234 Photographic Processes. 



of light in the camera, are then removed by a strong solution 

 of common salt, or a weak solution of hyposulphite of soda, 

 which takes away the golden tint ; and finally, the plate is 

 washed with hot distilled water. If the exposure in the 

 camera is allowed to exceed a certain limit, the mercurial 

 vapour will bring out a negative, instead of a positive picture ; 

 since a certain amount of actinic action gives to the iodide, etc., 

 of silver a power of condensing the mercury, while a greater 

 amount takes that power away. 



Process with Paper. A very simple process consists in 

 soaking paper of a suitable texture in a solution of common 

 salt, drying it in blotting-paper, and then brushing it with a 

 solution of nitrate of silver. If the paper is required to be 

 very sensitive, iodide of potassium is substituted for the com* 

 mon salt ; and if extremely sensitive, bromide of potassium is 

 used instead of either. The relative amounts of the salts 

 employed is of great importance. After exposure in the 

 camera, the picture is fixed with hyposulphite of soda. 



The Calotype. The paper is washed with nitrate of silver, 

 and then dipped in a solution of iodide of potassium : in this 

 state it is unaffected by light, but it is rendered highly sensi- 

 tive by washing with a solution of nitrate of silver, to which 

 acetic acid and gallic acid have been added. When removed 

 from the camera, the picture will gradually develop itself in 

 the dark ; but it is brought out at once, by gallo-nitrate of sil- 

 ver and heating at the fire. It is fixed with hyposulphite. 

 Pictures obtained by this and similar processes are negative ; 

 positives are " printed " from them, by placing them in contact 

 with sensitized papers, in a glass frame, and transmitting light 

 through them, for a sufficient time. 



Albumenized Paper. Paper which has been carefully coated 

 with albumen on one side, and dried, is washed on the albu- 

 menized surface with nitrate of silver. After exposuro in the 

 camera, it is fixed in the usual way. 



It has been found that the coatiug of albumen, containing 

 tho picture, may be removed from the paper, by steeping for a 

 few moment s in concentrated sulphuric acid, or in a concentrated 

 solution of chloride of zinc, and washing carefully with water. 

 The albumen then resembles an animal membrane, and may be 

 placed oil any other surface. 



The natural tone of a picture on paper is very disagreeable : 

 this is corrected by the liming bath, which consists almost es- 

 sentially of chloride of lt<>1(1, mixed with one or more other 

 salts. Chloride of gold and bicarbonate of soda constitute tho 

 mixture very commonly employed. The toning bath should 

 deepen the tint to a blue, a violet, or even a black, and it may 

 be used either before or after fixing. The dearness of gold 



