CYANOTYPE OR FERROTYPE. 



This name has been given, by Sir John Herschel, to several 

 processes in which cyanogen is used in combination with iron. 

 The term Ferrotype, which is sometimes applied to them, may with 

 more propriety designate the whole cf those photographic pro- 

 cesses, a numerous class, in which iron may be employed as the 

 developing agent. 



FIRST PROCESS. 



The paper is washed over, as in the Chrysotype, with a 

 solution of ammonia citrate of iron. It is now exposed to 

 light, and a latent picture impressed upon it. If the paper has 

 sensibly darkened, the picture will appear negative. It is now 

 brushed over very sparingly and equally with a solution of the 

 ferro-cyanate potash, in which is dissolved a little gum arable. 

 The negative picture quickly vanishes, and is more slowly replaced 

 by a positive one of a violet blue colour, on a greenish yellow 

 ground. If when dry the details are not sufficiently distinct, a 

 second wash will generally bring out the picture, which should be 

 beautiful and sharp. 



SECOND PROCESS. 



A paper is prepared with a mixture of equal proportions of 

 ammonia-citrate iron, and ferro-sesquicyanate of potash. When a 

 picture has been impressed, it is thrown into water, and dried, 

 and a negative picture results. If this picture is washed with a 

 solution of the proto-nitrate mercury, it is readily discharged, but 

 is susceptible of restoration by thoroughly washing out the mer- 

 curial salt, and drying the paper. A smooth iron, rather hot, but 

 not sufficiently so to scorch the paper, is now passed over it, and 

 the obliterated picture immediately re-appears, but of a brown 

 tint. These photographs gradually fade and disappear, but may be 

 again restored by the application of heat. 



THIRD PROCESS. 



One part by weight of ammonia-citrate of iron is dissolved in 

 eleven parts of water, and this is mixed with an equal quantity of 



