If removal of silver nitrate prints (called "de-silvering") is desired, this may be accomplished by placing the specimen in a 2-percent solution of mercuric nitrate in a tray similar to that used for the silver nitrate. To prepare a small amount of this solution, dissolve two-thirds of an ounce of mercuric nitrate crystals in 1 quart of distilled water and add one-third of a fluid ounce of nitric acid. Shake well. This solution, too, may be used several times before losing its strength and is not necessarily discarded after each use. It is not necessary to keep it in a dark bottle. The specimen bearing silver nitrate prints is immersed in this solution until all traces of the prints disappear. It should then be rinsed thoroughly in water to remove all mercuric nitrate. If this is not done the paper deteriorates, becoming brittle and crumbly. A tray of distilled water may be used for rinsing or a tray of ordinary tap water changed several times during the rinsing. The specimen is then laid out flat to dry. Wrinkles, such as are left in paper after ordinary drying, may be prevented by ironing with a moderately hot iron. An electric iron with a temperature control is desirable. If kept too hot it will scorch or wrinkle the paper somewhat. The bottom of the iron should be clean so that unremovable smudges will not be left on the paper. No ill effects have been noted from working in the 2-percent mercuric nitrate solution with bare hands for very short periods, but it is a caustic solution and it is suggested that the specimens be handled with tweezers or that rubber gloves be worn if contact is prolonged. CHAPTER XV _The Use of the Fingerprint Camera_ If a fingerprint is visible, an effort should be made to photograph it before any attempt is made to develop it. In every case a print developed with powder should be photographed before lifting. It sometimes happens that the print does not lift properly although it may be quite clear after development. The camera which is especially adapted to the purpose and which is easiest to handle and operate is the fingerprint camera, one type of which is shown in figure 424. This camera has several advantages in photographing fingerprints: It photographs the prints in natural size. It contains its own light source. It has a fixed focus. Cameras of this type are available in models operated by batteries and 110-volt current. It is believed that the battery-operated type has the greater utility, since house current may not be available at the crime scene. When not in use the batteries should be removed as they will eventually deteriorate and corrode the brass contacts in the camera. [Illustration: 424. The fingerprint camera.]