American cinematographer (Jan-Dec 1924)

Eighteen AMERICAN CINEMATOGRAPHER December, 1924 solution where anaerobic conditions would exist. Possibly gelatine dissolved from the film acts as food for the organisms. VI. Prevention of Fog 1. The Precipitation of Sodium Sulphide from Developers. (a) Precipitation by Silver Dissolved from Emulsions. When a developer is used continuously no trouble is liable to occur from sulphide fog, because the silver dissolved from the emulsion precipitates the sulphide as fast as it is formed. This accounts for the sludge of silver sulphide sometimes found in such a developer. However, when sulphide is being formed continually, and the developer is not used for a day or two, all the silver is precipitated and the excess sulphide fogs the next film which is developed. This is why the fogging tendency appears so suddenly in such a developer. Further, it was found that by developing some film in the fogging developer, the excess sulphide was removed and the fogging tendency disappeared. In this connection it is of interest to know the solvent power of a developer for the silver halides in the emulsions. The solubility of silver for the silver bromide in solutions of sodium sulphite was determined by Mees and Piper5, but when a film emulsion is passed through a developer other factors are present. Also, the silver halide first dissolved is reduced by the developing agent to free silver, either in a colloidal or a precipitated form. In the colloidal form it is partly responsible for the color of used developer solutions, and as a precipitate it forms a part of the black sludge sometimes found in the bottom of a tank of old developer. For example, the sludge which settled out of a tank of old No. 16 motion picture positive developer was found to contain 13.6% silver. In order, therefore, to determine approximately the solvent power of a developer for the silver halides in emulsions, the following procedure was followed : A solution was made up containing sodium sulphite, sodium carbonate and potassium bromide in the same concentration as in a developer but no reducing agent was added. 500cc. of the solution was placed in a tray and 12 sheets of film, 8 inches by 12 inches, were put through the solution. Six sheets were developed at a time shifting the sheets from the bottom to the top of the pile constantly for ten minutes. The solution was then allowed to stand over night so that the turbid matter settled out and the clear liquid Was pipetted off for analysis. In order to determine the concentration of silver in the solution, sodium sulphide was added and the turbidity caused by the silver sulphide was compared in a colorimeter with the turbidity produced under similar conditions by a solution of known silver content. The solution of known content was prepared from the same stock solution of sulphite, carbonate, and bromide by agitating a sheet of clear photographic gelatine for ten minutes, and allowing that solution also to settle over night. This procedure was necessary in order that the gelatine content might be similar to that of the solution to be tested, for the protective colloid effect of the gelatine had considerable influence on the state of division of the precipitated silver sulphide. For the comparison 5. Mees and Piper, Suplhite Fo£ and Solubility of Silver Bromide in Sulphite Solutions, Phot. ourn. 36. 221, (1912). Roy Davidge Film Laboratories NOW LOCATED AT 6701 Santa Monica Blvd. (Opposite Brulatour's) GREATLY INCREASED FACILITIES NEW EQUIPMENT COMPLETE COMMERCIAL PROJECTION ROOM PHONE GRanite 9503 FOR SALE Pa the No. 1110, completely equipped with six magazines, tripod, tilt head, mats and vignettes of every description. Guaranteed in perfect shape. 40, 50 and 85 mm. lenses. Bargain. JAMES C. VAN TREES American Society of Cinematographers SCHEIBE'S PHOTO-FILTER SPECIALTIES Are now popular from coast to coast, and in some foreign countries. If my many varieties do not always fill the bill, tell me your wants and I will make them on special order. Always at your service. GEO. H. SCHEIBE 1636 Lemoyne St. DUnkirk 4975 Los Angeles, Cal. "Walter. J. van Rossem 6049 Hollywood 51vd. Phone Holly 725 COMMERCIAL PHOTOGI^PHY Still Developing and Printing^* &Ho^Camaas-FOR RENT— Still FOR SALE BELL & HOWELL OUTFIT Camera, Tripods, Improved Thalhammer Iris, Six Magazines, 40 mm., 50 mm., 75 mm. and I 10 mm. Lenses, also Verito 75 mm., Carrying Cases and Trunk. Outfitted to Satisfy the Most Exacting Right price and right terms to right party. Call J. A. Dubray, 5448 Virginia Ave. Phone 433-305 Los Angeles