PHOTOGRAPHIC PROCESSES. 75 



finally left soaking in a disli of water acidulated with 

 nitric acid. 



When required, a plate is taken out rinsed in pure 

 water and polished with a soft rag or piece of wash- 

 leather, kept specially for that purpose. 



The easiest method of testing the cleanliness of 

 the surface is by breathing on it ; if the moisture of 

 the bi'eath is evenly distributed, the plate is clean. 



Collodion is difficult to prepare, but bromo-iodised 

 collodion, of first-rate quality, can be purchased from 

 any leading firm ; and will give better results than 

 that prepared at home. 



A vertical glass bath, with a cover to keep out the 

 dust, etc., will be found the most useful ; a pure 

 silver wire dipper, for immersing the plate in the 

 solution, being purchased along with it. 



The sensitizing solution placed in the bath is made 

 as follows : — 



Eecrystalized Nitrate of Silver, 80 grammes. 

 Pure Distilled Water, 1 litre. 

 Iodide of Potassium, 25 grammes. 



with the subsequent addition of a two per cent. 

 barium nitrate solution ; this and the following 

 formulas are taken from Abney's excellent treatise, 

 to which the reader is referred for fuller informa- 

 tion on the subject. 



The developing solutions are made as follows : — 



Ferrous Sulphate, 40 grammes. 

 Glacial Acetic Acid, 30 to 40 c.c. 

 Alcohol, 2 s. 

 Water, 1 litre. 



