mode of determining the strength of Commercial Nitric Acid, 186; table of strength of Nitric Acid of different densities, 378. Nitro-Sulphuric Acid, explained, 77; process for making by mixed acids, 186; by Oil of Vitriol and Nitre, 190; should not be used cold, 83. Nomenclature, chemical, 315. Norris, Dr., his dry Collodion process, 298. Notation, chemical, 318. Organic bodies, chemistry of, 324. Oxide of Silver, preparation and properties, 17; dissolves in the Nitrate Bath, rendering it alkaline, 88; properties of its solution in Ammonia, 362; preparation of ditto, 247; its solution in Nitrate of Ammonia used in Photography, 249. Oxymel, keeping process, 291; preparation of Oxymel, 360. Paper, Photographic, selection of, 240; peculiarity of English papers, 241. Paper, sensitive, for printing. _See_ Sensitive Paper. Perchloride of Iron, preparation of toning Bath with, 160. Permanence of Positives, mode of testing, 169. Photographic image, chemical composition of, 140; action of destructive tests on, 145. Photographic properties of Salts of Silver, 18; of Iodide of Silver upon Collodion, 74. Photographic researches by the Author, 140. Photography, historical sketch of, 6; the term explained, 61. Portrait lenses, theory of their construction, 59; rules for their use, 227; mode of finding chemical focus, 229. Portraits, drapery for, 66; directions for taking, 220; the position of the Camera, and other points of importance, 228; the time of exposure, 221. Positive printing, on Albuminized paper, formulæ for, 241; on plain paper, formulæ for, 245; on Ammonio-Nitrate paper, formula for, 246, 258; by development, formulæ for, 259; manipulatory details of printing, fixing, toning, washing, and mounting, 251; process of toning by Sel d'or, theory of, 134; practice of, 267; reasons for the want of correspondence between different formulæ, 257; use of Chloride of Gold in toning, 132, 271; theory of the preparation of the sensitive paper for Positives, 122; theory of the process of fixing, 129; of toning by Gold, 132; the Author's researches, 140; rationale of the printing process, 120; composition of the image, 140; fading of Positive prints, 160; destructive action of Sulphur on, 145; of oxidizing agents on, 148; of Chlorine, acids, boiling water, etc., on, 151; of combustion of coal-gas on, 153; effect of damp air on, 153; theory of mode of washing Positives, 162; comparative permanency of prints, 166; mode of testing permanency, 169. Positives, definition of, 106; Collodion Positives, theory of production of, 108; formulæ for solutions for, 201; development of, 111, 221; Collodion and Nitrate Bath best adapted for, 109; mode of whitening by Bichloride of Mercury, 112; solution for whitening, 207;