THE IRON PROCESSES 259 
Filter. The method of using is as above. Brighter prints 
are obtained by the addition of 0.5 per cent of oxalic acid to 
the above; the paper keeps better if 0.05 per cent of potas- 
sium bichromate is added. 
The use of sodium or ammonium ferric oxalate gives 
greater speed than the above, but the paper does not keep so 
well. The sensitiser should then be: 
Potassium ferricyanide 133 ¢ 1021 gr. 
Ferricammonium citrate (red) 166 ¢ L277 se: 
Sodium or ammonium ferric 
oxalate 33 g 25920, 
Water 1000 ccm 16 oz. 
All these solutions are sensitive to light. Other iron salts 
may also be used, for instance: 
A. Sodium or ammonium ferric 
oxalate 250 g 4 oz. 
Water 1000 ccm 16 oz. 
B. Potassium ferricyanide 250 g 4 oz. 
Water - 1000 ccm 16 oz. 
Filter both solutions, mix, and again filter. Or the follow- 
ing may be used: 
A. Tartaric acid 180 g 1382 gr. 
Ferric chloride, anhydrous 68 g 220%: 
Ammonia q. S. q. S. 
Water 1000 ccm 16 oz. 
Dissolve the acid in three-fourths of the water, then add the 
iron, and, when dissolved, add sufficient ammonia to neu- 
tralise the acid, about 175ccm (2%40z.) being required; 
excess of ammonia must be avoided. Filter, and add to: 
B. Potassium ferricyanide PANT a 1666 gr. 
Water 1000 ccm 16 oz. 
Mix the two solutions. This gives greater sensitiveness, but 
the prepared papers to not keep so well as the others. 
