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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1906 Excerpt:...image composed of the insoluble portions. (6) The altered substance may be incapable of absorbing moisture. This is the foundation of several mechanical printing processes, depending on the behavior of wet and dry surfaces toward greasy ink, upon the production of relief by swelling of the absorbing portions of the image, and so on. With the above variety of methods of treatment, it is not remarkable that the color of the image should differ widely in different processes. It may in a sense be considered even accidental. For example, let a sheet of paper be brushed over with a 15 per cent solution of uranium nitrate, and, after drying in the dark, be exposed to sunlight under a sheet of opaque paper with a few holes cut in it. The reductionproduct under the openings is barely visible, but will give a brown image when treated with a solution of potassium ferricyanide, because the color of uranium ferricyanide happens to be brown. Different developers will give different colors. The uranous reduction-product will reduce the salts of silver and gold, and if silver nitrate and gold chloride are used as developers, the image will be brought out in finely divided gray silver or bluish gold. It is possible to go still further, and to produce at will either a positive or a negative, in certain cases. A sheet of paper prepared as for blue-line prints (Chapter XV) and exposed under a perforated card will show blue spots on a white ground, or white spots on a blue ground, according as it is brushed over with solutions of potassium ferrocyanide or ferricyanide.1 Quite apart from its function of producing a visible image, it is therefore possible to regard the developer in general as a detector of the existence of a very small amount of chemical or physical change of cer...