264 Mr. R. Hunt on the Use ofHydriodic Salts 



rating hydrogen from its combinations. I cannot regard this 

 singular salt of silver as a definite compound : it appears to 

 me to combine with iodine in uncertain proportions. In the 

 process of darkening the liberation of hydrogen is certain ; 

 but I have not in any one instance been enabled to detect free 

 iodine ; of course it must exist either in the darkened surface, 

 or in combination with the unaffected under layer ; possibly 

 this may be the iodide of silver, with iodine in simple mixture, 

 which, when light acts no longer on the preparation, is libe- 

 rated, combines with the hydrogen of that portion of moisture 

 which the hygrometric nature of paper is sure to furnish, and 

 as an hydriodate again attacks the darkened surface, restoring 

 thus the iodide of silver. This is strikingly illustrative of the 

 fading of the photograph. The picture is light iodide of sil- 

 ver and dark oxide of silver; as the yellow salt darkens under 

 the influence of light it parts with its iodine, which immedi- 

 ately attacks the dark oxide, which is gradually converted into 

 an iodide, oxygen, as I have shown, being liberated. The fol- 

 lowing experiments go not only to prove this position, but 

 also serve to illustrate in some measure the action of light on 

 this compound. 



59. Iodide of Silver. Precipitate with any hydriodate, 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 disappear : we may thus 

 continually restore and remove the blackness at pleasure. 



60. If we well wash and then dry the precipitate it blackens 

 with difficulty, and if kept perfectly dry it continues dark ; but 

 moisten it and the yellow is restored after a little time. 



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

 of silver; in another, some solution of any hydriodic 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 

 iodide of silver formed in the other, which blackens as quickly 

 as it is formed. 



62. Place a similar arrangement to the above (61.) in the 

 dark, iodine is slowly liberated. No iodide of silver formed, 

 but around the wire a beautiful crystallization of metallic 

 silver. 



63. A piece of platina wire was sealed into two glass 

 tubes ; these when filled, the one with hydriodate of potassa 

 in solution, and the other with a solution of the nitrate of 

 silver, were reversed into two watch-glasses containing the 



