280 PHOTOGRAPHIC FACTS AND FORMULAS 
should be developed for not less than 5 minutes. The acid 
baths should not be stronger than from 0.5 to 1 per cent; the 
acid treatment with the three baths should not last longer 
than 30 minutes in all. The smaller quantities of the mercury 
solution give the best colours. For this process the best 
results are obtained with paper sized with agar-agar, which 
may be prepared as follows: 
Agar-agar 10g 77 gr. 
Water 1000 ccm 16 oz. 
Soak for 24 hours with an occasional stir, then pour off the 
water, and add enough fresh water to make the total bulk 
to the above quantities. Heat until boiling, and allow to boil 
for 10 minutes; filter through a piece of well-washed linen, 
and allow to cool until a firm jelly is obtained. This jelly 
should be squeezed twice through coarse canvas, so as to 
divide it up into little nodules. A small quantity should be 
placed on the paper, worked all over with a fairly stiff brush, 
then equalised with circular strokes with a softer brush, and 
dried. The quantity used depends on the surface of the 
paper, but this should not be strongly glazed when dry. 
Tue Hor Batu Processes.—In this process a hot devel- 
oper is used and the sensitiser should be: 
Ferric oxalate sol. 6 ccm 42 min. 
Potassium chloroplatinite sol. 4ccm 28 min. 
Gelatine-oxalic sol. 1 ccm 7 min. 
Greater contrasts in the prints may be obtained by the addi- 
tion of 5 to 10 drops of sodium chloroplatinate solution or a 
1 per cent solution of potassium bichromate. The same. 
developers as used for the cold bath papers may be used at a 
temperature of 50° to 75° C. (122° to 167° F.). For sepia 
tones the sensitiser should be: 
Ferric oxalate sol. 6 ccm 100 min. 
Potassium chloroplatinite sol. 4ccm 67 min. 
