Proceedings of the British Association. 317 
Arr. XIII.—Abstract of the Proceedings of the Eleventh Meet- 
ing of the British Association for the Advancement of Science, 
held at Plymouth, September, 1841. [Prepared from the Re- 
port in the London Atheneum.| Concluded from page 164. 
Sect. B. Chemistry and Mineralogy. 
Mr. R. Henr communicated a paper on the influence of the 
Serro-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 salts on the darkened chlo- 
ride of silver, with a view to the removal 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 toa 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 photographie 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 
