DEVELOPERS AND THEORY OF DEVELOPMENT 343 



Only small concentrations of these compounds are required, typical concentrations 

 lying in the range from 1 part per 10,000 to 1 part per 100,000. 



In addition to the inorganic restrainers and the organic desensitizers used for their 

 respective effects in the control of fog, a number of organic antifogging compounds 

 have been discovered. These compounds are not yet in common use in developers. 

 They include 



Antifogging Compounds 



2-Mercaptobenziminazole 



5-Nitrobenziminazole 



6-Nitrobenziminazole 



5-Chlorobenziminazole 



6-Chlorobenziminazole 



Aminobenzene 



Thioacetanilide 



Thioacetnaphthalide 



Cysteine hydrochloride 



Tetrazole 



2,4-Thioketothiazolidine 



These compounds are used in concentrations of 1 part in 10,000 to 1 part in 100,000. 

 In general they delay the initial appearance of the image and prolong the development 

 required. 



Other Compounds Added to the Solution. — In addition to the four components 

 typically found in developing solutions, many other substances have been added to 

 secure special results. Some of these additional compounds will be considered 

 here. 



Methanol. — Methanol has been added to some solutions particularly those where 

 a high concentration of reducer is desired, as it is frequently possible to maintain a 

 higher concentration in solution with both methanol and water present than with 

 water alone. The gelatin of the emulsion swells less when methanol is present in 

 sufficient concentration than it does in the simple aqueous solution. 



Sugar. — Sugar has been used to reduce the rate of diffusion of the developing solu- 

 tions into the emulsion. Lower gamma may be obtained, and some have also 

 felt that finer grain resulted from its use. There is little evidence that the grain is 

 finer than would result from the lower gamma, i.e., developed to the same gamma. 

 There is little difference in graininess from the use of the sugar in the solution. 



Citric Acid. — Citric acid is used as a clarifier. It tends to prevent precipitation 

 of insoluble compounds from some types of hard water. Being an acid, its presence 

 tends to reduce the pH, and hence it tends to reduce the rapidity of development. 

 This requires its use in small amounts only. 



Hardeners. — Various additions for developers have been suggested with a view to 

 hardening the gelatin during development. These suggestions have included chrome 

 alum, formaldehyde, and para-formaldehyde. Some success has been attained, but 

 there is a definite general preference to separate the permanent hardening of gelatin 

 from the developing bath. Very frequently this permanent hardening is provided by 

 a hardening fixing bath. At other times it is provided by a hardening stop bath 

 between development and fixation. Less commonly, it is obtained by use of a pre- 

 bath before development. For such use, dilute formaldehyde solution may be used, 

 but some workers have found undesirable fogging tendencies. This difficulty is by no 

 means universal, and the method is applicable to some emulsions at least. 



It is sometimes desirable to keep the swelling of the gelatin to a minimum during 

 development, particularly when its permanent hardening is produced by either a 



