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82 THE ADVANCE OF PHOTOGRAPHY
picture. The black lines kept back the light. Accordingly, at these places the gelatine remained soluble, but was rendered insoluble under the white places, by the action of light. After exposure he washed the plates in the dark with warm water. By this means the places that had remained soluble under the black lines were dissolved ; the others were retained on the plate. Thus Talbot obtained a drawing on the metal itself on a brown ground. This is worthless by itself, but it provides the means of producing a steel plate for engraving.
We have elsewhere explained the nature of steel and copper-plate engraving. Both processes consist in the production of a metal plate which contains, in incised lines, the drawing that is to be reproduced. These lines retain the ink which is rubbed upon the plate, and transfer it to the paper. The hard steel plates have the advantage of lasting for many more copies than the softer copper plate ; only the steel engravings are far inferior to copper-plate in artistic beauty, and therefore the former have lost favour. But the steel engraving is very important for preparing technical and scientific diagrams, paper money, and the like, as less artistic beauty is required in their case. It was steel plates of this kind that Talbot produced by the help of light.
We have seen that his steel plate was covered with an insoluble film of gelatine, and that the metal was uncovered at all places where the light had not operated. He poured on it a fluid which ate into the steel — for example, a mixture of acetic acid and nitric acid. This mixture, of course, only attacked the steel where it was exposed, and thus produced an incised drawing in the steel plate, so that the latter, after being cleaned, gives as good an engraving as if it were the work of the engraver.
Thus a new process was found, based upon the chemical action of light, which was gradually to replace the more difficult work of the engraver.