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 impossible. As the result of much experience, Mr. Swan has arrived at the conclusion that keeping the sensitive tissue for a long time in a moist condition, or drying it slowly, results in a decomposition analogous to that effected by light, producing uniform and complete insolubility. We are able to confirm his conclusions from our own experiments. After complete desiccation, the sensitive tissue may be kept for several days. We have kept it for a fortnight without change. However, Mr. Swan strongly recommends that the tissue be used on the first or second day after sensitizing. By keeping too long, a discoloration of the print results, precisely analogous to that produced by keeping sensitive chloride of silver paper too long. The print develops tardily, and the lights are not clear. Excessively prolonged immersion in the bichromate solution of course retards drying, and should therefore be avoided. As a rule, by sensitizing in the evening, a supply of paper may be prepared for printing next day; twelve hours' suspension in a dry atmosphere being amply sufficient for the necessary drying.

Perfect desiccation, so as to make the tissue horny and unmanageable, is not desirable. It is in such a case difficult to get perfect contact in all parts in the pressure frame, and difficult to mount the tissue before development. Should the tissue by accident be rendered too dry, it is desirable to hang it for a few minutes in a damp place, when it will quickly become sufficiently pliant to permit easy manipulation. On the other hand, it is obvious that the tissue must not be too damp, or retain the slightest capacity for adhesion, or ruin to the negative would be the necessary consequence.