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220 THE ADVANCE OF PHOTOGRAPHY
castor oil in solution. The collodion soon dries, and can then be separated from the glass, carrying the film of the picture with it. Such films contained in some cases as many as 1500 despatches. At the place of arrival the films were unrolled and then enlarged by the help of a magic lantern ; a number of writers thereupon set to work to copy the enlarged despatches, and ultimately forwarded them to their respective addresses. Thus Paris corresponded, by the aid of photography, for six months with the outer world, and even poor persons were able to let their relatives know that they still lived.
Pyro-Photography. — An ordinary photograph is, as paper, very combustible, and exposed to injury from corrosive substances. Encaustic images on porcelain and glass do not participate in this exposure to injury, and therefore attempts have been made to prepare fireproof photographs, especially for the decoration of glass and porcelain. Success has crowned these efforts in several cases. One of the simplest processes was that of W. Griine at Berlin.
Griine found that the collodion image — which, as we have seen, consists of minute parts of silver — is capable of manifold changes, and that, moreover, it is easily transferable, by means of the elastic collodion film, to other bodies. The film, with the picture, can be removed from the glass, placed in different solutions, and then transferred to curved surfaces, etc. If the collodion picture is placed in a metallic solution, a chemical change ensues. Assuming the solution to contain chloride of gold, then the chlorine passes over to the silver, of which the picture consists, chloride of silver is formed, and metallic gold is precipitated as a fine blue powder on the picture. Thus a gold picture is obtained.
With certain precautions this can be transferred to, and burnt into porcelain. By this means a dull image is obtained, which can be rendered brilliant by polishing.