HYDROXYPHENOL OLYCIN.—See Glycin.
HYGROMETER.—An instrument for measuring the degree of moisture of the atmo sphere. The principle of these hygrometers depends chiefly upon absorption or condensation. An instrument of this kind is often useful in the photographic laboratory to detect the amount of moisture in the atmosphere. A very simple one was constructed and used by the late W. B. Woodbury in his laboratory. It consisted simply of a piece of carbon tissue fixed in an upright support. The piece of carbon tissue absorbed the moisture of the air, the gelatine expanded, and the strip was made to move or curl downwards. On a piece of cardboard at the back a scale was drawn showing the amount of moisture present.
For more delicate purposes, however, the most efficient hygrometer is one recently invented by Dr C. Koppe.* Fig. 222 is an illustration of it. It consists of a frame, to which one end of a human hair, carefully freed from grease, is fixed, the other end is wound round one groove of a pulley wheel with two grooves. Round the second groove a filament of silk is wound in a reverse direction to that of the hair, and, at the lower end of the hanging silk or filament, a little weight is attached, so that the hair is kept constantly under tension. An index needle attached
to the wheel in front of a scale of 90 deg., divided into ioo parts, shows the amount of any elongation or contraction of the hair. When the hair grows dry the needle turns to the left, and when it becomes damp the needle turns to the right. If the surrounding air be completely saturated, the needle points to loo on the scale.
IIYPO.—An abbreviation for sodium thiosulphite, commonly called hyposulphite of soda, largely used as a fixing agent in photography. (See Sodium Hyposulphite.) HYPOCHLORITES.—The salts of hypochlorous acids, usually obtained in solution by neutralizing hypochlorous acid with bases. They are oxidizing agents, and for this reason have been recommended for eliminating the last traces of sodium hyposulphite from the fixed and washed prints. Like all other so-called "eliminators," however, they have a tendency to attack the picture itself.