Fig. 28.
This washing arrangement causes the prints to be alternately soaking in water, and draining. Whilst in the water they are perpetually being shaken apart by the movement of the tray, and thus every part of the print gets washed, and it is almost impossible for two prints to stick together. In all washing apparatus there is a danger of air-bells forming on the surface of the prints while in the water, but in this form there is the advantage that whilst draining the air-bells must break, and so water on rising to the level of the prints can obliterate any of the evil effects which would be caused by their being perpetually remaining on one spot. It is useless to attempt to describe other forms of the apparatus, since there are so many; we have chosen one which appears to us to be a satisfactory form.
The following tests for the elimination of hyposulphite are taken from another work of this series.[26]
"The following is a most delicate test.
"Make the following test solution:—
| Potassium permanganate | 2 | grains |
| Potassium carbonate | 20 | " |
| Water | 1 | quart |
"The addition of a few drops of this rose-coloured solution to a pint of water will yield a slightly pink tinge. If there be any trace of sodium hyposulphite present, this colour will give place to one of a greenish hue.
"If the permanganate be not at hand, the following well-known starch iodide test may be adopted:—
"Take about two drachms of water and a small piece of starch about the size of a small pea; powder and boil the starch in the water till the solution is quite clear; add one drop of a saturated solution of iodine in alcohol to this clear liquid. It will now become dark blue. Of this solution drop two drops into two clean test tubes, and fill up one with distilled water and the other with the water to be tested; a faint blue colour should be perceptible in the first test tube. In the second test tube, should hyposulphite be present, this blue colour will have disappeared, the iodide of starch becoming colourless in its presence. The best mode of comparing the two waters is by placing a piece of white paper behind the test tubes.