THE PHOTOGRAPHIC ROOM 43 the ideal system is to have a continually running supply, so that the bath is kept constantly fresh. The photograph is immersed, and a small hose made to play on it to drive off every particle of surface chemical. Separate baths should be kept for black-and-white prints and the ordinary blue prints, if satisfactory results are to be obtained. When thoroughly washed, the print should be drawn through a squeegee, permanently attached to the side of the bath, to take off as much of the surplus water as is possible. If the groundwork of black-and-white prints comes up slightly muddied, it may be chemically treated to bring it up white, but great care must be exer- cised in this treatment lest the black lines of the drawing should get obliterated. It may be of some interest to the operator, or draughtsmen with some knowledge of chemistry or photography, to briefly indicate the chemical reactions with either the ferro-prussiate blue paper with white lines or the ferro-gallic white paper with black lines. In the former the paper is coated with potassium ferricyanide and ferric compound of iron. When exposed to the influence of actinic light, either from the sun or the electric arc, part of the iron in the sensitive compounds is changed from the ferric to the ferrous condition, which with potassium ferricyanide gives an insoluble blue compound which is pre- cipitated on the paper. Side by side with this reaction a portion of the potassium ferricyanide is reduced to potassium ferrocyanide, which, with the unchanged ferric iron, also deposits a blue compound on the paper. The net result is that a complex mixture of blue compounds is laid down on that portion of the paper, i.e. the background, which has been sub- mitted to the ultra-violet rays. The portions unaffected because of the protection afforded by the ink on the tracing are washed away in the bath, leaving the white lines on a blue ground. Similarly with ferro-gallic photo paper, i.e. paper which gives black lines on a white ground. In this case the paper is coated originally with a solution of iron salts, the ferric compound being reduced by the action of light to the ferrous state. The paper is now treated with a solution of gallic acid, which changes the ferric iron on the parts shielded from light into a bluish black compound. The exposed portion, where the trans- formation from the ferric to the ferrous state has taken place, is unaffected. In water bath, or one bath ferro-gallic paper, the coated material carries its own developer in the form of a powder on the surface. With this paper, on immersion in water, after printing, the ferrous salt, with the gallic acid in contact with it, is washed away, leaving fixed on the paper the black compound of ferric iron and gallic acid. Drying of Prints.—The prints are caught by spring clips at the edges, and hung up to dry. It is better to let the prints dry naturally, as artificial drying is apt to distort them badly, especially where they dry last. Register of Prints.—A photo register is kept by the photographer, showing when each print was sent out, and what was its destination.