88 HYDROMETALLURGY. [ III. 



litres water, and dried between blotting-paper. Before use, it 

 is brushed over with a solution of 2 grm. nitrate of silver, 0.3 

 grm. suberic acid, and 5 grm. caustic ammonia, dried, and put 

 into the camera. After the lapse of 5-10 minutes, the paper 

 is removed, washed in water containing a few drops of am- 

 monia, then passed through a solution of 1 pt. hyposulphite 

 of soda in 3 pts. water, dried partly between blotting-paper 

 and lastly before a fire. 



Archer (Chemist, 1850, p. 360, 450) has recommended the 

 use of pyrogallic acid for developing the latent picture upon 

 iodized paper. As disappointment is apt to ensue from the 

 rapid decomposition of the acid, it is better to prepare the 

 wash extemporaneously, as follows. To a solution of 20 gr. 

 nitrate of silver in 1 oz. of strong acetic acid, add 3-4 gr. 

 l^ure pyrogallic acid, immediately before using it. All risk 

 of decomposition is thus avoided. The prepared paper is 

 placed at once into the camera, where the light produces the 

 picture without the necessity of a second washing. As the 

 paper is very sensitive, the manipulations must be exact and 

 dexterous in order to insure success. Ample directions are 

 given in the original essays. 



Blanquart-Evrard (Lend. Athenaeum, 1850, 743) has pro- 

 posed the employment of fluoride of potassium for imparting 

 extreme sensibility to the iodized paper. 



According to the same authority, when the paper is prepared 

 by washing it with a liquor formed by mixing the white of two 

 eggs with a pint and a half of whey, it is free from all in- 

 equalities, and may be kept an indefinite time without being 

 injured. A little Narbonne honey added to the albumen, says 

 Niepce de St. Victor, will increase the sensitiveness of the 

 plate or paper. 



Crayon Daguerreotype. — Mayall (Lond. Athenpeum, 1850) 

 gives the following directions for making the so-called crayon 

 photographs : 



" Take a daguerreotype image on a prepared plate, as usual, 

 and be careful to mark the end of the plate on which the head 

 is pi oduced. Remove the plate from the holder before mercu- 



