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272 THE ADVANCE OF PHOTOGRAPHY
dissolved away, and the ink covering the albumen on which the light has acted will represent the image. When all the details are apparent the development is complete ; the plate should then be rinsed and placed on one side to dry.
Should some of the lines of the negative be missing upon the zinc, then it is a clear proof that there has not been satisfactory contact in the printing-frame. If the finer lines wash out when the plate is being developed, then the exposure has not been sufficiently prolonged ; on the other hand, if a good portion of the surface refuses to part with the ink so that the finer details are covered up, then the exposure has been too long.
The plate has finally to be etched by means of a dilute solution of nitric acid, but albumen does not resist the action of this acid to a sufficient extent to enable one to at once proceed with the etching. It is, therefore, necessary to first "bake in " the plate with a very fine bitumen powder.
When glue is used in the place of albumen the various processes in the preparation and printing are very similar to those just described. The plate is first of all placed on a whirler, a small quantity of the glue is poured into the middle of the plate, which is then whirled round so that the superfluous glue is carried off as the whirling goes on. It must be very specially remembered in this case, however, that if the glue is allowed to get too hot during the drying processes it will be found impossible to develop the print. After exposure the metal plate is placed in an aniline dye so that its subsequent development with water can be easily watched. In order that the glue may be able to resist the etching it must be turned into a kind of enamel. This is done by simply burning in the glue, by holding the plate over a gas flame or hot plate. During this process care must be taken not to char the glue. The dye now also burns away and the plate goes to a very white tint. Afterwards on