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20 From The Edinburgh Review. ter. This is of so delicate a nature, that, 1. Researches on the Solar Spectrum, and the when applied to the examination of the subHii Spectra of the Chemical Elements. By stances composing our globe, it yields most G. Kirchhoff
, Professor of Physics in the University of Heidelberg. Translated tion. At the same time it is of so vast an
new, interesting, and unlooked-for informaby Henry E. Roscoe, B.A., Professor of Chemistry in Owens College, Manches- application as to enable us to ascertain with
ter. Cambridge and London : 1862. certainty the presence in the solar atmos2. Chemical Analysis by Spectrum Observa- phere-at a distance of ninety-five million
tions. By Professors Bunsen and Kirch- miles—of metals, such as iron and magnehoff. Memoirs I. & II. - Poggendorff's sium, well known on this earth, and likewise Annalen (Philosophical Magazine, 4th to give us good hopes of obtaining similar Series, vol. xx. p. 89, vol. xxii
. p. 1), knowledge concerning the composition of the London, Dublin and Edinburgh.
fixed stars. Here, indeed, is a triumph of It is unnecessary to insist, at the present science! The weak mortal, confined within day, upon the incalculable value of discover- a narrow zone on the surface of our insignifiies in natural science, however abstruse they cant planet, stretches out his intellectual may be, or however far-distant may appear powers through unlimited space, and estitheir practical application. If we put aside mates the chemical composition of matter for the moment that highest of all intellec- contained in the sun and fixed stars with as tual gratifications afforded by the prosecu- much ease and certainty as he would do if tion of truth in every form, the perception he could handle it, and prove its reactions in of which is one of the chief distinctions of the test-tube. human from mere brute life, and if we look How can this result, at first sight as marto the results of scientific discovery in bene- rellous and impossible as the discovery of fiting mankind, we find so many striking ex- the elixir vitæ or the philosophers' stone, be amples of the existence of truths apparently arrived at? How did two German philosoaltogether foreign to our every-day wants, phers, quietly working in their laboratory in which suddenly become points of great inter- Heidelberg, obtain this inconceivable insight est to the material prosperity and the moral into the processes of creation ? Are the con. advancement of the race, that we are less clusions which they have arrived at logical apt to utter the vulgar cry of “cui bono”
consequences of bonâ fide observations and respecting any scientific discovery; and if experiments—the only true basis of reasonwe are not advanced enough to love science ing in physical science—or do they not savor for the sake of her truth alone, we at least somewhat of that mysticism for which our respect ber for the sake of the power she be- German friends are famous ? Such quesstows. Not once, but oftentimes in the an- tions as these will occur to all who hear of nals of science, it has turned out that discov- this discovery ; and it will be our present eries of the most recondite truths have ere aim, in reviewing the publications which are long found their application in the physical placed at the head of this article, to answer structure of the world, and even in the com- these and similar questions, and to show mon interests of men; for in the range of that, far from being mystical, these results scientific investigation, it can never be said are as clear as noon-day, being the plain and how near the deepest principle lies to the necessary deductions from exact and laborisimplest facts.
ous experiment. And here we may express A great discovery in natural knowledge, our satisfaction at the change which has ocfor which no equivalent in direct benefit to curred within the last few years in the direcmankind has as yet been found, but which tion given to the powerful intelligence and nevertheless excites our liveliest interest and the indefatigable industry of Germany. The admiration, has lately been made in the rap- labors of the Germans in physical science idly advancing science of Chemistry. This have far surpassed in their results those specdiscovery, which is one of the grandest and ulative researches which had rendered “Germost important of all the recent additions to man philosophy” the synonym of all that science, consists in the establishment of a was unintelligible and perplexing: and it is new system of chemical analysis of a new impossible to overrate the services which power to investigate the constitution of mat- men like Liebig and Bunsen (the chemist)
and Kirchhoff have rendered to mankind. that these colors were no peculiar effect of
The colored band thus obtained by New
ton did not, however, reveal to him all the The only channel through which we on the characteristic beauties of solar light, because earth can obtain information of any kind in his spectrum the tints were created by the whatever concerning the sun and stars, con- partial superposition of an infinite number sists in the vivifying radiance which these of differently colored images of the round luminaries
pour forth into surrounding space. hole through which the light came. It was The light and heat which we receive from not until the year 1802 that Dr. Wollaston, the sun not only supply the several varieties by preventing the different colored lights of force which we find in action upon the from overlapping, and thus interfering with surface of the earth, thus rendering the whole each other, discovered that great peculiarity human family truly children of the sun; but in solar light which has led to such startling a knowledge of their nature enables us to as- discoveries in the composition of the sun itcertain the chemical composition of those far- self. Dr. Wollaston noticed, when he aldistant bodies upon which the existence of lowed the sunlight to fall through a narrow our race so intimately depends. The exam- slit upon the prism, that a number of dark ination of the nature of sunlight and star- lines cutting up the colored portions of the light has led to the foundation of a science spectrum, made their appearance. These of stellar chemistry; and it is likewise upon dark lines, or spaces, of which Wollaston the examination of the light given off by ter- counted only seren, indicate the absence of restrial matter, when through heat it becomes certain distinct kinds of rays in the sunlight; luminous, that the new method of spectrum they are, as it were, shadows on the bright analysis is founded—a method so delicate as background. to enable the analyst to detect with ease and It is, however, to the celebrated German certainty so minute a quantity as the one optician Fraunhofer, that we owe the first one hundred and eighty millionth part of a accurate examination of these singular lines. grain of substance.
By a great improvement in the optical arThe world owes to the great Newton its rangements employed, Fraunhofer, rediscorfirst knowledge of the nature of sunlight. In ering these lines, was able to detect a far 1675 Newton presented to the Royal Society larger number of them in the solar spectrum his erer-memorable treatise on Optics; and than had been observed by Wollaston. He amongst the numerous important discoveries counted no less than five hundred and ninety there disclosed and recorded, was one de- of these dark lines, stretching throughout monstrating the constitution of wbite light. the length of the spectrum from red to vioHe describes what he observed when he let, and in the year 1815 drew a very beaupassed a beam of sunlight, from a hole in tiful map of them, some of the most importhe shutter of a darkened room, through a tant of which he designated by the letters of triangular piece of glass called a prism. He the alphabet. Fraunhofer carefully measnoticed that, instead of a spot of white light ured the relative distances between these corresponding to the hole in the shutter, a lines, and found that they did not vary in bright band of variously colored lights, show- sunlight examined at different times. He ing all the tints of the rainbow, was thrown also saw these same dark fixed lines in reon the wall of his room. Newton concluded fected as well as in direct solar-light; for
on looking at the spectrum of moonlight and and thus to map them, exactly as the astronof Venus-light, the same lines appeared quite somer determines the position of the stars in unaltered in position. But he found that the the heavens, and the surveyor triangulates light of the fixed stars was not of the same and marks out the main features of a counkind as direct or reflected sunlight, as the try; so that future wanderers in this new spectra of the starlight contained dark lines field may find fixed and well-recognized entirely different from those which are inra- points from which to commence their own riably seen in the solar spectrum. From excursions. Professor Kirchhoff is far from these observations Fraunhofer, so early as thinking that his measurements, delicate and 1815, drew the important conclusion that numerous though they be, have exhausted these lines, let them be what they may, must the subject. The further we penetrate into in some way or other have their origin in the the secrets of nature, the more we find there
The explanation of the production of remains to be learnt. He saw whole series these lines was reserved for a subsequent of nebulous bands and dark lines which the time; but Fraunhofer opened the inquiry, power of his instrument did not enable him and all his conclusions hare been borne out to resolve ; and he thinks that a larger num. by recent and more elaborate investigations. ber of prisms must be employed to effect thiş
Since the time of Fraunhofer our knowl- end. He adds : " The resolution of these edge of the constitution of the solar spec- nebulous bands appears to me to possess an trum has largely increased. Professor interest similar to that of the resolution of Stokes, in his beautiful researches on Fluor- the celestial nebulæ; and the investigation escence, has shown that similar dark lines of the spectrum to be of no less importance exist in that part of the spectrum extending than the examination of the heavens thembeyond the violet, which require special ar- selves." True, indeed, does this appear, rangements to become visible to our eyes; when we learn that it is by the examination and Sir David Brewster and Dr. Gladstone of these lines that we can alone obtain the have mapped with great care about two thou- clue to the chemical composition of sun and sand lines in the portion of the spectrum stars ! from red to violet.
The exact measurement of the distances But it is to Kirchhoff, the Professor of between the lines was made by moving the Physics in the University of Heidelberg, that cross wires of the telescope from line to line we are indebted for by far the best and most by means of a micrometer screw with a finely accurate observations of these phenomena. divided head, and reading off the number of In place of using one prism, as Fraunhofer divisions through which the screw had to be did, Kirchhoff employed four prisms of most turned. The breadth and degree of darkperfect workmanship, and thus enjoyed the ness were also noticed, and thus the lines advantage of a far greater dispersion, or were mapped. In order to give a represpreading out, of the different rays
than the sentation in the drawing of the great variety Munich optician had obtained. The lines of the shade and thickness of the lines, they were observed through a telescope having a were arranged according to their degree of magnifying power of forty, and when the blackness, and drawn of six different thickwhole apparatus was adjusted with all the nesses. First, the darkest lines were drawn accuracy and delicacy which the perfection with thick black Indian ink; the ink was of optical instruments now renders possible, then diluted to a certain extent, and the Kirchhoff saw the solar spectrum with a lines of the next shade drawn, and so on to degree of minute distinctness such as had the lightest series. As soon as a portion of never before been attained ; and of the the spectrum had been drawn in this manbeauty and magnificence of the sight thus ner, it was compared with the actual specpresented those only who have been eye- trum, and the mistakes in the breadth and witnesses can form any idea.
darkness of the lines, as well as in their poKirchhoff's purpose was not merely to ob- sition, corrected by fresh estimations, and serve the fine vertical dark lines which in the drawing made anew. A second comparuntold numbers crossed the colored spec- ison and another drawing were then made, trum, stretching from right to left. He and this process repeated until all the groups wished to measure their relative distances, / of lines appeared to be truthfully repre,
sented. Copies from the same lithographic that each elementary substance—that is, & stones accompany the English edition of the substance which has not been split up, or memoir as are appended to the original, and decomposed, or out of which no two or more these are masterpieces of German artistic bodies differing in their properties have been skill. They are printed on six different obtained—whether it be a gas, a solid, or a stones, with ink of six different tints, and liquid, may by heating be made to emit a reproduce with marvellous fidelity the ap- kind of light peculiar to itself, and different pearance which the solar spectrum presents from that given off by any other substance. when viewed through the magnificent Hei- Here, then, is the basis of this new method delburg instrument.
of spectrum analysis—a science which deThese maps extend, however, over only monstrates the chemical composition of a one-third part of the visible portion of the body by the color or kind of light emitted solar spectrum, and it will, we fear, be long from it when heated. We now only need to before the other two-thirds are completely know, in order to understand the subject, surveyed, as the following note, telling of the the proper conditions under which bodies failing eyesight of the ingenious observer, can be made to develop this beautiful proptouchingly explains : “My drawing," he erty, by help of which their chemical natures says, “is intended to include that portion can be thus easily investigated, and analysis of the spectrum contained between the lines rendered not only independent of test-tubes, A and G. I must, however, confine myself but likewise of distance; for it is clear that at present to the publication of a part only so long as light can be seen, it matters not of this, as the remainder requires a revision, how far removed its source may be. The which I am unfortunately unable to under- sole condition which must be fulfilled in take, owing to my eyes being weakened by order to attain the object, is that the body the continual observations which the subject to be analyzed must be in a condition of rendered necessary.”
luminous gas or vapor; for it is only in the Before it can be understood how these gaseous state that each kind of matter emits dark lines reveal the chemical composition the light peculiar to itself. It is somewhat of the solar atmosphere, it must be shown difficult at first to understand how a gas or how the constitution of terrestrial matter can air can be heated until it emits light, and be ascertained by the examination of the yet familiar instances are not wanting of nature of the light which such heated mat- such a condition of things. Flame, indeed, ter emits. That certain substances, when is nothing else than heated and luminous heated or burnt, give off peculiar kinds of gas; and in the blue part of the flame of a light, has long been known; and this fact candle, and in the lambent blue flame which has been made use of by the chemist to displays on the top of a large fire, we have ex. tinguish and detect such substances. Thus amples of a truly gaseous body heated until compounds of the earth strontia, when burnt it becomes luminous. with gunpowder, produce the peculiar mix- The modes in which the various elements ture well known as the “red fire ” of the can be best obtained in the condition of lu. pyrotechnist; the salts of barytá give color minous gases are very different. For the to the green fires of the stage ; and we all compounds of the metals of the alkalies and see in the Christmas game of snap-dragon alkaline earths, it suffices to bring a small that a handful of salt (chloride of sodium) quantity of one of their salts into a flame of thrown into the dish imparts to the flame a a spirit lamp, or into a gas flame. The salt yellow color.
then volatilizes, or becomes gaseous; and This property of substances to give off this vapor, heated to the temperature at certain kinds of light was formerly only which it is luminous, tinges the flame with known to hold good for a few bodies; but a peculiar color. For the compounds of the the progress of science has taught us that other metals, such as iron, platinum, or it is not confined to one substance, but is silver, a much higher temperature is needed ; applicable to all. We only require to exam- whilst for bodies such as air and hydrogen, ine a body under the proper conditions, in which are gases at the ordinary temperature, order to see that when heated it emits a pe- a different mode of manipulation is neces. culiar and characteristic kind of light; so sary.
In order to become acquainted with the compound in front of the slit through which exact nature of the light which bodies in the the light falls on to the prisms, and thence condition of luminous gases emit, their light into the telescope, we shall see the spectrum must be examined otherwise than by the of sodium. We notice that it consists simnaked eye. The same kind of apparatus is ply of two very fine bright yellow lines placed used in this investigation which Fraunhofer close together, all the rest of the field being and Kirchhoff applied to the investigation perfectly dark. On investigation we find of solar light; in short, the distinctive qual. that all the compounds of the metal sodium ities of these luminous gases are ascertained give these two lines, and no other substance by their spectra. Then only is it that the is met with in whose spectrum these lines full beauty of this property of matter be- occur. So excessively delicate is this indicomes apparent, and the character of each cation of sodium-that is, so small a quanelementary body is written down in truly tity of sodium salt suffices to bring forth a glowing language-language different for flash of these bright lines—that we discover every element, but fixed and unalterable for sodium everywhere, in every particle of each one, as to the interpretation of which dust ; in the motes visible in the sunbeam. no variety of opinion can possibly exist. We cannot touch any substance without im
To Professors Bunsen and Kirchhoff sci- parting to it some soda salt from our hands. ence is mainly indebted for the examination Hence it appears that Professor Bunsen was of this hitherto hidden language of nature. able to detect the presence of one one hunThese philosophers undertook an investiga- dred and eighty millionth part of a grain of tion of the “ Spectra of the Chemical Ele- soda ; and we learn without astonishment ments," and nobly have they carried out that common salt, derived from the ocean their intention; unfolding a vast store of which covered two-thirds of the earth's surnature's secrets to the knowledge of man- face, is always present in the atmosphere in kind, and revealing the existence of much a very finely divided solid form, which doubtmore yet to be learnt in unlimited fields less produces most important effects on the which promise a rich harvest of discovery to animal economy, and probably on all the the patient and exact inquirer. Seldom in- phenomena of life. deed has it been the privilege of men in a If a small quantity of a potash salt, insingle discovery to found a science, or to stead of the soda, be placed in the flame, it open a subject so pregnant with important will be tinged purple; the potash spectrum results as that of spectrum analysis. consists of a portion of continuous light in
Those alone who are acquainted with the the centre, bounded by a bright red and a practical details of the science of Chemistry bright violet line at either end. This peculwill be able fully to appreciate the grand iar appearance is alone caused by the comchange which the introduction of this new pounds of potassium, and is produced by all method effects in the branch of their ence the salts of this metal. So, too, with each devoted to analysis. Qualitative analysis metal we notice peculiar bright-colored thereby undergoes a complete l'evolution ; bands, or lines, which are so distinct and the tedious operations of precipitation and characteristic that a glance through the tel. filtration must now be superseded by the escope reveals, to an experienced eye, the rapid observation of the spectra of the col- presence of each of the metals of the alkalics ored flames by which the presence of the and alkaline earths, when they occur or are most minute trace of the substance_far too combined together even in the minutest small to be found by the older and coarser quantities. For none of these bright lines methods—can be surely and clearly detected. overlap or interfere with any other; the lines Let us endeavor to form an idea of the ap- of each metal when all are present together, pearance of the peculiar spectra thus ob- appear perfectly distinct. It is a hopeless tained; the most complete or eloquent de- task to endearor by words to express the scription must, however, fail to give more beauty of the phenomena which in this than a bare idea of the reality.
branch of science present themselves to the In the first place, if we look through the beholder ; as well might we attempt to contelescope of Kirchhoff's instrument, having vey by description, to one who had not witplaced a flame colored yellow by a sodium nessed those scenes, the grandeur of the