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190 THE ADVANCE OF PHOTOGRAPHY
parts of the sensitive film. It should be remembered that intensification is of practically no use unless sufficient details are visible on the negative. Again, the plate should be properly fixed and thoroughly washed so as to remove all traces of lrypo before any attempt at intensification is made, otherwise stains are almost certain to result.
The mercurial intensifier is one of those most commonly used. Mercuric chloride is used to bleach the negative ; by its action a white compound of the chlorides of silver and mercury is formed, and then this is subsequently blackened by the action of some other reagent.
Dilute ammonia has frequently been used for this purpose, and although the results obtained by its action are of a somewhat uncertain nature, a dark brown colour is produced which renders the negative very suitable for obtaining good prints.
Some lorm of ordinary developer may be used to bring up the image after bleaching has proceeded to the desired extent, for if the negative only requires a slight degree of intensification it should be immersed for a short time only in the mercurial solution, Avhile on the other hand, if much intensification is desired, bleaching should be proceeded with until the whole film appears white when looked at from the back.
The great disadvantage of this intensifier is that it is a most violent poison, and hence should only be used with extreme precautions.
Another favourite method of intensification is that carried out by means of the uranium intensifier. By this method a solution containing potassium ferricyanide and some salt of uranium, usually the nitrate, is applied to the negative . The result of this is that a deep orange red deposit is formed upon the ordinary silver image, and so the opacity of the'negative is greatly increased. Here, again, the slightest trace of hy po will be found to cause stains or a kind of red fog. When using this method great care must be taken