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 of calcium, sulphuric acid, or other substance having great affinity for water.

When the tissue is dry, it is ready for printing; it is, therefore, removed from the glass and placed in the pressure-frame, with the collodion surface in contact with the negative. The proper exposure is ascertained by the aid of the photometer described on another page. Before development, the tissue is coated with India-rubber solution in the same manner as the paper tissue referred to in another chapter, and is, in like manner, mounted on paper coated with India-rubber. It is then developed, washed, dried, and transferred as already described; the film of collodion, in this instance, forming the surface of the finished print.

Instead of coating the glass-plate with collodion, it may be rubbed with ox-gall, or with the solution of wax before mentioned, and coated with the sensitive tissue compound. When this is dry, it may be coated with collodion, removed from the glass, and treated in the manner already described. Or it may, instead of being coated with collodion, have a sheet of wet paper applied to it, and pressed in contact so as to adhere. It is then suffered to dry, and treated as the paper tissue in all respects, its only difference consisting in the fine surface communicated by the plate-glass, which becomes, finally, the surface of the transferred picture, and possesses a little more delicacy of effect than that produced by the ordinary paper tissue.

The process I use to make enlarged carbon prints consists of six successive operations:

1st. Choice and Preparation of the Glass.—The glass