The conceptrations recommended later are averages which may be increased or diminished as desired. It must, however, be borne in mind that the use of comparatively highly concentratedreducers renders control difficult unless a glass dish, illumin ated from below, is employed.
459. Farmer's Reducer. The oldest known of the surface reducers (H. Farmer, 1884) is a mixture (prepared when required for use) of a solution of potassium with a solution of hyposulphite. In this very unstable mixture, which generally loses its activity (re cognized by the decoloration) in a time varying from a few minutes (mixture of highly concen trated solutions) to a few hours (mixture of very dilute solutions), the ferricyanide controls the activity of the mixture, the concentration of hyposulphite being always sufficient to give rapid solution of the silver salt which is formed.
Practised workers, in judging how to mix the two stock solutions, are guided by the depth of colour of the mixture. At first it will be suitable to mix in equal volumes a per cent solution of potassium ferricyanide and a TO per cent solution of sodium hyposulphite.
By making the mixture alkaline with a little carbonate of soda (Stiirenberg, 1903) or with ammonia (R. Namias, 1910) it will retain its activity considerably longer and its rate of at tack on the silver will be retarded a little ; the same time the somewhat persistent colora tion of the gelatine by the ferricyanide will be prevented.
In order to avoid irregular action, the dish must be rocked during the whole process of reduction. The negative must be rinsed every time it is taken out of the bath for purposes of examination, otherwise streaks of lighter density may occur.
As a solvent of the silver ferrocyanide, the hyposulphite may be replaced by cyanides (poisonous) or by the alkaline sulphocyanates (§398) ; the mixtures thus formed are very stable.
460. Permanganate Reducer. A very dilute, acidified solution of permanganate (R. Namias, 1899) behaves as a reducer. It is very econom ical and its action is not quite so entirely super ficial as that of Farmer's solution.
Starting from the same solutions already prescribed (§ 440) for the preparation of revers ing solution, the following mixture is made up when required— Potassium permanganate, 30 to 50 min. (3 to 5 c.c.) 0.4% solution Sulphuric acid, 2% solution 30 to 50 min. (3 to 5 c.c.) Water to make 2 oz. (I00 c.c.) Instead of the water, a 2 per cent solution of alum may be used in order to avoid the softening of the gelatine in warm weather.
The silver is dissolved in the form of sulphate ; a part of this is precipitated as the chloride by the chlorides present in the water used for making up the solutions or in rinsing the nega tive. In addition the negative becomes brown due to the manganese dioxide formed in the partial reduction of the permanganate. Both of these substances may be removed by im mersing the negative in a solution containing about TO per cent of bisulphitc of soda, followed by washing in several changes of water.
461. Other Surface Reducers. Of the many other reducers having practically identical pro perties, mention may be made of those which may be kept ready for use and which can be used several times.
The following may be included in this group: a mixture of ferric oxalate, sulphite of soda and hyposulphite of soda (Belitski, 1883), which is perfectly stable in the dark ; a solution of potas sium bichromate acidified with sulphuric acid (90 gr. of bichromate and I drm. 36 minims of acid to 20 oz., i.e. 10 grm. and ID c.c. respec tively to 1,0oo c.c.), which may be made up as a very concentrated stock solution (E. Gosselin, 1889) ; a solution obtained by adding to a solu tion of hyposulphite of soda a solution of cuprammonium sulphate which is prepared by adding ammonia to a solution of copper sulphate until the precipitate first formed is re-dissolved in a Large excess of ammonia (Prunier and Mathet, 1892) ; lastly a solution containing about 5 per cent of cerium (ceric) sulphate (poisonous) acidified with sulphuric acid (Lumiere and Seyewetz, 462. Proportional Reducers. Quirione Re ducer. The addition of sulphuric acid to the extent of about 3-4- drm. to 20 Oz. (20 C.C. to 1,000 c.c.) to a saturated solution (0.5 per cent) of benzoquinone (ordinary quinone) furnishes a reducer (Lumits_!re and Seyewetz, 1910) which has more recently been found (R. Luther, 1923) to act almost proportionally on the various densities. This solution, which is clear yellow, gradually turns brown, and finally gives a, brown deposit. An image which has been re duced in this bath acquires a slightly reddish tint. It is necessary, after reducing and brief rinsing, to bathe the negative for some time in a solution containing about 10 per cent of bisulphite of soda before proceeding to the final washing.