Intensification is photographically the opposite of reduction, the object being to increase contrast. This is done by the deposition of some other material on the silver image. A silver image, for instance, can be very much intensified by toning it with uranium, the reddish brown uranium ferrocyanide having very great printing strength and making quite a weak negative into one having a great effective contrast for printing purposes. Usually, however, intensification is formed by depositing silver or mercury upon the image, and most photographic intensifies depend upon the use of mercury.

Mercury is a metal which forms two series of salts, the mercuric salts, which are in a higher degree of oxidation, and the mer-curous salts.

Many of the mercuric salts are insoluble in water, but mercuric chloride is sufficiently soluble for practical use, and when a silver image is placed in a solution of mercuric chloride, this reacts with the silver and forms a mixture of mercurous chloride and silver chloride.

The bleached image, which appears white, can then be treated in various ways. If it is developed, for instance, both the silver chloride and the mercurous chloride will be reduced to the metal, and in addition to the silver, with which we started, we shall have added to every part of silver an equal part of mercury. Instead of using a developer we may blacken the image with ammonia, which forms a black mercury ammonium chloride and produces a high degree of intensification Mercuric Chloride is a virulently poisonous salt known popularly as "corrosive sublimate." Its only use in photography is for intensification, and it is obtained in white, heavy crystals which are soluble with some difficulty in water.

Portrait Film Negative, Artura Print By M. B. Nicholson Kansas City, Mo.

Portrait Film Negative, Artura Print By M. B. Nicholson Kansas City, Mo.

For many purposes separate bleaching and redevelopment is inconvenient, and for this reason the Eastman Intensifier has been placed on the market, this consisting of a mercury solution in which the intensification proceeds continuously so that it can be stopped at any time. This does not give quite so great an intensification as the use of the two solutions, but it is far more convenient in operation.

A very powerful method of intensification, used chiefly for negatives made by photo-engravers, is obtained by bleaching with mercuric chloride and blackening with silver dissolved in potassium cyanide. The use of the cyanide cuts the shadows very slightly at the same time that the highlights are intensified, so that a great increase in the contrast of the negative is obtained. This is usually known as the "Monck-hoven" Intensifier.

The only other intensifier which calls for notice here is the chromium intensifier. The silver image is bleached with a solution of bichromate containing a very little hydrochloric acid, bichromate being an oxidizer of the same type as permanganate or ferricyanide. The image is then redeveloped and will be found to be intensified to an appreciable extent. This intensifier has found increasing favor owing to the ease and certainty of its operation Potassiunr Bichromate is made by the oxidation of chromium salts. It forms orange red crystals, stable in air, and is easily soluble to a yellow solution. It is obtained in a pure form by crystallization. Potassium bichromate is used in photography both for bleaching negatives and for sensitizing gelatine, fish glue, etc. When gelatine containing bichromate is exposed to light it becomes insoluble in water and in this way images may be obtained in insoluble gelatine.