204« Mr. R. Hunt on the Use of Hydriodic Salts 



3. It occurred to me that it might be possible to saturate 

 the paper with a metallic solution, which should be of itself 

 entirely uninfluenced by light, on which the silver coating 

 might be" spread without suffering any material chemical 

 change. The results being curious, and illustrative of some 

 peculiarities to be explained when the hydriodates come un- 

 der our examination (65.), I shall record a few of them. 



4. Sulphate and Muriate of Iron. — These salts, when used 

 in certain proportions, overcame many of the first difficulties, 

 but all the drawings on papers thus prepared faded out in the 

 dark. 



5. Acetate and Nitrate of Lead. — The salts of lead I have 

 since perceived have been used by Sir John Herschel with 

 success in some of his negative processes. I found a tolerably 

 good result when I used a saturated solution of the above- 

 named salts; but papers thus prepared required a stronger 

 light than other kinds to give good results ; when I used 

 weaker solutions the drawing w r as covered with black patches. 

 On these a little further explanation is required. When the 

 strong solution has been used, the hydriodic salt which has 

 not been expended in forming the iodide of silver, which, it is 

 well known, is the lights of the photograph, goes to form an 

 iodide of lead. This iodide is soluble in boiling water, and is 

 thus easily removed from the paper. When the weaker so- 

 lution of lead has been used, instead of the formation of an 

 iodide, the hydriodate exerts one of its peculiar functions in 

 producing an oxide of the metal (65 — 67.). 



6. Muriate and Nitrate of Copper. — These salts, in any 

 quantities, rendered the action of the hydriodates very quick, 

 and when used in small portions appeared to promise much 

 assistance in quickening the process ; but experience has 

 shown their inapplicability, the edges of the parts in shadow 

 being destroyed by chemical action. 



7. Chloride of Gold. — I did not anticipate much from the 

 use of this salt. On trial it was found to remain inactive 

 until the picture was formed, when a very rapid oxidation of 

 the gold took place, and a consequent darkening of all the 

 bright parts (5.) (65—67.). 



8. Chloride of platina was found to act in all respects si- 

 milarly to the chloride of gold, the re-darkening of the lights 

 being much more rapid and intense (5. 7. 67.). 



9. A very extensive variety of preparations were tried with 

 like effects, and I was at length convinced, that the only plan 

 by which a perfectly equal surface could be obtained, without 

 impairing the sensitiveness of the paper, was careful manipu- 

 lation with the muriated and silver solutions. 



