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1949 SPRAY PROCESSING 689
oxidation. Put in another way, it may be said that in the case of negative developers the use of a small-reservoir spray system may result in some saving of chemicals, while a large-reservoir spray would just about double the total cost of developing agent consumed. In the case of positive developers at pB. 10 the use of a small-reservoir system might increase the total cost of developing-agent consumption 'iby about 10 per cent, whereas the large-reservoir system might increase the cost by about 70 per cent. Thus the possibility of making economical use of spray processing would seem to depend upon the extent to which the volume of circulating developer can be reduced without losing control over the rate of change of the composition of the bath. The theoretically ideal condition would be that in which the volume of circulating developer is reduced to zero, for in this case the "replenisher" itself (presumably of constant composition) is sprayed on to the film and then runs to waste. This advance awaits only a: suitable application technique.
In the case of negative developers, there is little to be gained by using nitrogen or an airtight system, unless the oxygen content, of the air can be brought much lower than 4 per cent. If nitrogen were used, the cost of doing this would be prohibitive. Although reducing -the oxygen content of the air, especially in the case of positive developers, could be achieved by making the cabinet airtight, the trouble of doing this and the instability that would be introduced on opening the cabinet doors and admitting oxygen make the device scarcely worth while.
Some advantage might be gained in a spray system using a large reservoir of developer, if the air bubbles trapped in the returning developer could be separated before they are carried back into the bulk of the solution in the reservoir. This separation could, perhaps, be coupled with a removal of dissolved gelatin by allowing the foam to be removed from the system at this point.
ACKNOWLEDGMENT
The author would like to acknowledge the assistance rendered by Miss S. J. Moorcraft and Messrs. A. F. C. Hirst and J. Smith in constructing the apparatus and carrying out the many hundreds of analyses involved.
REFERENCES
(1) E. Tausch, "Zur Chemie der Photographischen Entwickler." Thesis; Technische Hochschule, Berlin, 1934.