WHE) TRON PROCESSES 263 
light under a drawing; the image is seen in a bright yellow 
colour on a darker ground. 
Development is effected with a 20 per cent solution of 
potassium ferrocyanide; the paper may be pinned to a 
board, and a very soft brush, charged with the solution, 
passed over the surface, care being taken not to rub up the 
image. Or the edges of the paper may be turned back to the 
height of about an inch and the print floated, face down, on 
the solution. The edges are turned up to prevent any solu- 
tion touching the back of the print, which would cause blue 
stains. In about half an hour, if the print is floated, a corner 
may be turned back and the image examined; if it is fully 
visible, the print should be washed with a spray or in run- 
ning water for a short time, immersed in a 1 per cent solu- 
tion of hydrochloric or sulphuric acid, the surface gently 
rubbed with a soft brush to remove the slight blue deposit, 
then again washed in water, again brushed, and hung up to 
dry. Corrections can be made with solution of potassium 
oxalate, about 15 per cent, thickened if necessary with gum 
arabic. 
FERROGALLIC OR INK Process.—This is also sometimes 
known as Colas’s process. It gives black lines on a white 
ground. The sensitiser is a mixture of ferric chloride or 
sulphate, and tartaric acid, thickened with gum, or gelatine: 
Ferric chloride 50 g 1 oz. 
Ferric sulphate, basic 25)¢ 4 oz. 
Tartaric acid 45 ¢ 432 gr. 
Water 500 ccm 10 oz. 
Dissolve, and add: 
Gelatine 25g Y, oz. 
Water 250 ccm 5 oz. 
Soak the gelatine in the water for 30 minutes and melt with 
