GELATIN 415 



is being realised that both the amphoteric and colloidal proper- 

 ties must be taken into account. Both of these last properties 

 play a part in industrial applications ;| for example, the 

 action of gelatin as a protective colloid is of great importance 

 in the preparation of photographic emulsions ; but in the opera- 

 tions of developing and fixing, its behaviour as an amphoteric 

 substance must be considered, as may readily be realised when 

 one remembers that the usual developers are alkaline and 

 that acid fixing baths are often used ; the swelling of the gelatin 

 film will vary in the baths, and in the change from the 

 developer to the fixing bath the gelatin must, at some time, 

 pass through the isoelectric point {compare Sheppard, Brit. 

 Journ. Phot., 1922, in the press). Questions of the hardening 

 and tanning of gelatin also arise, but these have not been 

 touched upon in this article. 



