COATING THE PLATES.
Plate coating in factories is done by machinery, but in experimental work hand coating will answer quite well. It is pos sible to coat 60 plates in an hour by hand, and, in fact, until about fifteen years ago, the majority of the work was done in this way. A pneumatic holder, similar to that shown in Fig. 140, will be useful. This consists of a collapsible rubber bulb A with a flat top B. The bulb is pressed, the plate laid on the top, and the bulb released, when it holds firm by suction (see Fig. 141). In use, the holder should be placed in warm water until ready, as otherwise the chill is liable to cause a and poured from it, the amount used being noted, it will be found that about SO minims are required to each half-plate or its equivalent. Coating pots resembling a teapot were at one time obtainable, slight unevenness in the coating. A pool of emulsion, about equal to one-triird the area of the plate, which should be warmed, is then poured in the centre of the plate, and by tilting it very gently the emul sion is caused to flow first to the top right hand corner, next to the top left hand corner, then to the bottom left hand corner—taking extreme care to do this slowly, as otherwise the emulsion will run consisting of an inner and outer vessel, the latter being filled with warm water and the former with emulsion. The latter
may be poured from such a contrivance without fear of bubbles forming. The plate can now be put in the washing-tank, which should be filled preferably with dis tilled water, and allowed to remain for half an hour, giving one or two changes during that time. It may then be removed, and if the operator is possessed of a drying cupboard it should be placed in it until thoroughly dry.