TILE BT.ILE-PRINT PROCESS is peculiarly adapt ed to the reproduction of drawings and plans, and is used by architects, engineers. and mechan ics. Two solutions are prepared: the first con tains one part of citrate of iron in four parts of pure water; the second contains one part of red prussiate of potash in six parts of water. When ready for use. equal parts of the solutions may be mixed in a shallow dish, and applied to sheets of paper with a sponge or a camel's-hair brush. Any paper will serve, but that is best which has but little sizing. The solution should 1w applied and the paper should be dried and kept in the dark. The solutions themselves will keep. sepa rately. in the dark as long as desired, hut if mixed soon begin to deteriorate. The drawing or writing to be copied should be made with very black ink, upon paper or tracing-cloth. A pho tographer's printing-frame. with a plain glass and a back easily removed. is used in the follow ing manner: Place the drawing face down upon the glass; the prepared paper with its face against the back of the drawing; put the 1110V able back in place, reverse the frame, and expose to light. In direct sunshine. two to :keNen min utes will be long enough], the time to be ascer tained by trial; in diffused light, the exposure must be five to ten times as long. After expos
ure the print should be immediately washed in clear water; when the chemicals are removed, the sheet is fastened by its corners to a line to dry, and the surface may afterwards be finished by a hot iron, or by pressure. A little practice is needed to secure the best results, and in a good print the lines will be clear white. and the back ground a deep blue. A light blue background indicates a weak solution, or insufficient expos ure; over-exposure is shown by a grayish tint. Clear, quick sunshine will give sharper lines than can be obtained by slow, diffused light. The chemical change is evident; the light causes it reaction between the prussiate of potash and the iron, of which Prussian blue is the product; this occurs wherever the light has not been inter cepted by the black lines of the drawing, which therefore appear in white upon an intensely blue and unfailing background. Copies may be multiplied at will from negatives on glass or films, from engravings in books, from drawings. or from manuscripts.