A Litre of water _weighs_ a Kilogramme, or something less than 2-1/4 Pounds Avoirdupois. A Cubic Centimetre of water _weighs_ a Gramme. INDEX.[56] [56] The preparation and properties of the Chemicals used in Photography will be found in the Alphabetical List commencing at page 327. Aberration, chromatic, 54; spherical, 56. Accelerating agents, their mode of action in Collodion explained, 95. Acetate of Silver, its preparation and formula, 365; its formation in Nitrate Bath explained, 89; ensures absence of free Nitric Acid, 116; tends slightly to favour fogging and spots, 104; contra-indicated for glass Positives, 111. Acetic Acid, properties and mode of testing purity of, 327; useful in preventing fogging, 104; in rendering the development slow and even, 99; does not coagulate Albumen, 329; a good commercial form of acid, 212; Acetic Acid essential in Calotype, waxed paper, and Albumen processes, 177; also in printing paper Positives by development, 260. Aceto-Nitrate of Silver, term explained, 177. Achromatic Lenses, their construction explained, 55; the visual and chemical foci often coincident in, 60. Acids, nature of, 308. Actinism, explained, 61; importance of distinguishing Actinic from visual rays, 62; mode of finding Actinic focus, 229. Affinity, chemical, 312. Albumen, its chemistry, 328; forms a compound with oxide of Silver, 20; used in Positive printing to produce a fine surface layer, 122; to increase sensitiveness, 125; affects the colour of the prints, 127; protects the image from oxidation, 150; putrifies when exposed to moisture, 155; discolours the Nitrate Bath, 245. Albumen negative process, its invention, 10; theory of, 180; Collodio-Albumen process of M. Taupenot, 294. Albuminized paper, formula for, 241; slow in fixing, 131; not well adapted for toning by Sel d'or, 269; good for stereoscopic subjects and small portraits, 249; cannot be sensitized with Ammonio-Nitrate of Silver, 246. Alcohol, its chemistry, 330; sometimes too dilute for making Collodion, 84; mode of rectifying, 196; must not contain impurities, 96; effects of adding to Collodion, 84, 96; to developer, 205. Alkalies, nature of, 308. Alkalinity of Nitrate Bath, explained, 88; the evils it produces, 104; how to test for it, 377; how to remove it, 277. Amber varnish, 226. Ammonia, preparation and properties, 331; its use in fixing, 42; Mr. Shadbolt's formula for, 271; its action upon Chloride of Gold, 343; effect of concentrated Ammonia upon Oxide of Silver, 362. Ammonio-Nitrate of Silver, its chemistry, 262; used in Positive printing to increase sensitiveness, 125; to give black tones, 127; cannot be used with Albumen, 246; increases permanency of print, 169; old Nitrate Baths not easily convertible into Ammonio-Nitrate, 248;