(3) The sensitizing mixture is formed by a mixture of the diazo A and coupler C, reaction being prevented by the presence of an organic acid. Such papers can be developed by exposure to ammonia vapour without fixing or washing (G. Koegel, 1916-1922), or by wetting super ficially with a very dilute alkaline solution. For making copies of tracings (positive images with brown or purplish lines on a slightly tinted ground) Ozalid paper is commercially available, and cellophane film is sold as Ozafllm for copying X-ray negatives and cinema film. The process of dry development obviates any distortion of the support by wetting and consequently any variation in the scale of the copies. The ammonia must be allowed to evaporate completely before the copies are stored away.
(4) '1she diazo and the coupler C are coated on paper after one of them has been changed into an addition compound not susceptible to coupling. Development is effected by decom position of the addition compound by treat ment with water vapour, ammonia or alkaline solution (A. Feer, 1889 ; F. van der Grinten, 1926).
(5) The diazo A is decomposed by light and a substance D is formed which is capable of coupling with the unchanged substance A, either spontaneously after a comparatively long time, or immediately by alkalinization, the copies being positives after exposure under a negative (M. Andresen, 1895 ; G. Koegel, 1921).
694. Other Processes. Many other processes, producing pigment images or formed of dyes, cannot be described here, even briefly. Mention should, however, be made of various processes based on the differential reactions of ferrous and ferric salts with organic colloids (§ 62i), and also the processes employing the insolubilization of the gelatine of a gelatino-bromide emulsion by the oxidation products of the developer (in particular pyrogallol), when the development is done in a developer without sulphite or contain ing only very little sulphite (§ 350), or by using their reactions on amines or phenols, introduced in the developer or in the sensitive coating, to form insoluble dyes (R. Fischer, 1912). There are also various processes based on the fact that a solution of hydrogen peroxide, acting under suitable conditions on a negative or on a gela tino-bromide positive, dissolves the gelatine in the parts where there is no silver deposit (M. Andresen, 1898).