International Review of Educational Cinematography (Jan-Dec 1931)

— 1032 — very slowy under the action of the air, and the formation of products of oxidation is very low, even under the action of brome and bromide of silver. The necessary chemical conditions for obtaining the secondary image. In order to obtain a secondary image of organic coloured products simultaneously with the principal image the following conditions must be fulfiled : i) It must undergo the oxidating action of the bromide so that the coloured products can be fixed on the gelatine at the same time as the image is formed and in an equivalent proportion to that of the silver precipitated. 2) In spite of the absence of sulphite the developer must not be subjected to too rapid an action of the oxygen in the air : in this way the bath remains sufficiently clear during the development, and a general and highly undesirable colouring of the layer is avoided. Now, according to our experience the pyrocatechine developer is the only one known which corresponds to these conditions. Pyrocatechine and the formation of alkaline. Pyrocatechine produces a product of oxidation which, originally green, subsequently turns dark brown, and which has a strong indissoluble effect on the gelatine. It is well known that the product of oxidation of hydroquinone 's quinone. But although pyrocatechine is an product of oxidation of hydroquinone is cannot be said with certainty what kind of oxidation product is derived from i'. However this may be it is a fact that the watery solution of pyrocatechine can be added to alkaline carbonate to a degree reaching 10 % of the carbonate of potassium without the product blackening rapidly under the action of the air. The solution remains fairly clear for half an hour or more ; the rapidity of the blackening process under the action of the air also depends on the surrounding temperature. But with pyrocatechine not only alkaline carbonate may be used but also caustic alkali. In this case the process of oxidation by the air is much more rapid. The green product, which is first formed does not colour the gelatine layer, and so it is possible to develop the image without diminishing the transparence of the negative. As we shall see later this bath of pyrocatechine with caustic alkali and without sulphite, offers considerable advantages. Let us begin by considering the quantities of caustic alkali and alkaline carbonate to be used in the pyrocatechine bath. The deceased Dr. Konig considered that a bath of pyrocatechine containing caustic alkali in quantities sufficient to form a monophenol was adequate for the formation of the substitution of the hydrogen of one of the hydroxyles contained in the pyrocatechine formula. This substitution might even suffice for a bath containing sulphite of soda, although the action of the bath is very slow. For a bath without sulpite of soda the formation of a monophenol cannot be considered sufficient : the quantity of caustic alkali in this case must be sufficient — rather more than less — to produce the biphenol. Only in this way can a vigorous bath, corresponding to the necessary conditions for the forming of the secondary image, be obtained. The following formulas should be noted : C„ H4 (OH), Pyrocatechine : the two OH occupy the position 1-2. C6 H, (OH) (ONa) Alkaline derivative of pyrocatechine monophenol of sodium. C6 H4 (OM), Diphenol of sodium.