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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 83
We have mentioned in the first chapter a similar process, based on the application of asphalt, and later on a different one by Scamoni (see p. 222).
This discovery of Talbot was soon followed by a more productive one on the same ground.
Pretsch's Engravings. — An Austrian, Paul Pretsch, in 1854, prepared copper-engravings by a similar method, with the help of the electrotype process. He also took a film of gelatine, which contained chromate of potash, exposed this under a negative or a positive picture, and then washed it in hot water.
After doing this, all the places were retained which had become insoluble through the light, and after the washing and drying they stood out in high relief.
Accordingly, in copying under a positive, the lines which were black lg J '
in the original appeared in low relief, and the white parts in high relief.
This film was placed in an electrotype apparatus. This apparatus has the property of precipitating copper or other metals on a prepared surface. It consists of a galvanic cell, and a trough containing a solution of sulphate of copper. The reliefs which are to be copied, having been made conductors of electricity by a coating of graphite," are suspended from the zinc pole B (fig. 20) of the battery in the sulphate of copper : to the copper pole D, a plate of copper is attached. As soon as the electric current passes, the solution is decomposed, and the copper adheres to the relief. The thickness of the copper depends on the time