Paper which has been so chemically treated that the color of its surface may be altered by the action of light is known as sensitized paper. Under this general designation are included numerous papers differing from each other in the details of manufacture, though the name is most commonly applied to paper that has been floated in a bath of nitrate of silver, or coated with an emulsion of silver-nitrate of chloride. ♦For blue-prints♦ One of the most common of papers included under the general term is that known in general trade as blue-process paper, which is prepared by floating white paper in a solution of potassium ferrocyanide. It is used for copying plans and maps, as well as for printing photographic negatives. After exposure to the light for the proper length of time, under the subject to be reproduced, the print is finished by immersion in several changes of clean water. Very similar to the blue-process paper is the blue or ferro-prussiate paper, which is sensitized or made sensitive by being treated with a solution in water of red prussiate of potash and peroxide of iron. This may be applied as a coating to the surface of the paper, or the latter may be floated upon the solution. When exposed to light under a drawing, those parts of the sheet to which the light has access through the transparent portions of the drawing are more or less affected, according to the greater or less transparency, as well as to the length of the exposure. When this printing has proceeded as far as desired, the sheet is washed in clear water, and those parts that have been protected from the light, become white, while those exposed to the light and affected by it take on, when dry, a permanent blue. ♦Other sensitized papers♦ Another variety of what may properly be termed sensitized paper is the arrowroot-paper used in photography for positive prints. It is plain or non-glossy, and is coated with a weak solution of arrowroot in water, with sodium, chloride, and a trace of citric acid. Photographic paper, as such, includes a great variety of these sensitized papers, employed in various processes of the art; albumenized, salted, coated with emulsion, or otherwise treated. One of these, known as Pizzighelli paper, a sensitized platinum-paper, gives a neat surface, and soft, clear, gray tones, which are most artistic and pleasing for many subjects. ♦Carbon or transfer♦