright lines in the carbo-hydrogen spectrum coincide with dark lines in the sun, and argues in favour of some physical connection. He also proves, without using the prism, that a portion of chloride of sodium, weighing less than the 1,000,000th part of a grain, is sufficient to tinge a flame with bright yellow light, and this quantity of salt contains only one 2,570,000th part of a grain of metallic sodium. And he adds,

* Phil. Trans. (1852), vol. cxlii. p. 522. | Trans. (1856), vol. xxi. p. 411.

“When we consider the almost universal diffusion of the salts of sodium, and the remarkable energy with which they produce yellow light, it seems highly probable that the yellow line R, which appears in the spectra of almost all flames, is in every case due to the presence of minute quantities of sodium.”

Swan, then, surmounted the sodium difficulty which had puzzled such men as Herschel, Brewster, and Talbot. The * circumstances” which produced the “singularity" noticed by M. Melvill in the middle of the last century did not escape him, and a considerable stumbling-block was certainly removed from the path of succeeding explorers.*

One of the most accurate and philosophical recent researches on the spectrum is that of A. J. Angström,t upon the electric light. Did our space allow, we should have been glad to notice his results for their intrinsic interest. We do not find, however, in his memoir the slightest allusion to the practical employment of the luminous bands for the detection of substances. He proves the singular fact that the electric light, besides lines peculiar to each metal, gives a multitude of luminous lines, comparable in number and distribution to the dark lines of the solar spectrum, and the same for all metals.

Although Talbot, Wheatstone, and Stokes saw the utility of the prism in detecting the presence of bodies, they presented no complete system of observation. Dr. J. H. Gladstone, however, adopting the principle thus established, carried it out into practice with great ability in one particular branch. He detects the presence of a substance in a clear solution by its power of absorbing certain rays of a continuous spectrum; in short, he applies the spectrum to determine with more accuracy the characteristic colour of a solution, and shows that the lines and bands thus observable are a perfect test of the presence of a certain substance. He also establishes, from numerous observations, the important rule, recognised by nearly all investigators in this subject, that “all the compounds of a particular base, or acid, have the same effect on the rays of light.” This method of absorption is applicable to a greater range of substances

Swan has since thought it worth while to claim any credit that this entitles him to (Phil. Mag. [4], vol. xx. p. 173), although the German professors duly acknowledged acquaintance with his research.

† Poggendorff's Annalen, vol. xciv. p. 141; or the Lond, and Edin. Phil. Mag. [4], vol. ix. p. 327.

Quarterly Journal of the Chemical Society, vol. x. (1858), pp. 79-91, also P. 219.

than Kirchhoff's and Bunsen's method ; thus it may detect the presence of litmus and other organic colours. But it has none of the extreme delicacy, facility, and certainty which constitute the whole value of spectrum-analysis. Dr. Gladstone had not the good luck to choose the right branch of this inquiry ; but his concluding remarks prove that he was fully aware of the value of the prism. They are as follow :

“ These observations will be sufficient to prove that the varying chromatic phenomena, exhibited by different substances, may be taken advantage of in qualitative analysis, to an extent which has been hitherto unappreciated. My remarks have been almost confined to transmitted light; but the phenomena of reflected light offer a similar, and as yet almost unoccupied, field of investigation. What I have here marked down must be considered rather as a tentative inquiry than as a really valuable contribution to our knowledge of the effect of different chemical substances on the rays of light; but should any one be induced to take up the matter systematically, he might easily make such a series of observations as would furnish data for regular tables of comparison; and the prism would then take its place, as the blowpipe now does, among the recognised and almost indispensable instruments of the analytical laboratory."

We should notice that Mr. Crookes and Mr. Pearsall have also, in particular cases, used the prism on the analytical principle.

From the preceding historical sketch it would appear probable that Talbot discovered the very method of analysis now performing such great things; that Wheatstone recognised the principle, and that Gladstone developed it into a complete method in one branch, which did not prove to be luckily chosen. To all the other inquirers on the subject, the utility of the spectrum in analysis seems not to have occurred. It would be superfluous here again to eulogise the achievements of the German professors, whose published results we are reviewing.

Some persons, we believe, consider it a trifling, if not an illnatured, occupation to trace back the occurrence of thoughts and works which only, after a time, are recognised at their true worth. But the same principle might be applied to other parts of history. It amounts to saying, “Let bygones be bygones. We have a great deal of science and learning, and many pleasant things in the present day, and we shall not increase them by rooting up old records, and inquiring who secured these advantages to us.” It must not be so; the very history of science is a science of itself, showing the gradual development of modes of thought, and the errors and difficulties into which genius falls are some of the most interesting facts in the range of knowledge.

But independently of this interest, we have a strong opinion

that formal honour and, if possible, general reputation should be meted out to scientific men by laws as strict and impartial as those which secure the execution of a dead man's will. In the first place, it is obvious that the human race owes to scientific discoverers a debt which it is not merely impossible to discharge, but even to conceive. Suppose Providence had decreed that Watt, instead of being plagued by patent laws, should have enjoyed a small percentage of the advantages which he has caused and will cause to others; how long the life, how great the benefits that would have fallen to his lot! Watt, indeed, was partly a practical man; but the same could be said of most abstruse philosophers. Again, it is, we believe, generally true that an earnest discoverer is never so well rewarded as when the worth of his results is proved and acknowledged, and the expectation of the proof and acknowledgment is nearly equal to the fact. What is wealth, or a title, or a splendid funeral to a man who has laid the foundations of a new science, knowing that it will grow through all time more perfect and useful, and bear his name upon its front? But the beauty and grandeur of the edifice will seldom be apparent in the few first stones, except to him who has the whole design mentally before him. So he who publishes a single new and original thought may well know what it will grow to be, and yet feel that future times only will witness the growth. That a single thought may turn the affairs of men, no reflective person can deny. Every science, and consequently every material achievement of modern times, is the development of a few simple grand thoughts, apparently incidental in their occurrence, but generally the result of enormous labour. May there be none, then, who shall go without their due award of honour for any labour or ability which they apply to the advancement of the general good! Thus we shall best insure future energy. But in any case, let us remember the soliloquy of Falstaff: “ Honour pricks me on. .... What is that honour ? Air: a trim reckoning! Who hath it? He that died O'Wednesday. Doth he feel it? No. Doth he hear it? No. Is it insensible, then? Yea, to the dead. But will it not live with the living ? No. Why? Detraction will not suffer it; therefore I'll none of it.”

We have but little space left to consider one obvious but exceedingly interesting application of spectrum-analysis, viz. to the determination of the chemical composition of astronomical self-luminous bodies. If we receive from a distant body rays of light, which, when examined by the prism, are found to correspond, to the utmost degree of accuracy that our observing powers can attain, with the light given off by sodium and calcium, here the induction is quite irresistible, that sodium and calcium exist in that distant body. It is possible that this direct mode of observation may be applied to the stars; but when we come to consider the sun's spectrum in this relation, a most remarkable phenomenon meets us. The bright bands of coloured light observed in the spectra of sodium and the various elements correspond, not to bright bands in the solar spectrum, but to the dark lines long since observed by Fraunhofer. This coincidence is so striking in the case of the conspicuous line D correspond. ing to the line of the sodium spectrum, that Fraunhofer himself noticed the fact, as we have seen, and all subsequent observers have confirmed it. From this and other coincidences many natural philosophers probably had become convinced of a physical connection of the bright terrestrial and the dark solar lines. It remained, however, for Kirchhoff to show experimentally, as well as theoretically, the nature of this relation; and his results are published in the second paper at the head of this article.

Kirchhoff finds that any element in the gaseous state has the power both of radiating and absorbing light rays of those definite kinds which are shown in its spectrum. If, from its condition of heat, the light radiated is more intense than that absorbed, the body appears luminous; but if the rays falling upon it are the most intense, the body gives out less light than it absorbs. In the last case, the gaseous body will intercept light, or be opaque to rays of the definite kinds belonging to the gaseous substance. If, then, a compound ray of light containing rays of all kinds or colour pass through sodium in a gaseous state, the yellow rays which belong to sodium are intercepted, and the spectrum of the compound light would be marked with a dark band, which corresponds in position to the line Na. a of the sodium spectrum, and the dark line D of the solar spectrum. It must be understood, however, that this line is only dark relatively to the rest of the spectrum, for the sodium vapour which is absorbing yellow light may be giving out yellow light of less intensity. What is thus shown in the case of sodium is obviously applicable to potassium, lithium, and all other elements, however complicated their spectra.

The fact of the absorption of definite rays by certain elements was proved in the most convincing manner by the following experiments :

“The bright line produced in the spectrum of a gas-flame by the presence of a bead of chloride of lithium, is changed into a dark one when direct sunlight is allowed to pass through the flame. When the bead of lithium is replaced by one of chloride of sodium, the dark double line D (coincident with the yellow sodium line) appears with uncommon distinctness."

The same line D was also produced when light from a white

« PreviousContinue »