210 Mr. R. Hunt on the Use of Hydriodic Salts. 



sixth longer, is however in most cases of small consequence. 

 It is somewhat singular that if the glass plate is interposed 

 between the paper and the lens, the action is not more re- 

 tarded than if it had been placed behind it. The interfe- 

 rence of a transparent plate is little felt in the hydriodic pro- 

 cess. 



30. On Jixing these Photographs. The picture being 

 formed by the influence of light, it is required, to render it 

 unchangeable by any further action of the luminous fluid, 

 not only that the hydriodic salt be entirely removed from the 

 paper, but that the iodide of silver which is formed be also 

 dissolved out of the drawing. 



31. By well washing the drawing in warm water the hy- 

 driodate is removed, and the pictures thus prepared have 

 been stated to be permanent; and if they are kept in a port- 

 folio, and only occasionally exposed, they are really so ; for I 

 shall show presently (54-.) that they have the property of 

 being restored in the dark to the state in which they were prior 

 to the destructive action of light. I have now before me the 

 first drawing of this kind I ever executed, bearing the date 

 June 17, 1839. This drawing has been kept loosely in my 

 table drawer, and has often been exposed for many successive 

 days to the action of the sun ; yet the most delicate vena- 

 tions of the rose leaves are aS perfect as at first. Thus pre- 

 pared, however, these photographs will not bear continued 

 exposure without injury, about three months in summer, or 

 six weeks in winter being sufficient to destroy them. 



32. For a long period I was under the impression that 

 two iodides of silver existed, the one sensitive to solar influence, 

 but the other not so; and in my paper published in your Ma- 

 gazine for April, I stated such to be my opinion. I have, 

 however, since that period seen reason sufficient to question 

 the correctness of my conclusion. Under the former impres- 

 sion, not being successful in removing the iodide from the 

 paper without also injuring the oxidized or dark portions, 

 I endeavoured to effect a chemical change in the iodide of 

 silver. Some of the results being curious, I shall give them. 



33. By washing the photograph with a hot saturated so- 

 lution of the acetate of lead, the yellowness of the lights was 

 at first increased, but eventually considerably whitened, and 

 the dark parts assumed a peculiar crimson hue. The draw- 

 ing faded out entirely by the action of light in three weeks. 



34. When these drawings are dipped into a solution of the 

 bichloride of mercury, they fade out in precisely the same 

 manner as Sir John Herschel discovered the photographs on 

 Mr. Talbot's principle were obliterated, and in like manner 



