Mr. Hunt on Acli7io-Chemistry. 33 



when. both solutions are brought into the same actinic condi- 

 tion 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 phaenomena, which evidently point to some 

 law of chemical action which has not yet been made the sub- 

 ject of study. 



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 si- 

 milar change takes place, to a certain extent, in solutions of 

 this salt in distilled water, when exposed in clear glass ves- 

 sels, 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 sul- 

 phate 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 will be made the sub- 

 ject of a future communication. 



Actinic Influence on Electro-chemical Action. 



Some years since, when endeavouring to discover the che- 

 mical action exerted by the sun's rays on the iodide of silver, 

 I was induced to form some small galvanic arrangements, a 

 description of which and the results of my experiments, I 

 published in the Philosophical Magazine for October 184<0. 

 As these experiments bear strongly on my present researches, 

 I venture to extract a passage or two from that paper. 



" In a watch-glass, or any capsule, place a little solution 

 of silver; in another, some solution of any hydripdic 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 in a very short time it will be 

 seen that iodine is liberated in one vessel, and the yellow 



Phil. Mag. S. 3. Vol. 27. No. 1 77. July 1 845. D 



