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resemble chloride of potassium in properties, that it would have been impossible to have ascertained their existence in the minute proportion in which they occur but for the method of spectrum analysis.*

Kirchhoff and Bunsen ignited many of the salts of the different metals in flames of very varying temperature, including those of sulphur, bisulphide of carbon, diluted alcohol, carbonic oxide, hydrogen, and the oxyhydrogen mixture, and they found that the same metal always produced the same lines whatever flame they employed to heat it.

They do not, however, appear to have observed that as the temperature rises, a new series of bands become visible in certain cases. The spectrum of chloride of lithium, in the flame of a Bunsen burner, gives but a single intense crimson line; in a hotter flame, as that of hydrogen, it gives an additional orange ray; and in the oxyhydrogen jet, or the voltaic arc, a broad, brilliant blue band comes out in addition. A similar effect is perceived in the case of metallic iron and other metals a when heated by the voltaic arc, which at elevated temperatures furnish much more complicated spectra than when less intensely heated.

The first specimen, shown in Fig. 2, exhibits some of the fixed dark lines of the solar spectrum contrasted with the Li position of some of the most important bright lines furnished by the spectra of the alkalies and alkaline earths, when their salts are heaped upon a R loop of platinum wire introduced into the flame of a Bunsen gas-burner. The characteristic lines in each case are distinguished by the letters of Cs the Greek alphabet, the most strongly marked lines being those indicated by the earliest letter.

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* Bunsen and Kirchhoff found their method of separating potassium from rubidium and caesium upon the difference in solubility of the double salts which the chlorides of potassium, rubidium, and cæsium form with bichloride of platinum. Boiling water takes up 5'18 per cent, of the potassium salt (KCl, PtC12), o'634 of the rubidium salt (RbCl, PtCl2), and only o' 377 of the caesium salt (CsCl, PtCl2). The caesia and rubidia may be separated by taking advantage of the solubility of carbonate of cæsia in absolute alcohol, carbonate of rubidia not being soluble in this menstruum. The hydrates of potash, rubidia, and casia are all deliquescent, and absorb carbon acid from the air; they give crystalline precipitates with tartaric acid, and an opalescent precipitate with fluosilicic acid. The chemical equivalents of the three metals are as follows:-Potassium, 39'1; rubidium, 85.36; caesium, 123:35.

CHEMICAL NEWS, April 19, 1862,

yellow band Na a. In the lithium spectrum a crimson band, Li a, is the prominent line; Li B is seldom visible; but at the elevated temperature of the voltaic arc an additional blue line becomes very intense.

In the spectrum of cæsium a good deal of diffused light is visible, but the two lines in the blue, C's a and Cs B, are strongly marked, and may be seen when a quantity of the chloride not exceeding 1000th of a grain of the pure salt is used, orth of a grain if diluted with fifteen hundred times its weight of chloride of lithium, Rubidium is not distinguishable in quantities quite so minute. The lines Rb a and Rb 8, in the red, and Rb y in the blue, are almost equally characteristic, but about 30th of a grain of the chloride is required Fig. 2 Green. Orange. Red. CB a A

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to render them visible. The spectra of the alkaline earths are equally definite though more complicated.

the chlorides, bromides, and iodides of the different The salts which are most readily volatilised, such as metals, give the most brilliant spectra. But it is only in the case of the alkalies and the alkaline earths that the spectra thus obtained are characteristic. Where the spectra of the other metals are required, recourse must be had to Wheatstone's method of taking electric sparks, between wires consisting of the metal of which the

spectrum is required, and the electric sparks may conveniently be procured by the employment of Ruhmkorff's coil. The temperature obtained in this way is very intense, and developes lines not produced by the heat of ordinary flames.

If photographs of these spectra be taken, I have found that the impression obtained in atmospheric air or in nitrogen is nearly the same whatever metal be employed; but if the spark be passed in hydrogen, or in carbonic acid, all the extra violet lines disappear, proving that these lines of the photograph were due not to the metal, but to the nitrogen of the air.

FIG. 3.

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When it is desired to render the lines produced by the spectra from different metals visible to a large audience, they may be shown by the employment of the voltaic battery. About forty pairs of Grove's construction will answer well. The wires of the battery must be connected with the carbon electrodes of a Duboscq's electric lamp. The metals to be burned are supported upon the lower or positive electrode made of the hard carbon deposited in the gas retorts; and when the salts of the metals are to be employed, two or three vertical holes are to be drilled into the upper end of the charcoal point, and into these the salt for experiment is introduced. On completing the connection with the battery, the arc is produced as usual. The general arrange-W ment of the apparatus is shown in Fig. 3. The light is allowed to escape from the lamp through a narrow vertical slit 8, of which a distinct image must be produced upon the white screen W W, destined to receive the spectrum at a distance of from fifteen to twenty feet from the lamp. When the arrangement is thus far completed, a hollow prism p1, filled with bisulphide of carbon, is interposed between the condenser C and the screen, and the lamp with the condensing lens is turned round until an image of the spectrum falls upon the screen, the prism being brought to the angle of minimum deviation, when the incident and refracted rays form equal angles with its faces. When this is properly adjusted, the bands peculiar to each spectrum are distinctly visible on the screen. If it be desired to obtain a longer image of the spectrum, this may be effected by making the refracted rays fall at the proper angle upon a second prism P, before they reach the screen.

From what has been already stated, it is obvious that the principal facts in relation to the occurrence of the bands of the

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spectrum were known before Kirchhoff and Bunsen directed their attention to the subject. But it has been invested with a new interest by the discovery of the new bases cæsia and rubidia, and particularly by a theory of Kirchhoff's which embraces and generalises the greater number of the phenomena, though it does not account for all of them. This theory we shall now consider.

It is to be remembered that the spectrum produced by the ignition of a solid or of a liquid always yields a continuous band of light, containing rays of all degrees of refrangibility within the range of its two extremes; but the same body, when converted into vapour, may produce a luminous atmosphere which may emit light of certain definite refrangibilities only, so as to produce a spectrum consisting of a series of bright bands of particular colours, separated from each other by intervals more or less completely dark. Bearing these facts in mind, the theory proposed to account for Fraunhofer's lines will be readily understood.

In 1858, Mr. Balfour Stewart published in the Edinburgh Phil. Trans., vol. xxii., a paper on the law of exchanges in radiant heat, and in the following year the subject was taken up by Kirchhoff, who arrived at the same conclusions as Mr. Stewart, independently; and the German philosopher extended his theory to the phenomena of light as well as those of heat. The conclusion at which he arrived may be thus stated,-That when any substance is heated or is rendered luminous, rays of a certain and definite degree of refrangibility are given out by it; whilst the same substance has also the power of absorbing rays of this identical refrangibility.

Sodium, for example, when ignited, emits an intensely brilliant yellow light, which is concentrated into two closely contiguous bands or bright lines coincident in position with Fraunhofer's double black line D in the solar spectrum. But if through the flame coloured by sodium, the more powerful light of the charcoal points or ignited lime be transmitted, the continuous spectrum due to this stronger source of light is interrupted by a black line coincident with the solar black line D. Kirchhoff has also ascertained that certain of the bright bands in the spectra of potassium, lithium, barium, and strontium, may in like manner be reversed, and I have found that some of the strongest lines in the blue in the spectrum of copper may be similarly reversed.

Now, Kirchhoff has applied these facts to the explanation of Fraunhofer's dark lines. He supposes that in the luminous atmosphere of the sun the vapours of various metals are present, each of which would give its characteristic system of bright lines; but behind this incandescent atmosphere containing metallic vapour is the still more intensely heated solid nucleus of the sun, which emits a brilliant continuous spectrum containing rays of all degrees of refrangibility. When the light of this intensely heated nucleus is transmitted through the incandescent photosphere of the sun, the bright lines which would be produced by the photosphere are reversed, and Fraunhofer's black lines are only the reversed bright lines which would be visible if the intensely heated nucleus were no longer there.

Kirchhoff has proceeded to test this theory by submitting the solar spectrum to a still more minute investigation than Fraunhofer, or even than Brewster and Gladstone, who have recently mapped more than two thousand of the dark lines. (Phil. Trans. 1860.)

The annexed diagram shows a small portion of Kirchhoff's detailed drawing, including the part of the specitrum extending from E to b (Fig. 4), and he states that

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for every bright line in the spectrum of iron is a corresponding black line in the solar spectrum. Seventy such lines occur between D and F, and in the small

PROCEEDINGS OF SOCIETIES.

portion contained in the figure there are seventeen such ROYAL INSTITUTION OF GREAT BRITAIN. lines indicated by the mark Fe. The strong lines near b, marked Mg, coincide with the brilliant green lines in the spectrum of magnesium. Chromium and nickel also give less distinctly marked lines, indicated in the case of chromium by the letters Cr.

Kirchhoff, from these and other more extended observations, draws the conclusion that in the atmosphere of the sun the vapours of sodium, potassium, magnesium, iron, chromium, and nickel are present; but that lithium, aluminum, zinc, cobalt, copper, and silver are not present.

Fascinating as this theory is, it must be remembered that it is yet upon its trial, and that it does not explain the facts at present known respecting the vapours of hydrogen, mercury, chlorine, bromine, iodine, and nitrogen. M. Morren even questions the accuracy of some of Kirchhoff's observations. Thus, he states that in a measurement which he made of the red band of potassium, conjointly with Plücker, they found that it did not correspond with the solar line A, but that it is considerably more refrangible. (CHEMICAL NEWS, Dec. 7, p. 302.) The present occasion, however, is not the most appropriate for discussing these and other points which still require careful experimental elucidation; yet it seems to be not unfitting at the present moment to put some ardent minds upon their guard; for it appears by some to have been hastily assumed, in a spirit of self-confidence, that we already have the key to everything upon this subject. A possible way to new truth has indeed been opened: here, doubtless, as in all other cases where we are permitted to obtain a further glimpse into the machinery of the universe, we shall but see fresh proofs of exhaustless wisdom on the part of the great Author of nature; and instead of boasting of our triumphs, we shall, if we pursue our researches in a right spirit, be taught a lesson of reverent humility as we are allowed to raise a fresh corner of the veil which separates that which is known to man from the infinitely greater portion which still lies beyond.

Royal Institution of Great Britain. General Monthly Meeting.-Monday, April 7, 1861.- William Pole, Esq., M.A., F.R.S., Treasurer and Vice-President, in the Chair. Alfred Denison, Esq.; Alexander Staveley Hill, Esq., D.C.L.; William Martin, Esq.; Daniel George Rees, Esq.; Augustus Thorne, Esq., F.R.G.S.; John Tyndall, Esq., F.R.S., were elected Members of the Royal Institution. Jonathan S. Crowley, Esq.; MajorGeneral Charles J. Green; Rev. G. Musgrave Musgrave; A. C. B. Neill, M.D.; Sir Joshua Rowe; E. H. Sieveking, Esq., were admitted Members,

A Course of Three Lectures on Spectrum Analysis, by Dr. H. E. Roscoe, Professor of Chemistry in Owens College, Manchester.

LECTURE I. (Saturday, March 29, 1862.) LADIES AND GENTLEMEN,-When I consented to give a course of three lectures on Spectrum Analysis, it was not without some misgivings as to how I should be able to fulfil my promise in spite of the great interest which these discoveries have awakened in this country, and the love which I feel for the subject and for its chief authors; because I felt that a course of lectures should be systematic and educational, and that I was about to bring before you a branch of science which from the nature of things, owing to the recent date of its birth and the enormous extent of its application, cannot at present be perfectly systematised. If, then, in bringing before you the glorious phenomena which present themselves in the science of spectrum analysis, we are not able to explain all that we see, if we find remarkable exceptions to the laws which we are able to develope, and if we notice that the facts appear disconnected, or even sometimes contradictory, we must remember that this page of the book of Nature has only just been opened, and that it would be presumptuous as well as foolish to expect that we should understand it in its full bearings at once. We must be content to proceed in these matters slowly but surely, knowing that each step brings us forward, and that, at last, the true beauty of the scene will be evident to all. Hence, then, if I appear abrupt in the treatment of the subject, or if my story should seem to be discontinuous, I must beg your in mind the saying quoted by Arago, that "clearness is kind indulgence; and I for my part shall endeavour to bear the politeness of those who speak in public," and like him I shall hope to prove not unpolite. (Applause.)

subject matter of our discourse this day, it will be well to Before we pass, ladies and gentlemen, to the chief review those important discoveries in the branches of science connected with our special subject, which have served as stepping-stones, as it were, by means of which we have reached our present position; and although these may be well known and familiar to some of you, yet I think it may be well to slightly mention the most important of them.

The most important discovery which opened the way to our present knowledge regarding the properties of that vivifying radiance which the sun pours down upon the earth, was made by Newton. In the year 1675, Newton presented to the Royal Society his wonderful and ever memorable treatise on Optics; and amongst the numerous important discoveries which were contained in those researches was one regarding the composition of solar

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light. Newton was the first to explain that the white solar light consisted of different coloured rays. He passed a beam of white solar light through a triangular piece of glass called a prism; and instead of the white point of light which he allowed to enter into his dark room, he saw on the opposite side of the wall a bright band, consisting of all the different colours, blending in beautiful harmony one with another, from red to violet, passing through all the intermediate stages of orange, green, and blue; and Newton showed-and this was the most important part, perhaps, of his discovery-that this splitting up of the white light by the prism could only be accomplished once; that a second prism did not split up this light again into another variety of coloured rays. He proved that if, for instance, the yellow light from the first spectrum be allowed to pass through an opening in a dark screen, and behind this screen another prism was placed, his yellow light was not again split up, but was shown on a further screen as yellow light. In fact, the second prism did nothing more than spread out again the light which was first obtained. Newton also showed that coloured light thus obtained, when brought together again, produced on the eye the sensation which we term whiteFor this purpose he passed the beam of white light through the opening in a shutter, obtained the spectrum by means of a prism, and then, looking at this spectrum through a second prism, instead of observing the coloured band, observed a spot of white light, thus showing that the whole of these differently coloured rays, when brought again together by means of the prism, produce on the eye the sensation of white light.

ness.

That white light can be thus split up by means of the prism, I can easily show you by the help of the electric lamp which, in Dr. Tyndall's hands, reads you here so many beautiful lessons. Now, in order to explain this coloured spectrum which we get from all white light, Sir David Brewster supposed that the solar spectrum was made up of three super-imposed monochromatic spectra, each having a specific colour, and each having its point of greatest intensity at a different position in the spectrum. Thus, for instance, Brewster supposed that the different colours in the solar spectrum were produced by the superimposition of a blue, yellow, and red spectrum. This, however, was proved by Helmholtz to be incorrect. Helmholtz proved that each particular ray has its particular colour, and that light of each degree of refrangibility is monochromatic; that we cannot separate, by any possible means, the green ray into a yellow ray and blue ray; that that ray is green and remains green, and is not made up of a mixture of yellow and blue. This he showed by experiment; and he likewise proved that the super-imposition of the yellow and the blue did not produce on the eye the effect of green.

Here we have the white electric light which is extremely intense-so much so that we can scarcely bear to look at it. By passing the beam of light through the two prisms which we have here, we split it up into differently coloured rays. This is the kind of effect which we observe, of course, with the white solar light. The differences between the white solar light and this light obained by the ignited carbon points, is the subject which I wish to bring before your attention shortly.

We see here that the red is the least refrangible portion of the spectrum; but we pass on through every gradation of red, yellow, green, and blue to the violet, or most refrangible rays. Now, just as we find rays possessing this peculiar colour-as we find a particular kind of green ray, a particular kind of red ray, and a particular kind of blue ray, so we likewise find that beyond the red and beyond the violet we have still rays present although they cease to affect the eye. This can be easily shown by placing a thermometer at the extremity of the visible red rays. We shall then see that beyond the red part which acts upon the eye we have really rays, because the thermometer

rises; and further on, beyond the violet, we shall find that we have rays capable of producing another kind of actioncapable of producing chemical action. Now, these heatproducing rays, and these chemically active rays, both of which exist in portions of the spectrum, invisible to the eye, do not essentially differ from the rays of light. There is no difference of kind between the heating rays, the chemically active rays, and the light-giving rays. We might suppose, and in fact it is the case, that I, for instance, was able to discern rays up to a particular point in the red, and my friend might be able to see further. The rays do not stop at a given interval where I cease to see them, it is only that my eye, or my retina, or my brain ceases at that point to be sensitive for them. The eye or the brain of some one else might probably see a great deal further. Therefore, we must dismiss from our minds any idea of an essential difference between the heatgiving, the light-producing, and the chemically active rays. They differ from one another only as the lightgiving rays differ from one another, viz., in wave length and in intensity of vibration.

If we now examine the solar spectrum with care, we find that the light which we have just seen does not represent exactly what we observe when we look at the solar spectrum; for we notice that the solar spectrum contains a number of dark bands-dark bands which intersect the differently-coloured portions of the solar spectrum, and show the absence of certain kinds of rays in the solar light. These lines were first noticed by Dr. Wollaston in the year 1802. He observed only seven of them, and he supposed that they divided the seven various colours of the spectrum, as they were then termed, into special parts. In the year 1814 the great German optician, Fraunhofer, examined this subject, and re-discovered these lines, which now go by, the name of "Fraunhofer's lines; since he investigated the matter very thoroughly, and used a much more perfect optical instrument than Wollaston possessed. He measured the position of these fixed lines on the solar spectrum, which we must remember are always present in sunlight. He measured the position, and drew a very beautiful map of these fixed dark lines in the solar spectrum. I can give you a representation of Fraunhofer's drawing of the dark lines in the solar spectrum by means of a photographic reduction from his original drawing in the Transactions of the Munich Academy" for 1815. This drawing is not coloured; it simply represents the different lines. Fraunhofer drew no less than 576 of these lines. He noted that the same lines always occur in the same portion of the solar spectrum; and he named these lines and measured their refractive indices; that is to say, their relative positions in the solar light. This line, which is a very important one, with which we shall have a. great deal to do in our future Lectures,-the line D, is in the orange; the line b is in the green; the line E is likewise in the green; the line F is just in the limit of green and blue, and the line G is in the blue; whilst the two double lines, which are called H, are in the violet, and are scarcely visible to the unassisted eye when looked at in the ordinary way. This curve on Fraunhofer's drawing shows the intensity of the light in the various portions of the spectrum.

Several physicists have examined this subject since Fraunhofer's time, and have drawn diagrams representing the lines in the solar spectrum. Amongst these, as the most recent, are those of Sir. David Brewster and Dr. Gladstone, of which I have here a diagram. Here you see a photographic reduction of the lines as drawn by Sir David Brewster and Dr. Gladstone. This top diagram represents the whole length of the solar spectrum; and you see that it is cut up into an enormous number of lines; several hundred certainly have been drawn in this way by Sir David Brewster and Dr. Gladstone. But the most complete drawing which we now possess of the lines in the solar spectrum are those recently made by Professor

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Royal Institution of Great Britain.

Kirchhoff. These have been made by the help of a very perfect and a very beautiful apparatus, a representation of which I can show you.

The sunlight was allowed to enter through the fine vertical slit which was placed at the end of the tube A. This tube was fixed on a horizontal iron plate c, and an object-glass was placed at the end of the tube A, so as to render the solar rays passing through the slit parallel as they entered the first refracting surface of the prisms. These prisms were made of the most perfect workmanship by Steinheil, the celebrated optician of Munich, and were placed on this horizontal iron table c with very great care, so that their refracting surfaces were all exactly perpendicular to the surface of the plate. They were then placed at the angle of minimum deviation of the light that was about to be observed, and arranged so that the

beam of white light on passing through these prisms was refracted or bent out so far that the whole length of the spectrum, drawn upon the scale which Kirchhoff saw in this instance, would be twenty feet. Kirchhoff, looking through the eye-piece of the telescope B, saw what he describes to be-and I can, from my own personal knowledge, bear witness to it-one of the most magnificent sights that he ever beheld; and he obtained in this way the lines with a degree of distinctness and a degree of accuracy which certainly had never been attained before. By means of a micrometer screw D, attached to this telescope, it can be moved round the axis e, and the cross wires in the eye-piece can thus be brought from line to line. In this manner Kirchhoff measured in a portion of the spectrum the positions of the dark lines which it contains, going from line to line, just as a carpenter measures a room with a two-foot rule. Making his measurements in this way, Kirchhoff drew representations of what he saw, and I can show you on the screen a photographic reduction of these interesting drawings.

The first one which we will take is the one beginning with the line D, that line which, as I mentioned to you, is contained in the orange portion of the spectrum, and which we shall have to refer to again and again. There are the lines D. You will notice that it is no longer a single line, but that it is split up into a double dark line; and so beautiful was the instrument which Kirchhoff used, that he could notice between these two black lines another less distinct line, which, however, we see here. In order to obtain a mode of reference for the positions of the various dark lines thus seen, Kirchhoff drew a scale divided into millimètres, and under this scale he drew his dark lines, taking any one as

(CHEMICAL NEWS, April 19, 1862.

an arbitrary starting-point. Now you will notice that these lines possess different degrees of blackness. This indicates the variety of shade or of distinctness which these lines exhibit in the spectrum. This effect Kirchhoff produced in his drawing by means of differently-shaded lines. He used six differently-tinted inks; and with these he drew lines of different thicknesses; so that in this photograph we have a very fair representation of the variation in intensity which these lines exhibit. I do not call your attention for the present to the lines which are drawn below; of these we shall have to speak more especially on a future occasion. It is with these upper set of lines that we have at present to do-the dark lines which occur in the solar spectrum. This portion of the spectrum you see here is placed under the other, in order to facilitate the impression; for these drawings are chromo-lithographs, if one may call them so, or tinto-lithographs; they were printed from Kirchhoff's own drawing, on six different stones, with six differently-shaded inks; and thus a chromo-lithograph with different shades of the same colour was obtained; and in order to facilitate the printing, instead of the spectrum being altogether in one line, it was drawn in two sheets, a quarter of that portion of the spectrum which had been examined being put in each length. We have, then, four lengths. Here you see two of them, and here you see the other two lengths. Now you notice that at the right hand corner, at the top, those two very dark lines are the lines to which I called your attention in Fraunhofer's diagram, b. The line in the middle of the lower half is the line F which we noticed occur in the portion just bordering on the blue.

These dark lines which I have now shown you, exhibit only one-third of the total length of the spectrum. I cannot look at these beautiful drawings, in which the more one looks at them, the more one finds to admire, without some feelings of regret, for I know at what cost to the talented observer these lines were marked and these beautiful drawings were made. By the constant and trying observation which the subject rendered necessary, Professor Kirchhoff has, I am sorry to say, permanently injured his eyesight, so that for the present, at any rate, he is perfectly unable to continue these glorious investigations which he has so well opened out.

A full description of Professor Kirchhoff's interesting researches, including the plates of the dark solar lines, has just been published by Messrs. Macmillan, of Henrietta Street.

Whilst we are on this subject, I wish to point out to you that these dark lines not only exist in the visible portions of the spectrum, but also exist in the portions which contain the heating rays and which contain the chemical rays. I cannot show you any of the lines which occur in the ultra-red portion of the spectrum; but I can show you those which occur in the ultra-violet. Thanks to the beautiful researches of Professor Stokes on fluorescence, these lines have become perfectly well known, and I can show you a diagram of some of the lines which occur in the ultra-violet portion of the spectrum. These lines, H, are those we noticed in Fraunhofer's diagram as being in the violet. When we allow the solar rays to pass through a quartz prism, we find that rays pass through to which glass is perfectly opaque. The diagram is a copy of the effect produced on a film of sensitive collodion which was exposed to the action of these ultra-violet rays passing through quartz prisms. The white spaces indicate the position in which there is no light; they are the Fraunhofer lines in the ultra-violet sunlight. You see here that the lines stretch out a long way beyond the visible portion of the spectrum, or that to which the eye is ordinarily sensitive.

I will call your attention now, ladies and gentlemen, to the portion of the spectrum on which Professor Kirchhoff has shown us those beautiful lines. This being a representation of the whole of the solar spectrum, those lines,

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