THE PRISM IN QUALITATIVE ANALYSIS. VI.-On the use of the Prism in qualitative analysis. BY DR. J H. GLADSTONE, F.R.S. THE ordinary methods of qualitative analysis depend on a system of exclusion. We first determine that the substance analysed does not contain any member of certain large groups but consists of one or more members of certain other groups and these we sub-divide excluding one division after another till we arrive at the individual member or members and these me at length distinctly recognized by special characteristics or perhaps special tests. In this process the indications of colour have hitherto played but a very subordinate part. They have indeed been esteemed most valuable when we come to special charac- teristics or tests but in any previous stage of the process they are not relied on.Thus to take an instance :-if in mineral analysis we meet with a metal not precipitable by hydrochloric or hydro-sulphuric acids but thrown down by sulphide of ammonium as well as by ammonia or potash as a gelatinous oxide soluble in the fixed alkalis the green colour of the oxide at once confirms the conclusion that the metal is chromium; but if we see an unknown salt or mixture of salts of a similar green colour we do not conclude that it contains cliromium nor if we fail to recognize that colour do we necessarily infer that chromium is absent. We know indeed on the one hand tliat there are many other green salts and on the other hand we speak of some of the salts of chromium as not green but red or blue.Now this arises in a great measure from merely observing the colour as it appears to the untutored and unaided eye. It should be remembered that the colour of an object is the resultant of the various rays of the spectrum which it reflects or transmits or perhaps more philosophically of the various rays which it does not absorb. Yet that colour may be produccd by the union of totally different rays ; thus-one substance may appear indigo because it reflects the indigo ray another because it reflects the blue with a little red. This can evidently be determined only by the prism; and in examining the coloured solutions of various salts by that instru- ment I have been led to the conviction that chromatic phenomena may be relied on in analysis to an extent which I think has been GLADSTONE ON THE USE OF rarely if ever suspected.There is this advantage too in such a mode of determining the character of a body that not a particle of it need be used up in the analysis nor altered or destroyed by the addition of reagents-a matter of no slight importance when we have to deal with organic compounds or valuable mineral substances. Mr Pesr sall and perhaps others have used the prism with success in particular cases ; Professor Stokes has claimed for it a place in the laboratory;* and it is to draw the attention of chemists still more decidedly to this mode of analysis and to lay down some gerieralized observations that I have written the present paper. The use of the prism is not difficult.Many different methods may be pursued. Professor Stokes examines the chromatic effect of some metallic oxides by fusing them with a bead of micro- cosmic salt and ‘‘viewing 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.”Jy In examining a powder or small crystal I have found the following plan answer :-A plate of glass is held in the sun-light in such a position that few if any rays are reflected by its surface to the eye while through it is seen a piece of black cloth or paper in the shade. On the glass is then placed a thin line of the powder or a small crystal or two which mill of course appear brightly illuminated against a dark ground.By viewing this through a prism the line of powder will be expanded into a ribbon or the crystal into a broad image which by its varied colours will show what rays are reflected. Another way of examining a solid substance is by looking at it with a prism through n slit in a black card. But these are inelegant and inexact methods of observation as com- pared with what is practicable when liquids are examined We have then to deal especially with transmitted light. The common method is to view by a prism a slit in the window shutter seen through a glass vessel containing the liquid; but this method generally fails to give intelligible results unless attention be paid to the strength of the solution and the thiclmess of the stratum * Since the above was written my attention has been called to the fact that Mr.Talbot (Ed. Journ. Sci. v 77 and Phil Mag. iv 114 and ix 3) and Mr. Crookea have recommended the prism as a means of distinguishing the different flames given by metallic salts; but this plan of procedure though doubtless very accurate in certain cases is of limited and. difficnlt application. -i. Phil. Trans. 1852 p. 522. THE PRISM IN QUALITATIVE ANALYSIS. through which the ray passes ;for in most cases the different rays of light are not absorbed immediately on their entering a liquid but when they have penetrated it to a greater or less distance and they disappear not suddenly but by becoming gradually fainter and fainter. Hence for holding the solution I am in the habit of employing not a vessel with parallel sides but a hollow wedge of glass held in such a position that the slit in the window-shutter is seen traversing various thicknesses of the liquid from the thinnest possible film to a stratum of perhaps three-quarters of an inch in thickness.This line of coloured light does not then appear uniform as will be at once foreseen it varies in intensity from almost pure white to a deep shade ;and it frequently displays variations of a much more unexpected character being of a totally different coloiir in one part to what it is in the other ;for instance blue at one end of the line and red at the other every shade of purple intervening. When this colourcd line of light is analysed by a prism very remarkable appearances often present themselves the whitish portion of‘ the line where the light traverses very little liquid will be expanded into a ribbon differing but little from the spectrum given by unaltered light; but as the line is viewed through deeper and deeper portions of liquid some rays are seen to diminish in intensity others gradually to die out while others again almost immediately disappear giving place to perfect darkness.If the prism be a tolerably good one the slit narrow and the day not too dull the most visible of “Fraunhofer‘s lines” will be readily seen running like black threads along this coloured ribbon and serving to mark with perfect accuracy what rays are transmitted and what absorbed. The appearances thus presented to the eye can be easily transferred to paper thus giving at one glance the optical character of the rays which penetrate every different thickness of the solution.From the numerous observations I have made in this manner it may be laid down as a general rule that aZl the compounds of a partieular base or acid have the Same efect on the rays of light. Occasionally but very rarely a glaring exception occurs ;and much more frequently there are certain variations in degree-the same rays being absorbed but to a slightly different extent as compared with one another. This truth has been partially recognized for a long time thus every chemist knows that the salts of nickel are green and those of zinc colourless ;but there are other cases in which it is not so a GLADSTONE ON THE USE OE evident-in fact where apparent exccptions occur.Some of these anomalies are here discussed. Chromium XaZts,-It has been already stated that some salts of chromium are called not green as the generality are but red or blue. This remark applies to both solid and dissolved salts. Yet when these various-coloured solutions are examined in the way described above they all give the same or very nearly the same spectrum and that a very characteristic one. See Fig. 1. The thinnest stratum of liquid suffices to cut off all the yellow rays and nearly all the indigo and violet as the thickness increases the transmitted light appears more and more con-centrated about two points the one at the least refrangible end of the spectrum the other between the lines F and b.It is there of a bluish green tint and is at first supported on either side (if such an expression may be allowed) by blue and by yellowish green which however soon disappear leaving it also gradually to be absorbed while the red which was at first accompanied by orange passes through the deep liquid alone and with almost undiminished brilliancy. The result of this great penetrating power of the red ray as compared with the green and blue is that the chloride nitrate sulphate sulphocyanide and other salts of chromium which appear green in tolerably dilute solution appear red when we look through a strong or a very deep solution. This circumstance has been noticed by Schretter and others and has been explained by Sir John Herschel,* who gives the name of dichromatism to the phenomenon.Now the acetate though it absorbs the same rays as the above-mentioned salts does not transmit the green so readily; hence it appears usually red and it is only in very thin or weak solution that it assumes the green appearance while the I‘ red potassio-oxalate ” absorbs the green so speedily that that colour never preponderates and the thinnest stratum never approaches it nearer than a bluish red. Yet these present a pris- matic appearance scarcely distinguishable from that of the salts previously described. Again there are two apparently isomeric modifications of some chromium salts which differ in colour as well as in chemical properties.Thus it is the blue and not the green sul- phate,? that combines with sulphate of potash to form potash * Ed. Phil. Trans. Vol. ix. Berzelius and Bchretter have both proved this. THE PRISN IN QUALITATIVE ANALYSIS. chrome alum but this double salt though it appears so unlike one of the green compounds to the unaided eye gives the same peculiar prismatic appearance. The blue potassio-oxalate,” also though the thinnest stratum of its solution is a faint bluish purple exhibits the same two maxima of transmission the only perceptible difference being that the blue space near F if; very brightly illu- minated. An easy method of observing these two modifications is to take an ordinary green solution of nitrate of chromium divide it into two parts and boil one of them for a few minutes with an excess of nitric acid.When cool again the contrast between the two by daylight will be very great the one a deep green the other a bluish purple; but if examined in the way described above they will present very nearly the same prismatic appearance the only difference being that the acid solution transmits more light at least more of the red and blue rays. Thus the various chromium salts though optically very different to the unaided eye present almost exactly the same appearance and that a very characteristic one when examined in the manner suggested. Cobalt Salts.-We are accustomed to speak of blue and red salts of cobalt; but this difference depends really on the state of hydration and the prism reveals an analogy otherwise unsuspected between the two colours.The chloride and other salts when anhydrous are blue. Alco-holic solutions of the chloride and acetate are also blue. The chloride and other salts when hydrated whether in the crys- tallized condition or actually dissolved in water are red. The only exception to this with which I am acquainted is the sulphocya- nide which crystallizes of a magnificent blue and gives a saturated aqueous solution also of a blue colour. The addition of more water however gradually changes this to the same red as any other solution of a cobalt salt ; while the subsequent addition of alcohol restores the blue colour to a greater or less degree. A blue salt of cobalt such as smalt blue glass or an alcoholic solution of the chloride is dichromatic and if the light reaching the eye has traversed a sufficiently large quantity it appears red.The alcoholic solution examined in the manner described above exhibits a very remarkable appearance. It is represented in Fig. 10. The extreme red ray pefietrates with the greatest facility indeed it is much more visible where the stratum is thick than where it is thin because in the latter case the neighbouring orange G2 GLADSTONE ON THE USE OF by its great luminosity renders it almost imperceptible. Imme-diately succeeding this narrow red ray is a thin space of perfect absorption followed by auother red ray which penetrates a short distance and by the orange which penetrates a little farther.Then coincident with the line D but considerably broader is a space of absolute blackness while the rays between it and E advance far into the liquid yellow and bright green. The suc- ceeding rays till near F are greatly reduced in brilliancy on their first entering the liquid yet they succeed in penetrating it to a considerable distance as a faint bluish green. The blue rays are admitted very readily the indigo still more so and the violet advances further yet into the liquid the lines F d and G being very discernible. A red solution of a cobalt salt gives the appearance represented in Fig. 11. The red and orange rays are transmitted with the greatest ease the line D is strongly marked the yellow penetrates not quite so far while the rays between E and F are very faint though they are not soon wholly absorbed ;the blue passes readily and the indigo and violet show a constantly increasing power of penetration.The lines d and G are very distinct. On comparing the prismatic appearances of these two classes of cobalt salts it will be at once perceived that both the anhydrous and the hydrated salts transmit the extreme rays of each end of the spectrum most freely. They are analogous also in the faint greenish blue which penetrates for some distance though with but feeble luminosity between E and F. Through a strong and deep solution of either the red ray alone passes. There are how- ever some equally well marked points of contrast the orange ray is almost immediately absorbed by the blue salt but is transmitted with the greatest facility by the red; and the maximum of absorp- tion is in the one case about B and in the other about F.Copper Salts.-Anhydrous copper salts present a variety of colours hydrated crystals or solutions are generally blue but sometimes green; yet the green salts themselves become blue when dissolved in a sufficient amount of water. Alcoholic solutions not merely of the green chloride but of the blue acetate are green. When examined by the prism they give the appearance delineated in Fig. 5. A thin stratum even of a very dilute solution cuts off at once the least refrangible red rays and the whole of the more refrangible half of the spectrum. The rays from a little before D to a little before F are transmitted THE PRISM IN QUALITATIVE ANALYSIS.and penetrate indeed with almost undiminished luminosity through a very great thickness of solution appearing to the eye of an almost uniform bright green colour. At first they are accompanied on either side by a little red and blue but these are soon absorbed. This description applies equally to a saturated aqueous solution of the chloride. The blue salts of copper and all aqueous solutions if sufficiently diluted give an appearance not differing widely from the normal spectrum. The rays about the blue portion are admitted very freely while those at each end are partially but not wholly absorbed. They differ considerably in the extent to which these other rays are suppressed; the consequence of which is that solu- tions of some salts for instance the acetate appear of a much deeper blue than solutions of other copper salts though of cor- responding strength.Fig. 13 is taken from the acetate. Most if not all double salts of copper (as for instance the double sulphate of copper and potash or ammonia,) are nearly identical with the simple salt both in the eharacter and intensity of thecolour. The true ammoniacal salts of copper which appear so intensely blue must be distinguished from these as they appear to contain the copper in some state of combination with the elements of ammonia and they give a very different prismatic appearance. The maximum of absorption is about .D,the red and the green rays are transmitted a short distance while the indigo violet and still more refrangible invisible rays are suffered to pass most freely.Ferric Xalts.-Au examination of the various ferric salts is very instructive in respect to the limits within which there may be diversity in the action of different compounds of the same base on the rays of light. It is well known that they are for the most part red yet there is an immense difference in the degree of redness exhibited by different salts when dissolved. Thus the nitrate is almost colourless unless in strong solution; the acetate is vastly redder; while the meconate and sulphocyanide are most intense in their colour. Yet the ferric chloride is orange-yellow and a solution of the citrate sometimes appears green. In all these cases however there is an absorption more or less complete of all the more refrangible rays leaving the least refrangible to penetrate almost any thickness withoiit much diminution.This great transmissibility of the red ray causes GLADSTONE ON THE USE OF those salts which appear of another colour in dilute solution to be dichromatic. On closer inspection several groups of ferric salts may be dis- tinguished ; all of which however agree in this permanence of the extreme red. 1. Those ferric salts which are red at any thickness for instance the sulphocyanide. An extremely thin film of a strong aqueous or alcoholic solution of this compound permits all the rays of the spectrum to pass; but suddenly all are cut off except those less refrangible than the line D and after awhile the extreme red alone penetrates.See Fig. 14. If a weaker solution of ferric sulpho-cyanide be examined there will be observed a tendency of the blue ray to manifest itself ;for this salt as is well known is altered by the addition of water. See Fig. 15. 2. Solutions of the ferric chloride and citrate give the same prismatic spectrum at least as far as configuration is concerned though they appear very different to the unaided eye. The chloride is of an orange-yellow and whether the solution be strong or weak deep or shallow it appears of very nearly the same tint and depth of shade. The citrate on the contrary if little be presented to the eye at once is green; if more it is warm brown; if still more red. In each case the prism shows that all the rays from the least refrangible to E are freely transmitted ;at b the blue begins to fringe the green but it is soon absorbed; while every ray beyond that is cut off absolutely the violet by even a thin stratum of a very dilute solution.See Fig. 6. There is a difference as will be anticipated between the intensity of the rays transmitted by these two salts the chloride admits the orange and yellow to almost any distance with apparently undi- minished luminosity ; while the citrate transmits the green rays subduing the intensity of the others to such a degree that the yellow space at least appears green also. 3. The compounds of ferric oxide with meconic acid and its derivatives form a very distinct group. The meconate itself is dichromatic appearing red en masse but of a peach-blossom colour when in a very thin stratum.The prismatic appearance is given in Fig. 16. The red and orange rays are transmitted freely the yellow and green are rather soon absorbed; about F no ray mhat-ever enters the solution but the more refrangible are admitted gradually fade mid disappear all at about thc same distance and THE PRISM IN QUALITATIVE ANALYSIS. a7 rather abruptly. The comenute of iron gives a prismatic appear- ance which is almost identical see Fig. 17; but the more refrangible half of the spectrum is not so luminous I think; and the indigo ray penetrates farther than its neighbours. The maximum of absorption is about half may between b and F. The ferric pyromeconate appeared indistinguishable by the prism from the comenate.There are two compounds of comenamic acid and ferric oxide the one affording an intense red the other a still more intense purple solution. Yet different as these are to the unaided eye they give the same configuration of spectrum when examined with a prism and this configuration differs from that of the comenate only in the same manner as the latter differs from that of the meconate. It is represented in Fig. 18. The acid comelaamate which is red transmits the red orange green and blue as its congeners do but allows the violet to penetrate farther even than the indigo. The basic cornenamate,which is blue-purple in thin and red-purple in thick stratum transmits the same rays as the acid salt; but the luminosity of the more refrangible half of the prismatic appearance is much greater and it penetrates farther.The ferric gallate ferrocyanide and ferridcyanide are perfectly different to other salts of the same base in their action on the rays of light. Chrornates,-It is well known that chromic acid and its com-pounds are all highly coloured but that the colour is not the same in dl cases; yet the prism while it indicates a perfect differ-ence between the compounds of chromic oxide and those of chromic acid exhibits a close analogy between the various chromates. Thus bichromate of potash is red while the neutral chromate is yellow; but a glance at Figs. 19 20 will show that their prismatic appearances bear a certain analogy.The red salt in thin stratum permits the least refrangible half of the spectrum to pass but cuts off at once every ray more refrangible than F; as the stratum of liquid increases it absorbs the slight amount of blue and thc red also until nothing is visible except a bright band of orange and yellowish green extending equal distances on each side of D but not quite including either Bor E. The yellow salt admits a somewhat wider spectrum as seen through the hollow wedge it presents an uniform band of light extending the whole length and consisting of the ordinary spectrum as corn- GLADSTONE ON THE USE OF prized between A and F; only when it is very thin a little more bhie and red appear. Both chromic acid and bichromate of silver though red in the solid state give orange-yellow solutions and prismatic appearances alnnost identical with that presented by yellow chromate of potash.Litmus.-As an instance from the organic world litmus may be selected. Every tyro in chemistry is familiar with the fact that this substance is blue when neutral more blue when alkaline red when acid and of a peculiar wine-red colour when affected by either carbonic or boracic acid. Now the prismatic appearances presented by these differently coloured kinds of litmus are only modifications of a common type. If a neutral solution of litmus be placed in the hollow wedge its dichromatic character becomes at once evident the thin por-tion appears bhne the thick portion red while every shade of purple intervenes.If a line Qf light passing through these various thick- nesses be examined by a prism it shows that the red penetrates any distance almost unchanged that the orange is more readily absorbed that the rays a little less refrangible than D are absolutely stopped that the green is transmitted to a considerable distance and is very luminous that the blue is transmitted not quite so far and the indigo and violet are still more quickly absorbed. See Fig. 21. If to this solution of litmus an alkali whether fixed or volatile be added the passage of the red yellow green and violet rays is little if at all affected but the liquid becomes more opaque to the orange ray and far more transparent to the blue and indigo. See Fig. 22. The extent to which this alteration takes place depends on the amount of alkali present.If to the solution of neutral litmus boracic acid be added it manifests its presence by causing the indigo and violet to be more freely transmitted than even the blue but besides this it produces little change. See Fig. 23. If however an ordinary acid be mixed with the litmus a more exten- sive alteration is effected the maximum of absorption takes place not in the yellow but in the bluish green space about midway between 6 and F the red and orange rays are transmitted with the greatest readiness the yellow less rendily the green is speedily absorbed while the blue indigo and violet though faint penetrate some distance into the liquid. See Fig. 24. The transparency of this solution to the more refrangible part of the spectrum depends THE PRISM IN QUALITATIVE ANALYSIS.on its degree of acidity; indeed if very slightly acid a thin stratum of litmus appears of a light bluish purple. Salts each constituent of which is co1oured.-When two bodies combine each of which exerts an influence and a different influence on the rays of the spectrum it might be expected at the first thought that the resulting colour would be the colour of the first constituent plus the colour of the second constituent ;but a moment’s reflection will show that this cannot be the one con- stituent mill absorb certain rays and the other certain other rays and the salt itself will transmit only those rays which are not absorbed by either or in other words those rays which are trans- mitted by both.Thus the resulting colour may sometimes to the unaided eye bear no kind of relation to the original colours. As an instance acid chromate of chromium may be taken. This salt compounded of two substances which give respectively yellow and green solutions is not a bright green but a brownish red yet this is just what theory requires as will be evident on noticing the rays not absorbed by either Fig. 1 or 19. And even where the combination of the two substances does give the reputed resultant of the two original colours as in the chromate of copper (Fig. 9) which is yellowish green the resulting colour really does not con- tain the whole of either of the original prominent tints but is due to the rays that lie between them and are common to both.Compare Figs. 13 and 19. This persistence of the chromatic effect of substances with whatever they are combined renders it sometimes impossible to decide whether two coloured constituents have combined or not; thus on mixing permanganate of potash with ferric chloride we still see the very remarkable series of bands that characterizes the first of these solutions (Fig. 12) and the complete absorption of the blue caused by the second (Fig. 6) but there is nothing in this circum- stance either to prove or disprove the formation of permanganate of iron. Nor does the fact that the mixture of the blue perman- ganate with the yellow ferric salt has produced a red solution afford any indication of a chemical change; that colour might be and indeed mas anticipated from a consideration of what rays were absorbed by each.Indeed if blue sulphindigotate of potash be mixed with yellow chromate of potash the resulting colour is likewise red and not green although certainly no combination has taken place between them. How easily might a chemist be GLADSTONE ON THE USE OF misled and suppose that the unexpected colour indicated the production of a new substance ! This examination of apparent exceptions to the general rule that “all the compounds of a particular base or acid have the same effect on the rays of light,” shows that even in these cases the rule still holds good or at least that the differences are only modi- fications of a common type.The following conclusions of practical value in analysis may be drawn 1. When the light transmitted by a coloured solution of unknown composition is examined by a prism little can be inferred from the fact that a particular ray is absorbed unless indeed we happen to recognize some of those very peculiar black bands which characterize certain bodies as the permanganates ; but from the fact of a particular ray being transmitted we may con-clude as almost certain that none of those bodies which in ordinary combination absorb that ray are there present in any kind of combination. 2. Beside this negative inference we may frequently arrive at a positive conclusion. It is a rare circumstance that the mere colour of a solution will inform even the most experienced eye of what it is that imparts the colour; but directly it is examined by the hollow wedge and prism some familiar spectral appearance may be recognized which cannot be mistaken and is at once dis- tinctive.Thus suppose we have an inorganic salt which gives a green solution it may be a compound of nickel or of protoxide of iron or of uranium of sesquioxide of chromium or possibly of copper of ferric oxide or of the protoxide of molybdenum or it may be a ferridcyanide or a compound salt such as chromate of copper. True these greens are not all alike to the unaided eye but the differences of their character are not easily described or remem-bered ;but let the prism be applied to them and they are at once distinguished and that without losing a drop of the solution.The different appearances are represented side by side in the plate viz. Figs. 1to 9. Fig. 1. A salt of sesquioxide of chromium in weak solution. Already described. Fig. 2. A salt of nickeI. A spectrum in which the extreme ends the red and violet are soon cut off while the whole middle porticn is transmitted without change. THE PRISM IN QUALITATIVE ANALYSIS. Fig. 3. A protosalt of iron after being exposed for a short time to the atmosphere. The pale bluish green solution of a pure ferrous salt admits all the rays freely but the most refrangible are reduced in intensity. Fig. 4. A protosalt of uranium. These remarkable absorption bands were pointed out first by Stokes. Fig. 5.Green chloride of copper. Already described. Fig. 6. Citrate of the sesquioxide of iron in dilute solution. Already described. It differs from the copper salt principally in the constant presence of the red ray. Fig. 7. Green chloride of molybdenum. Similar to the chloride of copper but transmitting red for some distance and a larger amount of blue. Fig. 8. A ferridcyanide. The potassium-salt (the only one examined) appears green except in very deep solution when it is red the prismatic appearance of thin strata resembles that of citrate of iron but afterwards absorption takes place of the rays just beyond D,and those more refrangible are gradually absorbed afterwards till at last the rays between E and b alone penetrate; then these disappear and the spectrum between A and D passes onwards alone.Fig. 9. Chromate of copper. The yellow and green are con- stantly transmitted ; the red and orange are gradually absorbed; the more refrangible half of the spectrum is cut off at once. These observations Till 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 inves- tigation. What 1 have here marked down must be considered rather as a tentative inquiry than as a really valuable contribution to our knowledge of the effect oi 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 obser-vations as would furnish data for regular tables of comparison and the prism would then take its place as the blow-pipe does now among the recognized and almost indispensable instruments of the analytical laboratory.