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HISTORICAL SURVEY 25 or, still better, of ammonia. The slightest touch with the finger or rub of the sleeve, the smallest drop of saliva which might chance to escape from the mouth in coughing, would spoil the polished surface; nay, even the atmos- pheric air produces with time the same effect. If a cleaned plate is left only twenty-four hours in the air, it gradually condenses vapours on its surface, and another cleansing is rendered necessary. The cleaned glass is then coated with collodion. The collodion itself is, as we know, a solution of nitro-cellulose in a mixture of alcohol and ether, in which certain iodides and bromides—for instance, iodide of potassium and bromide of cadmium—have been dissolved. This solu- tion must also be prepared with the greatest attention to cleanliness. The purity of the materials employed is likewise of the greatest importance. The solution must be allowed to stand a long time, and carefully poured off from any sediment. The coating of a plate with collodion requires a certain manual dexterity, and only succeeds with those who have witnessed the process and had some practice. It is usual to hold the plate horizontally by one corner, or by a pneumatic holder under the centre of the plate, and to pour over the centre of it a pool of the thick fluid, and then to allow this to flow to all the four corners in succession by a gentle inclination of the plate, ultimately pouring off the superfluous fluid at one of the corners. A considerable part of the fluid originally poured upon it remains, and adheres to the plate. Whilst pouring off the excess of collodion streaks are liable to be formed, which would spoil the picture ; to avoid this, the plate whilst being drained must be con- stantly kept in motion until the last drop has run off. The fluid stiffens into a soft, moist, spongy film. At this moment the plate must at once be immersed in the solution of nitrate of silver (called the silver bath).