176 Miscellanies. 



solution of sea-salt, and when dry, to rub it over on one side with so- 

 lution of lunar caustic, by which chloride of sfZrer is formed, and 

 adheres to the paper. As thus prepared, it acquires a dark color on 

 exposure to light; the depth of color depending on the strength of the 

 solutions ; hence it may vary from lilac to deep purple, approaching 



to black. 



In preparing paper by this method, it is very difficult to get the chlo- 

 ride uniformly spread over the surface, and accordingly, when exposed 

 to light, it often gives a variety of shades; indeed, in many places it 

 continues white. It was this that induced me to try the use of other 

 salts of silver; and the one which I have found to answer best is the 

 phosphate, procured in the usual way, by the addition of the phosphate 

 of soda to the solution of lunar caustic. In preparing the paper by 

 this method, I generally employ one part of phosphate of soda dissol- 

 ved in about eight of water, and tlie nitrate of silver dissolved in about 

 six of water. The paper is first soaked in the phosphate, and then 

 dried, after which the nitrate is put on on one side by a brush, the pa- 

 per again dried and afterwards again put through the salt, by which 

 any excess of silver is converted to phosphate. As thus prepared, 

 it acquires a-yellovv tinge, which becomes black by exposure to light. 

 It is equally sensitive as the chloride, and, in my opinion, gives a 

 much more pleasing variety of shades. 



Instead of preparing the paper by the process described, I frequently 

 employ the phosphate precipitated before applying it, for which pur- 

 pose the nitrate solution is dropped into that of the phosphate of soda, 

 the yellow precipitate is allowed to fall to the bottom, and the super- 

 natant fluid is poured off; what remains must be kept in stone bottles 

 or in a dark place, as it is extremely sensitive to light. In preparing 

 the paper with it, it is put on with a broad flat brush, and then dried in 

 the usual way. Though there is a little difficulty at first in getting the 

 phosphate uniformly spread over the surface, yet by a little practice 

 a uniform ground is easily given, and when once acquired, the method 

 has the advantage of being much cheaper than those previously re- 

 commended. I sometimes add a little mucilage to the fluid, which 

 keeps the phosphate suspended in it. There are other methods of 

 preparing the paper, which though they do not give it so sensitive, yet 

 are cheaper than those stated ; I allude to the use of the phosphate in 

 solution in ammonia, or, which is cheaper, in the carbonate of ammo- 

 nia which is procured by adding concentrated solution of carbonate of 

 ammonia to the phosphate collected by precipitation as already de- 

 scribed, A still cheaper fluid may be prepared by adding a strong so- 

 lution of nitrate of silver to a concentrated solution of carbonate of 



ammonia, by which a carJo/zafe of silver is obtained in solution, and 



