XLVIII. On the Colour of Salts in Solution, each constituent of which is coloured. By J. H. GLADSTONE, Ph.D., F.R.S. &c.* [With a Plate.]

FROM

ROM the general rule, that a particular base or acid has the same effect on the rays of light, with whatever it may be combined in aqueous solution, it may be inferred that when two bodies combine, each of which has a different influence on the rays of light, a solution of the salt itself will transmit only those rays which are not absorbed by either, or in other words, those which are transmitted by both. Thus if a red acid and a blue base combine, the resulting salt will certainly not be purple, but it may present the colour of some ray intermediate between red and blue-perhaps green.

This, indeed, was laid down by me as a general rule in a paper read before the Chemical Society, "On the Use of the Prism in Qualitative Analysis+;" but only one instance was given, namely chromate of copper. Since then I have examined several salts of the character above described, and many haloid salts, which have led to some unexpected, and I think suggestive results.

The method employed was that described in the paper already referred to. It is briefly as follows:-The solution to be examined is placed in a hollow wedge of glass, which is interposed between the eye of the spectator and a narrow slit in the windowshutter, in such a manner that the thin line of light is seen traversing the different thicknesses of liquid. This line of light is then analysed by placing a good prism between the hollow wedge and the eye. In this way it is seen at once what rays are absorbed by increasing thicknesses of the solution. The diagrams in Plate II. give the appearances thus presented; the lowest portion representing the prismatic spectrum as seen through the thinnest possible stratum of liquid, the higher portions showing the gradual absorption of different rays. The diagrams are not coloured, but the fixed lines will indicate with far greater accuracy than colour would, the different parts of the spectrum transmitted. The least refracted, that is, the red ray, is always to the right hand; then follow of course the orange, yellow, green and blue, while the violet ray is on the extreme Ïeft +.

* Communicated by the Author; having been read at the Meeting of the British Association at Dublin, 1857.

† Quart. Journ. Chem. Soc. vol. x. p. 79.

Since writing the above, my attention has been directed by the Abbé Moigno to a description contained in his Répertoire d'Optique Moderne, 3 partie, p. 1258, of some experiments by J. Müller, in which he has represented the transmission of light through varying thicknesses of a

CHROMATES.

Chromic acid in solution, and its combinations with colourless bases, such as potash, cut off instantly the more refrangible half of the spectrum, admitting only the blue rays near F for a short distance; but they transmit the less refrangible half perfectly, See fig. 1. Plate II. The red bi-salts, as those of potash or baryta, absorb the green in the neighbourhood of F likewise, and a little red. Chromates were prepared by saturating the acid with the hydrated oxide or carbonate of the base.

Chromate of Copper is a green salt. The spectrum presented by it is represented in fig. 5, where evidently the absorption of the red ray is due to the base, that of the blue and violet to the acid. Compare figs. 1 and 9.

Chromate of Nickel is of a yellowish-green, and presents nearly the same spectrum as the copper salt, as might be anticipated on comparing figs. 1 and 10.

Chromate of Ferric Oxide is orange passing into red as the depth increases. It transmits only the red, orange, and yellow rays, which a comparison of figs. 1 and 4 will show to be transmitted in common by both constituents.

Chromate of Uranium is yellow, like any other salt of that base; but its prismatic appearance is totally different, the chromic acid having cut off all the remarkable luminous bands beyond F. Compare figs. 1 and 2.

Chromate of Chromium is of a reddish-brown colour. The rays transmitted by it are represented in fig. 7, from which it will be seen that the whole of the blue or green maximum of an ordinary chromium salt (see fig. 6) is cut off by the acid; but an absorption of light between D and E does not occur to such an extent as might be expected from the presence of chromic oxide. From the chemical reactions of this body, however, I have some doubt about its right to be considered a salt.

PERMANGANATES.

Permanganate of potash gives the very characteristic spectrum represented in fig. 3.

Permanganate of Uranium gives a purple solution. Its prisliquid by diagrams similar to my own. These experiments are an important step in the history of photo-chemical research, and would have been noticed by me in my previous paper had I been aware of their existence. M. Müller's mode of representing the optical effects produced by the transmission of light through coloured bodies is an improvement on the original attempt of Sir John Herschel, and should give precisely the same figures as my method; yet he arrives only, after a series of observations in each case, at the same result, which I, by using the hollow wedge, render apparent to the eye at once. His observations agree closely with mine; but they are very few in number, and have not led him to any of the generalizations deduced in my previous or my present paper.

matic spectrum shows the alternate bands of light and darkness due to the permanganic acid; but I did not recognize those due to uranium in the blue space. This, however, is not to be wondered at, since the permanganates admit very little blue light, and the uranium bands never show themselves till the light has traversed a considerable amount of salt. See figs. 2 and 3.

CARBAZOTATES.

Carbazotic acid gives a prismatic spectrum almost identical with that of chromic acid, but the line of distinction between the transmitted and the absorbed portions is about midway between b and F.

Carbazotate of Copper is a yellow salt, but its spectrum differs from that of carbazotic acid by the gradual absorption of the red ray-the universal effect of copper in solution.

Haloid Salts.

Chlorine, bromine, and iodine are, as all know, highly-coloured bodies; and their solutions in water are also coloured. Yet the acids which result from their combinations with hydrogen are colourless; and their compounds with most metals, including all those of the alkalies and earths, are colourless likewise, whether in the solid or the dissolved condition. Yet, as will be seen from the following observations, the peculiar effect exerted on light by the dissolved halogen itself appears also in its compounds with gold, platinum, and palladium: it appears, too, in its compounds with copper, nickel, and analogous metals; but in these cases only when the solution is very strong, the chromatic effect of the halogen wholly disappearing as water is added.

BROMIDES.

Bromine-water is red, and gives the prismatic appearance of fig. 13. The light transmitted near F is very dull.

Terbromide of Gold is intensely red in solution, and gives a spectrum identical with that of bromine. On comparing fig. 15, it will be seen that a compound of gold with a colourless acid transmits all the rays transmissible by bromine-water. The gold salt figured is the nitrate.

Bibromide of Platinum is intensely red, and likewise gives a spectrum identical with that of bromine. Fig. 16 will show that the remark made above in reference to the gold salt, applies here likewise.

Bromide of Palladium and Potassium also gives the ordinary bromine spectrum.

Bromide of Copper gives a saturated solution of a deep green colour; it transmits the spectrum represented in fig. 17, which

is precisely similar to that of bromine, except that the red ray is gradually cut off, as with other copper salts. On diluting this solution a complete change of colour ensues; it acquires the blue colour and the prismatic appearance of a compound of oxide of copper with a colourless acid, as represented in fig. 9.

Oxybromide of Iron is intensely red. It gives the same prismatic spectrum as bromine-water does.

CHLORIDES.

Chlorine gas absorbs the more refrangible rays; chlorine-water, though yellow, cannot be obtained strong enough, even at a temperature of nearly 0° C., to show any very decided effect on the spectrum; nothing, in fact, beyond a diminution in the intensity of the violet. Yet in some of its compounds its absorbent power is well marked.

Terchloride of Gold gives a yellow solution, and absorbs more blue than the yellowish-green nitrate does. Compare figs. 19 and 15. The acid hydrochlorate of this salt is indistinguishable from it by the prism, though to the unaided eye it appears of a somewhat purer yellow.

Bichloride of Platinum transmits scarcely any rays more refrangible than b, unless the stratum be very thin, when a little blue and violet pass. See fig. 20. The sulphate, fig. 16, admits more rays, especially in the neighbourhood of F.

Bichloride of Palladium is red: it transmits the red and orange rays freely, the yellow and green but partially, and the blue only where the stratum is very thin. See fig. 23.

Chloride of Copper in saturated aqueous solution is green it shows the absorption of the red ray due to the metal; but the rays more refrangible than b are likewise absorbed. See fig. 21. If water be added to this solution, it suffers the same change as the green bromide does, becoming blue, and admitting all the more refrangible rays, like any ordinary salt of copper.

Sesquichloride of Iron gives a solution of a reddish-orange colour when concentrated, and the prismatic appearance of fig. 8. If water be added, it becomes more yellow in colour, and admits the same rays as the nitrate does. See fig. 4. Of course in this and similar cases it will be understood that the light is made to pass through the same amount of salt, whether it be diffused through much or little water; a circumstance that in ordinary cases makes no difference in the absorption or transmission of rays.

Chloride of Nickel, when dissolved in very little water, is of a yellowish-green. It then absorbs, not only the red ray, as nickel salts always do, but nearly all the light beyond F besides. When more water is added it becomes of a bluish-green, and shows the absorption due to the metal alone. See figs. 11 and 10.

Chloride of Cobalt also varies in colour according to the state of dilution. The addition of water increases the transmissibility of both the yellow and the violet rays. See fig. 12.

IODIDES.

Iodine-water is too weak to show much absorption when placed in my hollow wedge, but an alcoholic solution of iodine gives the spectrum represented in fig. 14.

Iodide of Gold and Potassium gives a prismatic appearance resembling that of iodine. The remark made in respect to the terbromide of gold applies in this case also.

Iodide of Platinum and Potassium is of an intense red, and presents much the same prismatic appearance as the gold-salt; but while the light near F is wholly absorbed, it transmits some blue and more violet, which certainly iodine itself does not. See fig. 22.

Iodide of Palladium and Potassium is also intensely red, and resembles the corresponding platinum salt, except that the violet is not transmitted so freely as the blue.

Sesqui-iodide of Iron is of a deep red colour, and gives a spectrum resembling that of iodine.

Iodide of Nickel, when dissolved in a little water, is of a dark green, and affords a spectrum closely resembling that of iodine, except that the red ray is gradually absorbed by the metal. See fig. 18. When water is added to this, the same amount of salt transmits more and more blue, and assumes the appearance of fig. 10, a compound of nickel with a colourless acid.

Iodide of Cobalt, in saturated solution, is of a dark green, passing into a very deep red as the stratum increases.

it becomes pinker.

Double Salts.

On dilution

When the two bases in a double salt are both coloured, a similar result is obtained.

Double Chloride of Copper and Platinum is a definite salt, forming green crystals. A saturated solution of these gives the spectrum represented in fig. 24; but on addition of water, the same amount of salt transmits a somewhat broader band of green, and more blue, giving then the appearance of fig. 20, with the red ray gradually absorbed. This remarkable compound therefore shows, when in strong aqueous solution, the absorption due to each of its three constituents,-copper, platinum, and chlorine, see figs. 20 and 21; though, on dilution, the absorption due to chlorine is somewhat modified, as in the case of the uncombined copper-salt.

These results show that each coloured constituent of a salt