Observations on Photographic Processes. 75 



or four times in a suitable vessel and dry it by pressing it gently 

 between blotting paper. It is then ready for the camera. One 

 minute is sufficient for a building on which a February sun is 

 shining, four or five minutes for general views. A rather more 

 sensitive paper is prepared by using a mixed solution of five grains 

 of iodide of potassium and five grains of chloride of sodium in 

 one ounce of water instead of the iodide of potassium. Five 

 grains of bromide of potassium in one ounce of water forms a 

 rather less sensitive preparation. These papers may also be made 

 more sensitive by washing them again, after the iodide of potas- 

 sium, with nitrate of silver, though it will hardly repay the labor. 

 The series of salts of silver, according to their sensibility, when 

 afterwards brought out by gallic acid, appears to be iodide with 

 chloride, iodide, iodide with bromide, bromide, bromide with 

 chloride, chloride, fluoride nitrate ; ferrocyanide, sulphocyanide, 

 cyanide. The series with mercury differs essentially from this. 



After exposure these papers are still white, but they need only 

 to be fastened as before to another piece of board, and washed 

 over once quickly and evenly with a saturated solution of crys- 

 tallized gallic acid (only three or four grains in one ounce of wa- 

 ter) or with solution of galls, to bring out in a few moments the 

 hidden impression. A weak impression may be brought out by 

 several washings, letting the paper dry between each. Too long 

 an exposure however in the camera is better than too short an 

 one. When it has come out sufficiently, dip it in water and fix 

 it by washing it with the iodide or other solution used in the 

 preparation. Then wash it in water and dry between blotting 

 paper as before. 



In views from which copies are to be taken by superposition 

 over other sensitive paper, it is better perhaps to use the bromide 

 throughout, as it leaves the paper whiter and more permeable to 

 the chemical rays, or to fix the paper prepared with iodide by 

 washing with a bromide solution or hyposulphite of soda. Care 

 must be taken in making a proof not to use the nitrate of silver 

 or galls too profusely, otherwise the paper will be stained through. 

 Papers merely washed with nitrate of silver, used as soon as dry, 

 and brought out afterwards by galls, may answer well for copies. 



These papers should not be exposed to light or heat during any 

 part of the process, and there should be no delay between the use 

 of the nitrate of silver and iodide of potassium ; for the nitrate 



