(303.) If a solution of the manganesiate of potash is washed over paper, it imparts to it a brown colour. The sun's rays have a tendency to discharge this colour, and thus give a positive image. The addition of a small quantity of the nitrate of silver to paper prepared with this salt of manganese, renders it brown, but upon exposure it assumes a very intense blackness.

(304.) If the deutoxide of manganese is dissolved in a solution of the cyanuret of potassium, and paper washed with this solution, it will be found that an exposure to sunshine for half an hour, will produce no visible change over the surface; but on looking through the paper, it becomes evident that a considerable deposition of a brown precipitate has taken place within its pores over the spaces which were not protected from the action of the solar

rays.

(305.) Many of the salts of manganese, spread upon paper, exhibit some evidence of change under solar action. The muriate in particular undergoes decomposition, and some oxide of manganese is deposited. It is, however, to be remarked, that in those cases in which no apparent change is produced even by prolonged exposure to the sun's rays that is, no change of colour-we are not to conclude that no disturbance has taken place. In nearly all cases it will be found that a sufficient amount of change has been effected to be rendered evident by the application of some reagent. In particular, in the instances of the manganesic salts, as in those of copper, the subsequent application of the nitrate of silver produces very decided evidences of change. The chloride of gold, in some cases, appears to be reduced by these salts which have undergone solarization, in a manner strikingly dif ferent from the effect produced by salts which have not been exposed to the sun. It has also been noticed that the balance of affinity has been so much disturbed by the solar agency, that ammonia, the hydrosulphurets, and the ferroprussiates, produce different effects upon the

PHOTOGRAPHIC PROPERTIES OF LEAD.

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exposed and covered parts. May not these very curious phenomena be dependent upon the absorption of the active principle which is found to exist in the sun's rays, and every radiant source?

(306.) LEAD. The puce-coloured oxide of lead, in a moist state, under the influence of the sun's rays, parts with oxygen, and is converted into the deutoxide. It has been found that this is more decidedly the case with the oxide prepared by chlorine from the acetate of lead than with that which is prepared by the action of nitric acid on minium or red lead; from which it would appear that the oxygen is in looser combination in the one case than in the other. It was observed by Sir H. Davy, that this change was effected by the least refrangible rays; hence we may suppose it to depend on the calorific rays rather than the chemical rays.

(307.) In a very early stage of his photographic inquiries, Sir John Herschel was led to employ mordant washes of lead, and of organic matter precipitated by lead. These were found to have a very decided effect in quickening the change which takes place upon the nitrate of silver when exposed to sunshine; and in some cases it was found that a sensibility quite equal to that of the processes proposed by Mr. Talbot was produced. Papers, unfortunately, which are prepared with lead, have a tendency to darken spontaneously, or at least to turn very yellow, or sometimes grey, in the dark.

(308.) If red lead is boiled with cyanide of potassium, and paper washed with this solution is exposed to the Light, it undergoes some change, which is rendered evident by nitrate of silver. If gallic acid is washed over paper saturated with this solution, it becomes at first pink, and then brown. Upon exposing this to the sun, it loses colour; but although this change is produced with some rapidity, it is not found to proceed on to the point of whiteness, however long the paper may be exposed to sun

shine. If acetate of lead is washed over a paper, and then a weak solution of gallic acid, it will be found that no change of colour is produced for some time in perfect darkness; but a very short exposure to Light brings on a very decided darkening, which goes on even without the continuance of the solar influence. Numerous combinations of lead and other metals have been examined, with a view to detect, if possible, some of those cases of unstable affinity which yield the most interesting photographic results. Lead certainly has the power of increasing the liability of gold and platinum to change, and of carrying on the degree of darkness ultimately produced, much further than when these metals are used alone; and hence it is very probable that lead may in some processes be of considerable use, where the object is to produce originals. from which a great number of transfers may be taken.

(309.) NICKEL.-The salts of this metal, spread upon paper, pass slowly into a fine light brown, giving tolerably good negative pictures. Nitrate of nickel, spread upon paper, and then washed with ammonia, affords some very interesting photographic results. It is difficult to decide upon the exact proportion in which the ammonia should be used. It appears that a sufficient quantity should be applied to effect the precipitation of the oxide of nickel, but not enough to re-dissolve it. If an excess of ammonia is employed, the paper appears absolutely insensible; but if the proportions are well adjusted, a preparation which darkens quite as readily as the nitrate of silver, results. The subsequent application of the nitrate of silver considerably darkens the parts which have been already changed by Light.

(310.) Upon submitting paper thus prepared to the prismatic spectrum, it was found that the change was entirely dependent upon the influence of the rays of greatest refrangibility, the prolonged action of the other rays producing no apparent effect.

(311.) TIN.-It has been found that chloride of tin, carefully neutralised, will, when in contact with any organic matter, undergo a considerable change in the sunshine; a dirty appearance being given to it in half an hour, so as to contrast very decidedly with the original whiteness. Its oxide, washed, after exposure to the sun, with nitrate of silver or chloride of gold, exhibits very decided evidences of change.

The purple of cassius changes in the sunshine to a steel grey. If the dyers' muriate of tin is applied to paper, and then dried without much heat, it may be used to produce results of some interest. If, after exposure for half an hour to good sunshine, the paper is washed with very dilute chloride. of gold, the purple of cassius will be formed with much greater readiness over the shaded than the exposed parts, giving thus positive images. If the solution of gold is applied too copiously, the darkening is carried on with great rapidity over the whole of the sheet; but although at first it appears that the sheet is of one uniform colour, it will, by keeping, show distinctly the covered spaces.

(312.) COBALT.-The changes which some of the salts of this metal undergo when exposed to heat are well known. These appear to depend entirely upon the abstraction of moisture. Under the influence of the prismatic spectrum, these changes of colour take place, but only under the red rays, by long exposure. Another kind of change is apparent under the spaces on which the blue rays fall, and which, very different from that produced by the heat rays, is permanent. On paper the nitrate, muriate, and carbonates of cobalt, have been found to deepen in colour very considerably under exposure. In a few instances, when the sun has been very fervent, the pale blue of these salts changed to a good violet; but when removed from the light, this colour passes into a deeper blue. Washed with nitrate of silver, the blue

passes into a brown; and if again exposed, positive images are produced.

(313.) The very intense and beautiful brown which is given to the ordinary argentine preparations, by the presence of cobalt, may possibly be of some advantage in practice. If paper is washed with muriate of cobalt, and then nitrate of silver, in proper proportions to allow the silver to be in excess, it will be found to be superior to the ordinary muriated papers; and it is so far economical, that a very much weaker solution of the nitrate of silver may be used, and as deeply coloured an impression made as when a larger dose of the silver has been applied. Nitrate of silver does not darken very readily when spread on paper; but if mixed with nitrate of cobalt, it will be found to change more easily, and give a deeper colour than the silver salt alone would do.

(314.) ARSENIC. Some of the arsenical combinations. will be found to be influenced by the solar rays, and particularly the arseniate of potash. If a paper washed with this salt is exposed for an hour or two, the subsequent application of nitrate of silver brings out an image of any body used to obstruct the Light.

(315.) Bismuth, Cadmium, Rhodium, and some other metals, have been found, under certain conditions, to afford evidence of the same kind of changes as we have seen takes place in so many other bodies. The attentive study of the peculiarities of the rarer metallic compounds will, it appears, probably afford many curious results. As far as my researches have been carried, they afford good presumptive evidence that every chemical combination undergoes some peculiar modification when exposed to the power of the solar rays.

(316.) CHROMIUM.-The bichromate of potash is particularly distinguished by its photographic properties. These were first pointed out by Mr. Mungo Ponton, who proposed the following interesting process: Well-sized