sun. on looking at the spectrum of moonlight and of Venus-light, the same lines appeared quite unaltered in position. But he found that the light of the fixed stars was not of the same kind as direct or reflected sunlight, as the spectra of the starlight contained dark lines entirely different from those which are invariably seen in the solar spectrum. From these observations Fraunhofer, so early as 1815, drew the important conclusion that these lines, let them be what they may, must in some way or other have their origin in the The explanation of the production of these lines was reserved for a subsequent time; but Fraunhofer opened the inquiry, and all his conclusions have been borne out by recent and more elaborate investigations. Since the time of Fraunhofer our knowledge of the constitution of the solar spectrum has largely increased. Professor Stokes, in his beautiful researches on Fluorescence, has shown that similar dark lines exist in that part of the spectrum extending beyond the violet, which require special arrangements to become visible to our eyes; and Sir David Brewster and Dr. Gladstone have mapped with great care about two thousand lines in the portion of the spectrum from red to violet. But it is to Kirchhoff, the Professor of Physics in the University of Heidelberg, that we are indebted for by far the best and most accurate observations of these phenomena. In place of using one prism, as Fraunhofer did, Kirchhoff employed four prisms of most perfect workmanship, and thus enjoyed the advantage of a far greater dispersion, or spreading out, of the different rays than the Munich optician had obtained. The lines were observed through a telescope having a magnifying power of forty, and when the whole apparatus was adjusted with all the accuracy and delicacy which the perfection of optical instruments now renders possible, Kirchhoff saw the solar spectrum with a degree of minute distinctness such as had never before been attained; and of the beauty and magnificence of the sight thus presented those only who have been eyewitnesses can form any idea. Kirchhoff's purpose was not merely to observe the fine vertical dark lines which in untold numbers crossed the colored spectrum, stretching from right to left. He wished to measure their relative distances, and thus to map them, exactly as the astronomer determines the position of the stars in the heavens, and the surveyor triangulates and marks out the main features of a country; so that future wanderers in this new field may find fixed and well-recognized points from which to commence their own excursions. Professor Kirchhoff is far from thinking that his measurements, delicate and numerous though they be, have exhausted the subject. The further we penetrate into the secrets of nature, the more we find there remains to be learnt. He saw whole series of nebulous bands and dark lines which the power of his instrument did not enable him to resolve; and he thinks that a larger number of prisms must be employed to effect this end. He adds: "The resolution of these nebulous bands appears to me to possess an interest similar to that of the resolution of the celestial nebulæ; and the investigation of the spectrum to be of no less importance than the examination of the heavens themselves." True, indeed, does this appear, when we learn that it is by the examination of these lines that we can alone obtain the clue to the chemical composition of sun and stars! The exact measurement of the distances between the lines was made by moving the cross wires of the telescope from line to line by means of a micrometer screw with a finely divided head, and reading off the number of divisions through which the screw had to be turned. The breadth and degree of darkness were also noticed, and thus the lines were mapped. In order to give a representation in the drawing of the great variety of the shade and thickness of the lines, they were arranged according to their degree of blackness, and drawn of six different thicknesses. First, the darkest lines were drawn with thick black Indian ink; the ink was then diluted to a certain extent, and the lines of the next shade drawn, and so on to the lightest series. As soon as a portion of the spectrum had been drawn in this manner, it was compared with the actual spectrum, and the mistakes in the breadth and darkness of the lines, as well as in their position, corrected by fresh estimations, and the drawing made anew. A second comparison and another drawing were then made, and this process repeated until all the groups of lines appeared to be truthfully repre sented. Copies from the same lithographic that each elementary substance—that is, a 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 reproduce with marvellous fidelity the appearance which the solar spectrum presents when viewed through the magnificent Heidelburg instrument. These maps extend, however, over only one-third part of the visible portion of the solar spectrum, and it will, we fear, be long before the other two-thirds are completely surveyed, as the following note, telling of the failing eyesight of the ingenious observer, touchingly explains: "My drawing," he says, "is intended to include that portion of the spectrum contained between the lines A and G. I must, however, confine myself at present to the publication of a part only of this, as the remainder requires a revision, which I am unfortunately unable to undertake, owing to my eyes being weakened by the continual observations which the subject rendered necessary." Before it can be understood how these dark lines reveal the chemical composition of the solar atmosphere, it must be shown how the constitution of terrestrial matter can be ascertained by the examination of the nature of the light which such heated matter emits. That certain substances, when heated or burnt, give off peculiar kinds of light, has long been known; and this fact has been made use of by the chemist to distinguish and detect such substances. Thus compounds of the earth strontia, when burnt with gunpowder, produce the peculiar mixture well known as the "red fire" of the pyrotechnist; the salts of baryta give color to the green fires of the stage; and we all see in the Christmas game of snap-dragon that a handful of salt (chloride of sodium) thrown into the dish imparts to the flame a yellow color. This property of substances to give off certain kinds of light was formerly only known to hold good for a few bodies; but the progress of science has taught us that it is not confined to one substance, but is applicable to all. We only require to examine a body under the proper conditions, in order to see that when heated it emits a peculiar and characteristic kind of light; so liquid, may by heating be made to emit a kind of light peculiar to itself, and different from that given off by any other substance. Here, then, is the basis of this new method of spectrum analysis-a science which demonstrates the chemical composition of a body by the color or kind of light emitted from it when heated. We now only need to know, in order to understand the subject, the proper conditions under which bodies can be made to develop this beautiful property, by help of which their chemical natures can be thus easily investigated, and analysis rendered not only independent of test-tubes, but likewise of distance; for it is clear that so long as light can be seen, it matters not how far removed its source may be. The sole condition which must be fulfilled in order to attain the object, is that the body to be analyzed must be in a condition of luminous gas or vapor; for it is only in the gaseous state that each kind of matter emits the light peculiar to itself. It is somewhat difficult at first to understand how a gas or air can be heated until it emits light, and yet familiar instances are not wanting of such a condition of things. Flame, indeed, is nothing else than heated and luminous gas; and in the blue part of the flame of a candle, and in the lambent blue flame which plays on the top of a large fire, we have examples of a truly gaseous body heated until it becomes luminous. The modes in which the various elements can be best obtained in the condition of luminous gases are very different. For the compounds of the metals of the alkalies and alkaline earths, it suffices to bring a small quantity of one of their salts into a flame of a spirit lamp, or into a gas flame. The salt then volatilizes, or becomes gaseous; and this vapor, heated to the temperature at which it is luminous, tinges the flame with a peculiar color. For the compounds of the other metals, such as iron, platinum, or silver, a much higher temperature is needed; whilst for bodies such as air and hydrogen, which are gases at the ordinary temperature, a different mode of manipulation is necessary. 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 quan elementary 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 imparting to it some soda salt from our hands. Hence it appears that Professor Bunsen was able to detect the presence of one one hun soda; and we learn without astonishment that common salt, derived from the ocean which covered two-thirds of the earth's surface, is always present in the atmosphere in a very finely divided solid form, which doubtless produces most important effects on the animal economy, and probably on all the phenomena of life. To Professors Bunsen and Kirchhoff science is mainly indebted for the examination of this hitherto hidden language of nature. These philosophers undertook an investiga-dred and eighty millionth part of a grain of tion of the "Spectra of the Chemical Elements," and nobly have they carried out their intention; unfolding a vast store of nature's secrets to the knowledge of mankind, and revealing the existence of much more yet to be learnt in unlimited fields which promise a rich harvest of discovery to the patient and exact inquirer. Seldom indeed has it been the privilege of men in a single discovery to found a science, or to open a subject so pregnant with important results as that of spectrum analysis. Those alone who are acquainted with the practical details of the science of Chemistry will be able fully to appreciate the grand change which the introduction of this new method effects in the branch of their science devoted to analysis. Qualitative analysis thereby undergoes a complete revolution; the tedious operations of precipitation and filtration must now be superseded by the rapid observation of the spectra of the colored flames by which the presence of the most minute trace of the substance-far too small to be found by the older and coarser methods can be surely and clearly detected. Let us endeavor to form an idea of the appearance of the peculiar spectra thus obtained; the most complete or eloquent description must, however, fail to give more than a bare idea of the reality. In the first place, if we look through the telescope of Kirchhoff's instrument, having placed a flame colored yellow by a sodium If a small quantity of a potash salt, instead of the soda, be placed in the flame, it will be tinged purple; the potash spectrum consists of a portion of continuous light in the centre, bounded by a bright red and a bright violet line at either end. This peculiar appearance is alone caused by the compounds of potassium, and is produced by all the salts of this metal. So, too, with each metal we notice peculiar bright-colored bands, or lines, which are so distinct and characteristic that a glance through the telescope reveals, to an experienced eye, the presence of each of the metals of the alkalies and alkaline earths, when they occur or are combined together even in the minutest quantities. For none of these bright lines overlap or interfere with any other; the lines of each metal when all are present together, appear perfectly distinct. It is a hopeless task to endeavor by words to express the beauty of the phenomena which in this branch of science present themselves to the beholder; as well might we attempt to convey by description, to one who had not witnessed those scenes, the grandeur of the high Alps, or the majesty of the flight of a hesitated not a moment, but began to evapcomet through the heavens. Suffice it to orate forty tons of the water in order to get say, with Kirchhoff, that the appearances enough material to separate out his new here noticed " belong to the most brilliant metal, and examine all its chemical relations. optical phenomena which can be observed." No sooner, however, had he obtained more Professor Bunsen thus describes what he saw than a mere trace of the new substance, than when he placed a mixture of the salts of all he found that with it was associated a secthe metals of the alkalies and alkaline earths ond new metal. From the forty tons of the into the flame, and observed the spectra thus water in question Bunsen got only about one produced :hundred and five grains of the chloride of one metal, and one hundred and thirty-five grains of the chloride of the other; in such minute quantities do these substances occur ! Yet, thanks to the skill and patient industry of the great chemist of Heidelberg, these difficulties were triumphantly overcome, and we now possess a chemical history of these two new metals as complete and well authenticated as that of the commoner alkalies. The names wisely chosen for these substances indicate the nature of their origin, and point "I took," he 66 says, a mixture, consisting of chloride of sodium, chloride of potassium, chloride of lithium, chloride of calcium, chloride of strontium, chloride of barium, containing at most one one thousandth part of a grain of each substance. This mixture I put into the flame, and observed the result. First, the intense yellow sodium lines appeared on a background of a pale continuous spectrum; as these began to be less distinct, the pale potassium lines were seen, and then the red lithium line came out, whilst the barium lines appeared in all their vividness. The sodium, lithium, potassium, and barium out the property by help of which they were salts were now almost all volatilized, and after discovered. Bunsen calls one of them "Caa few moments the strontium and calcium sium," from cæsius bluish gray, because the lines came out as from a dissolving view, spectrum of this metal is distinguished by gradually attaining their characteristic bright- two splendid violet lines; the other he named ness and form." "Rubidium," from rubidus dark red, owing to the presence of two bright red rays at the least refrangible extremity of its spectrum. Since the publication of the discovery of these metals, their salts have been found to be pretty commonly diffused; but, owing to their close resemblance to the compounds of potassium, they were not recognized as separate substances; in fact, had it not been for this new method, we should not have been able to distinguish them from the well-known alkali potash. Cæsium and Rubidium occur in the water of almost every salt spring; and they have likewise been found in the ashes of plants, especially in those of beet-root, so that they must be contained in the soil; but in all these cases the quantity in which they are found is very minute. The mineral lepidolite contains a certain quantity of Rubidium, which now may be obtained by the pound; but Cæsium is still extremely rare. It is satisfactory to learn that in a similar way the existence of another new metal has been pointed out by Mr. Crookes. This body is characterized by a spectrum containing one bright green band, and has been called "Thallium."* The most striking example of the value of this new power of analysis, and of its probable results, is that of the discovery of two new alkaline metals by Bunsen. This distinguished chemist, in examining the spectra of the alkalies contained in the mineral waters of Dürkheim in the Palatinate, observed some bright lines that he had not seen in any other alkalies which he had investigated. He was sure that no other metals but those of the alkalies could be present, because by well-known chemical processes, he had separated every other kind of metal. Hence he concluded that these new lines indicated the presence of an alkaline metal whose existence had as yet been overlooked. In fact, just as Adams and Leverrier, from the perturbations of the planet Uranus, predicted the existence of Neptune, so Bunsen, from the perturbations seen in the spectra of the alkalies, predicted the existence of a new member of the large family of the elementary bodies. So certain was Bunsen of his method, and so confident was he that his bright lines could not fail him, that, although the weight of substance from which he obtained his result only amounted to the one one thousandth part of a grain, he *This new element has lately been prepared in somewhat larger quantities by M. Lamy from the scientific basis, by applying to it the modern methods of exact research. For the purpose of obtaining the peculiar In an article like the present it is impossible to enter minutely into the details of such discoveries, or even to mention more than the most striking points by way of il-spectra of iron, platinum, copper, and most lustration. Enough has, however, been said of the other metals, these metals must be to show the enormous fertility of this field exposed to a much higher temperature than of research, and to give an idea of the prin- that of a gas flame, to which they impart no ciples upon which the method depends. We color. This high temperature is best atanticipate, more especially, important results tained by the use of the electric spark. So to the art of medicine from the application great, indeed, is the heat developed by this of this analytical process to mineral waters, as agent, that a single electric discharge past they are termed, noted for their therapeutic through a gold wire dissipates the metal at qualities. The composition of these waters, once in vapor. Our illustrious Faradaytheir apparently inexhaustible faculty of re- the founder of so many branches of electriproduction, their modes of affecting the hu- cal science-first showed that the electric man frame in various states of health and dis- spark was produced by the intense ignition ease, are only known as yet empirically. Yet of the particles composing the poles; and it is impossible to doubt or deny that waters, Professor Wheatstone proved that if we look like those of Carlsbad, Aix-la-Chapelle, or at the spark proceeding from two metallic Bagnères de Luchon, contain certain agents poles, through a prism, we see spectra of the most powerful sanative character, containing bright lines which differ acwhich the means of chemical analysis hith-cording to the kind of metal employed. erto employed do not appear to have reached." These differences," said Wheatstone, writIt is extremely probable that the application of spectral analysis to the elements contained in these springs will bring them within the range of accurate medical knowledge, and perhaps extend the resources of medicine itself. ing in 1834, "are so obvious, that any one metal may instantly be distinguished from others by the appearance of its spark; and we have here a mode of discriminating metallic bodies more ready than a chemical examination, and which may hereafter be employed for useful purposes." This has, indeed, turned out to be a true prediction. The field of spectrum analysis was not wholly untrodden until it was explored by the two German professors. Even so long ago as 1826, Mr. Fox Talbot, a gen- The large number of bright lines which tleman whose name is honorably associated are seen in the spark spectrum are not all with discoveries in that most beautiful of the caused by the glowing vapor of the metal modern applications of science to art-Pho- forming the poles; a portion of them protography-made some experiments upon the ceed, as Angström first pointed out, from the spectra of colored flames, and pointed out particles of gas or air, through which the the advantages which such a method of anal- spark passes, becoming luminous also, and ysis would possess. Professor Wheatstone, emitting their own peculiar light. Thus, if Mr. Swan, Sir David Brewster, and Profes- we examine the spectrum of an electric spark sor W. Allen Miller in our own country, and passing from two iron poles in the air, we see Angström, Plücker, Masson, and others on at least three superimposed spectra, one of the Continent, have likewise contributed to the iron, one of the oxygen, and a third of our knowledge of this subject; but whatever the nitrogen of the air. By help of a little may have been done by others for the estab- mechanical device, it is easy to distinguish lishment of the new method, it must be ad- between the air lines and the true metallic mitted that the names of Bunsen and Kirch-lines, and in this way to detect the various hoff will justly go down to posterity as the founders of the Science of Spectrum Analysis; for they first established it on a firm residues of the Belgian sulphuric acid chambers. He finds that in its specific gravity and outward properties it closely resembles the metal lead, but that it possesses very peculiar chemical characteristics. metals. So certain and accurate is this method that Professor Kirchhoff has, without difficulty, been able to detect and distinguish the presence of minute traces of the *The spectra of the permanent gases, as well as those of the other non-metallic elements, have been accurately examined by Professor Plücker, of Bonn. |