S6 Mr. Hunt on Actino-Chemistry. 



solutions remained as clear as at first. One of the vessels with 

 its contents was then removed into the sunshine, when the 

 solution immediately became cloudy, and was very speedily 

 decomposed, the precipitate falling heavily. By experiments 

 with the spectrum, I have since found that the precipitation is 

 due almost entirely to the more refrangible rays. I have not 

 been enabled to decide with that degree of accuracy I could 

 desire, in which ray the maximum effect is produced. The 

 precipitates formed in the blue, indigo and violet rays were 

 nearly of the same weight, but it did appear that the precipi- 

 tation was most speedily produced by the mean blue ray. 



If we dissolve the brown precipitate from the mineral cha- 

 meleon in a solution of cyanide of potassium, we have a clear 

 fluid. Place one portion in darkness, and expose another 

 to good sunshine, the solution preserved in the dark will re- 

 main quite clear for many days, whereas that exposed to ac- 

 tinic influence throws down a brown precipitate after a few 

 hours' exposure. If the solution is washed over paper, we 

 j)rocure by exposure good negative images of leaves or any 

 other body superposed. 



A few grains of sulphate of the protoxide of iron were 

 dissolved in rain water. If kept in perfect darkness, the solu- 

 tion remains clear for a long time ; it becomes, however, even- 

 tually cloudy and coloured from the formation of some per- 

 oxide of iron, even in tubes hermetically sealed. A few mi- 

 nutes' exposure to sunshine is sufficient to produce this change, 

 and the oxide formed, instead of floating in the fluid, and as 

 in the former case rendering it opake, falls speedily to the 

 bottom. 



Some years since, at the Meeting of the British Associ- 

 ation at Plymouth, I published an account of an exceedingly 

 sensitive " photographic " process, the agents employed being 

 iodide of silver and the ferro-cyanide of potassium. I have since 

 then made some experiments with the hope of ascertaining 

 the rationale of the chemical changes which take place. This 

 has not been done in anything like a satisfactory manner ; 

 they have, however, led me to observe a curious effect pro- 

 duced in mixed solutions of iodide of potassium and ferro- 

 cyanide of potassium under the influence of sunshine. If 

 a mixture of these salts is kept in the dark, no change takes 

 place for a long period, but if exposed to sunshine, the colour 

 of the solution is much deepened, becoming of a bright golden 

 yellow, and a light brown powder is precipitated. I have re- 

 served the examination of this powder for some future period : 

 I believe it to be a combination of iron and iodine, and I have 

 good reason for believing that a very remarkable change takes 



