"It frequently occurs that though sodium hyposulphite cannot be detected in the washing water, it may be present in the paper itself. The paper on which most prints are taken being sized with starch, if a very weak solution of iodine be applied with a brush across the back of a print, a blue mark will indicate the absence of the hyposulphite. Care must be taken that the iodine solution is very weak, otherwise a part of the iodine will first destroy the trace of the salt, and then the remainder will bring out the blue re-action."
We finish this chapter by quoting our maxims to be observed in printing.
"Maxims for Printing.
"1. The prints should have the highest lights nearly white, and the shadows verging on a bronzed colour before toning.
"2. Place the prints, before toning, in the water, face downwards, and do not wash away too much of the free nitrate of silver.
"3. The toning solution must be neutral or slightly alkaline, and not colder than 60°.
"4. Tone the prints to purple or sepia, according as warm or brown prints are required.
"5. Move the prints, in both the toning and fixing solutions, repeatedly, taking care that no air-bubbles form on the surface.
"6. Take care that the fixing bath is not acid.
"7. Use fresh sodium hyposulphite solution for each batch of prints to be fixed.