442 THE POPULAR SCIENCE MONTHLY. 



mersion in the developing liquid, instead of pouring the latter on the 

 collodion layer, as in the case of plates of glass, an operation that re- 

 quires some dexterity and long practice. 



This paper will retain all its sensibility for about two years, pro- 

 vided it be sheltered from light and moisture ; it is not affected either 

 by cold or heat. Hence it is destined to be of great use to travelers 

 who explore remote regions. 



After the light-impression has been made on tho paper, it remains 

 to develop the image. This operation presents no difficulty, success 

 depending, so to say, only on the time of exposure. First, the paper 

 is dipped in common water, care being taken to make the immersion 

 complete. There it must remain for at least five minutes, or until the 

 paper, which was beginning to curl, becomes perfectly flat. In the 

 mean time the following solution is prepared in quantity only sufficient 

 for the pictures to be developed at once, for oftentimes it decomposes 

 in the course of a day or two : 



Distilled water 1 litre. 



Glacial acetic acid 20 grammes. 



Citric acid 20 " 



Pyrogallic acid 3 " 



Into a basin with flat bottom, and of a size corresponding to that 

 of the proof treated, is poured enough of the above solution to com- 

 pletely submerge the proof ; a depth of three or four millimetres is 

 amply sufficient. Into this is dipped the proof after taking it from the 

 water and draining it, the collodionized side uppermost. After inclin- 

 ing the basin in every direction, so as to cause the liquid to pass sev- 

 eral times over the proof, a portion of it is poured into a glass, and 

 then we add to it a few drops of the following solution : 



Distilled water 100 grammes. 



Crystallized nitrate of silver 5 " 



Stir well, so as to mix thoroughly. The whole is poured into the 

 basin, which again is inclined as before. The image now appears ; 

 seven or eight minutes suffice to completely develop it, with the sky or 

 the lighted parts of an intense black. 



When the proof is sufficiently developed it is put in water, and 

 then dipped in a solution of hvposulphate of soda, 40 per cent., to fix 

 it ; it is then freelv washed in water in the usual way. It is now dried 

 between leaves of silk-paper or blotting-paper. 



Treated in this way the proof is perfectly secure : it is not affected 

 by changes of temperature, and may be exposed either to damp or to 

 drought without the least injury. To preserve it, we have only to 

 place it in a book or in a portfolio, so that it may not be creased or 

 rubbed on the collodionized side. 



When we would take positive proofs, we detach from the paper the 

 layer of collodion, thus getting the image on a thin transparent pellicle. 



