into the same actinic condition they do not precipitate more freely than those do which have been kept in the dark. It appears to be necessary that the actinic states should be dissimilar to ensure the production of these curious phenomena, which evidently point to some law of chemical action which has not yet been made the subject of study.

(485.) Bichromate of potash, it is well known, is decomposed by the agency of the solar rays when in contact with organic matter. A solution of this salt spread upon paper forms a photographic agent of some interest. The paper, which is of a fine yellow colour when first prepared, becomes brown upon exposure to the sunshine, from the chromic acid of the salt being acted upon by the organic matter of the paper. A similar change takes place, to a certain extent, in solutions of this salt in distilled water, when exposed in clear glass vessels, for some time, to bright sunshine. The evidence we have of this is the gradual formation of minute bubbles of air, which are redissolved, and the progressive increase of free chromic acid in the solution, which may be detected by any of the usual methods.

A combination of the bichromate of potash and of sulphate of copper, either on paper or in solution, exhibits the above change very clearly, in the gradual formation of a chromate of copper of a peculiar character, which, although brown at first, becomes nearly white by prolonged exposure to solar influence; indeed, upon paper the whiteness is quite complete. Some very remarkable changes have been detected in combinations of these salts, which require further examination than they have yet received.

(486.) From two sets of experiments, on the electro-chemical action of the solar rays, I draw these conclusions:

1st. That electro-metallic precipitation is prevented by the influence of the sun's rays.

2nd. That light is not the retarding agent, but that the

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exercise of electrical force is negatived by the direct influence of actinism.

I placed in a test-tube a strong solution of nitrate of silver; in another tube, this being closed at one end with a thin piece of bladder, I placed a solution of iodide of potassium; which was supported in the solution of nitrate of silver, by being fixed in a cork, and a piece of platina wire was carried from one solution into the other. An arrangement of this kind was kept in the dark; iodine was liberated in the inner tube, and a crystalline arrangement of metallic silver was formed around the platina wire in the outer one. A similar arrangement was placed in the sunshine. Iodine was liberated in the inner tube, but no silver was deposited.

Having exposed the above solution to the sunshine of July during a long day, the tube was placed in a dark cupboard, but the actinic influence which had been exerted on the solution of silver had produced a permanent change in its condition; after several days no trace of any metallic deposit could be detected, but the whole of the iodine again entered into combination, whereas this was not the case in the unexposed glasses.

In the inner tube I placed the solution of silver, and in the outer one the hydriodate of potash. One arrangement was kept in the dark, the other was exposed to good sunshine. In both instances the liberated iodine gave an intense yellow to the solution, and in both cases the quantity of metallic silver deposited was precisely the same.

This yellow fluid being analysed by the prism, was found to obstruct all the rays above the green, whilst it permitted the permeation of the yellow and orange rays in great quantity and power. It is therefore evident that the luminous rays of the solar spectrum have no power in retarding electro-chemical action.

(487.) The following experiment gives a pleasing illustration of the excitation of electric currents by solar agency, and their opposition to ordinary chemical action.

Precipitate with any iodide, silver, from its nitrate in solution, and expose the vessel containing it, liquid and all, to sunshine; the exposed surfaces of the iodide will blacken remove the vessel into the dark, and, after a few hours, all the blackness will have disappeared. We may thus continually restore and remove the blackness at pleasure. If we wash and then well dry the precipitate, it blackens with difficulty, and if kept quite dry it continues dark; but moisten it, and the yellow is restored after a little time. In a watch-glass, or any capsule, place a little solution of silver; in another, some solution of any iodine salt; connect the two with a filament of cotton, and make up an electric circuit with a piece of platina wire: expose this little arrangement to the light, and it will be seen, in a very short time, that iodine is liberated in one vessel, and the yellow iodide of silver formed in the other, which blackens as quickly as it is produced.

Place a similar arrangement in the dark; iodine is slowly liberated. No iodide of silver is formed, but around the wire a beautiful crystallisation of metallic silver. Seal a piece of platina wire into two small glass tubes; these, when filled, the one with iodide of potassium in solution, and the other with a solution of the nitrate of silver, reverse into two watch-glasses, containing the same solutions; the glasses being connected with a piece of cotton. An exposure during a few hours to daylight will occasion the solution of the iodine salt in the tube to become quite brown with liberated iodine: a small portion of the iodide of silver will form along the cotton, and at the end dipping in the salt of silver. During the night the liquid will become again colourless and transparent, and the dark salt along the cotton will resume its native yellow hue.

(488.) M. Edmond Becquerel, in 1839, first called attention to the electricity developed, during the chemical action excited by solar agency. He provided a

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blackened box divided by a diaphragm into two cells; in each cell he placed the fluid to be examined, and plates of platina or gold were dipped in each, and connected with a galvanometer. The cells being filled with acidulated water, and platina plates in each cell, it was found that when the red, orange, yellow, or green rays fell upon the fluids, no action was excited. The blue and indigo induced a feeble action, but a very decided deflection of the galvanometer was produced by the impact of the violet rays. In 1840, I repeated the experiments of Becquerel, with many modifications, using tubes bent into the form of U; or floating one photometric fluid upon another. The results I then arrived at completely confirmed those of M. E. Becquerel; and I was led to adopt this method of measuring the permeability of bodies to the "chemical rays."*

(489.) Becquerel supposes the light to act on the corpuscules adhering to the surfaces of the plates. It is very convincingly proved that none of this excitement is due to calorific action. With an arrangement similar to that just described, the following results were arrived at by Becquerel when using screens of glass:

Here the incorrect results obtained by coloured glasses are very decidedly shown. All yellow glasses are permeable to some chemical rays; therefore, the intensity of 6.5 is to be attributed to these, and not to the luminous rays themselves. A plate of polished brass being exposed under similar conditions in various parts of the spectrum, the following results were obtained:

* See Philosophical Magazine, February, 1840.

Here it would appear that the maximum effect was produced by the green rays.

The plate of brass being oxidised the effects were as follows:

Plates exposed to iodine, chlorine, and bromine, were tried by M. E. Becquerel, and similar results obtained.

(490.) Not considering these results so satisfactory as I could desire them to be, I instituted during the summer of 1843 a series of experiments with plates of different metals, which were excited in several ways. These plates were connected with wires from the galvanometer, the dif ferent prismatic rays were passed separately through a slit in a card, and the transient and permanent deflections carefully noted. The galvanometer, which was by no means a sensitive one, was never deflected by any ray below the green, unless by the extreme red, and this was no doubt a thermo-electrical action; but in the green a weak action was always detected, which increased powerfully as we ascended into the rays of still greater refrangibility; the maximum shifting with the kind of preparation employed, between the mean blue ray and the most refrangible violet.

(491.) This action is only to be regarded as one of the evidences of chemical disturbance, exciting electrical currents; yet at the same time, it opens the question of the identity of the agent producing this disturbance and