Gallic Acid, its preparation and properties, 27; used in paper processes, 178; becomes mouldy by keeping, 261. formula for developing paper Positives with, 261. Gallo-Nitrate of Silver, 177; discolours rapidly when developing dishes are not clean, 179. Gelatine, its properties, 341; forms a compound with an Oxide of Silver, 21; employed in dry Collodion process, 299; modified form of, 302; affects the colour in printing processes, 128; used in Positive printing to form an even surface layer, 126; as a cement to mount Photographs, 257. Glass plates, rules for cleaning, 39; details of cleaning, 213; mode of coating with Collodion, 215; with Albumen, 180. Glutinous Collodion, explained, 83. Glycyrrhizine, its nature, 342; its action in Collodion, 114; formula for solution of, 209. Gold, Chloride of. _See_ Chloride of Gold. Gold salts, their use in Photographic printing explained, 131; in the Daguerreotype process, 175. Gradation of tone, in Collodion Photographs, affected by the density of the film, 109, 113; by use of Glycyrrhizine, 115. Gradation of tone, in paper Positives, conditions affecting it in prints obtained by direct exposure, 123; in Positives printed by development, 266. Hadow, Mr., researches on Collodion, 77; formula for making Pyroxyline, 187. Heliography, invented by M. Niépce, 7. Historical sketch of Photography, 6. Honey keeping process, 289. Hunt, Mr., introduces Protosalts of Iron in developing, 111. Hypo Bath. _See_ Fixing and Toning Bath. Hyposulphite of Silver, its peculiar changes in colour, 129; the sweet compound which it forms with Hyposulphite of Soda, 44. Hyposulphite of Soda, preparation and properties, 43; theory of its fixing action, 43; blackens Nitrate of Silver, 129; causes a milkiness with acids, 137; its decomposition by constant use in fixing, 138; the salts it forms with Chloride of Gold, 133; its conversion into sulphuretting Tetrathionate by Iodine and Perchloride of Iron, 139; test for presence of, 169.