Proceedings of the British Association. 317 



Art. XIII. — Abstract of the Proceedings of the Eleventh Meet- 

 ing of the British Association for the Advancement of Scie7ice, 

 held at Plymouth, September, 1841. [Prepared from the Re- 

 port in the London Athenceum.^ Concluded from page 164. 



Sect. B. Chemistry aiid Mineralogy. 



Mr. R. Hunt communicated a paper on the influence of the 

 ferro-cyanate of potash on the iodide of silver, producing a highly 

 sensitive photographic preparation. The author being engaged 

 in experiments on that variety of photographic drawing which is 

 formed by the action of the hydriodic sahs on the darkened chlo- 

 ride of silver, with a view to the remov^al of the iodide formed 

 by the process, from the paper, was led to observe some peculiar 

 changes produced by the combined influences of light and the 

 ferro-cyanate of potash. He found that the ordinary photographic 

 paper, if allowed to darken in sunshine, and then slightly acted 

 on by any hydriodic salt, and washed when dry, with a solution 

 of the ferro-cyanate of potash, became extremely sensitive to 

 light, changing from a light brown to a full black, by a moment's 

 exposure to sunshine. Following out this result, it was discov- 

 ered that perfectly pure iodide of silver was acted on with even 

 greater rapidity, and thus it became easy to form an exquisitely 

 sensitive photographic paper. The method recommended is the 

 following: highly glazed letter paper is washed over with a so- 

 lution of one drachm of nitrate of silver to an ounce of distilled 

 water; it is quickly dried and a second time washed with the 

 same solution. It is then, when dry, placed for a minute in a so- 

 lution of one drachm of the hydriodate of potash in six ounces 

 of water; and being placed on a smooth board, gently washed 

 by allowing pure water to flow over it, and dried in the dark at 

 common temperatures. Papers thus prepared may be kept for 

 any length of time, and are at any moment rendered far more 

 sensitive than any known photographic preparation, except the 

 Calotype, which it quite equals, by simply washing it over with 

 • a solution formed of one drachm of the ferro-cyanate of potash 

 to an ounce of water. These papers may be washed with the 

 ferro-cyanate and dried in the dark : in this dry state they are 

 absolutely insensible, but they may at any moment be rendered 

 sensitive by merely washing them with a little cold water. The 



