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 this means, he may be sure of the strength of his sensitizing solution without the trouble of frequently weighing the crystals of his bichromate salt.

No definite time can be given for immersing the tissue in the sensitizing solution, because the time should vary with the season of the year and temperature. In warm weather, in summer, one minute, or even less, is sufficient; in a moderate temperature, in spring and fall, say a little less than two minutes; and in cold weather, in winter, three minutes. The only disadvantage from an unnecessary long time in the solution is, that it loads the tissue with a superfluous quantity of water, which requires too long a time to dry out or evaporate, and makes the tissue very tender. But the tissue cannot be “over-sensitized” (in the sense in which the term is used in sensitizing paper for silver printing), by too long immersion in the solution.

In making the second transfer, Mr. Swan recommends a solution composed of two ounces of gelatine, half an ounce of glycerine, and one pint of water. As it contains no sugar, it may, under some circumstances, be found preferable to the other formula.

Another sensitizing solution has been recommended, and is made by dissolving three ounces of bichromate of potash in thirty-five ounces of water, and adding strong ammonia until the solution becomes alkaline. Use litmus paper to test its neutrality, and when neutral add a quarter of an ounce of ammonia, to be sure that the solution is strongly, but not too strongly, alkaline.

It is recommended to all, to try it. It is thought, by some persons, to possess advantages over the other in rendering the tissue more flexible, and causing it to lay more closely to the negative. On the other hand, these qualities are disputed.