76 PHOTOGRAPHIC PROCESSES. 



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 breath 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 : 



Eecrystallized 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 information 

 on the subject. 



The nitrate of silver bath should be exposed to the 

 light, strong sunlight if possible, when not in use. 

 When required, it is filtered, and a few crystals 

 added until it regains its proper strength. An 

 argentometer is very useful for ascertaining the 

 amount of silver in solution. 



