Osmose. An action that takes place when two liquids of differing densities are separated by a permeable medium. (See Dialyser.)
Over-Exposure is the undue prolongation of the exposure of the sensitive surface. The effect of over-exposure on the sensi-tive surface is that the image starts up quickly, and the plate shows signs of fogging [see Fog] before proper density is obtained, the resulting negative being thin, but full of detail.
Oxalate Developer. See Developer and Bromide Paper.
Oxalic Acid (Ger., Oxalsaure; Fr., Acide Oxalique). H2C204, 2H20 = i26. Prepared by oxidation of sugar or sawdust by nitric acid. It is recommended for acidifying the oxalate of potash solution for ferrous oxalate developer, but its chief use is in the preparation of oxalate of potash, ferric oxalate, and other stable salts. It is poisonous (60 grs. being often sufficient to cause death); its antidote, chalk, or lime in any form, with which it forms insoluble oxalate of lime. Solubility: 1 in 15*5 of cold water, 1 in 1 of boiling; sparingly soluble in alcohol.
Ox-Gall. The fresh gall of the ox purified, and evaporated to a suitable consistency. It is used photographically to make water-colours take to the surface of albumenised prints. It can be obtained from the artist's colourman as a sticky, rather offensive-smelling mass. It is soluble in water and spirit, and can be dissolved in either, or, preferably, a mixture of both, and applied to surface of print with camel's-hair pencil.
Oxygen. O = 16. A gaseous element, formingby weight of water, andof the air; it is the most abundant element known, entering into the composition of all animal and vegetable tissues, and forming about half the weight of the solid earth. It is used in the oxy-hydrogen light, and is usually prepared on a small scale by heating a mixture of chlorate of potash and black oxide of manganese.
Oxy-Hydrogen, Oxy-Calcium, or Drummond's Light. A
cylinder or ball of lime is heated by a blowpipe flame fed with oxygen, and glows with an exceedingly intense light; this light, in one form or another, being the chief illuminant for high-class magic-lantern effects. It can also be advantageously used for enlarging.