Dr Fjrfe on Photography. 151 



a yellow iodide of silver is thrown down. The same is the 

 case when the iodide is put on paper, previously covered with 

 the chloride, and, provided the solution is strong, it acts also 

 on the chloride when darkened, thus converting it to yellow 

 iodide, which is not in the least affected by light ; hence, by 

 putting the paper with the impression through solution of the 

 iodide, provided it is weak, the white chloride only is acted 

 on, and being converted to iodide, is no longer liable to change. 

 As, however, the iodide will act on the dark chloride, it is of 

 the utmost consequence to attend to the strength of the solu- 

 tion, which should be such that it will not attack the faint 

 parts of the impression. After the paper is passed through 

 it, it should be kept for some time in water, to wash off the 

 superfuous iodide of potassium, which, if left on, would gra- 

 dually destroy the whole of the impression ; indeed, even 

 with this precaution, I find it extremely difficult to preserve 

 them. The second method recommended by Mr Talbot is 

 merely immersing the paper in solution of sea-salt. This 

 process does not, however, seem to answer well ; I have re- 

 peatedly failed in preserving the specimens in this way, and 

 even when they are preserved, they are completely altered in 

 their appearance, and deprived of their original brilliancy. 



I have already stated, that I prefer the phosphate of silver 

 for taking the impressions, not only because it is equally sen- 

 sitive as the chloride, but gives a greater variety of shades. 

 In addition to these, it has another advantage ; the impres- 

 sions are easily preserved. After various fruitless attempts, 

 I at last found that the darkened phosphate is not soluble in 

 ammonia, though, as is well known, the yellow phosphate is 

 easily dissolved. I had, therefore, recourse to this for their 

 preservation, and though I did not completely succeed at first, 

 yet I at last did so, by attending to the precaution of washing 

 off the ammoniacal solution, because, when left on, the impres- 

 sion gradually becomes darker and darker, and is ultimately 

 destroyed, owing to the action of the light on it. The method 

 I now follow is to put the paper into a diluted solution of 

 water of ammonia (one of the spirit of hartshorn to about six 

 of water), and leave it there till the yellow parts become 

 white, shewing that the phosphate is dissolved, after which 



