470 



Messrs. S. E. Sheppard and C. E. K. Mees. [Dec. 20, 



which gives K, whence y M may be obtained by substitution ; but it 

 must be noticed that the expression only holds when 72/71 = < 0'8 

 approximately, i.e., when the tangential part of the curve is past. 



Comparison of Developers. By means of the 7 formulae the relative 

 efficiencies for different developers can be compared. The following 

 results give the comparison for ferrous oxalate, fluoride, and citrate : 



Ferrous Fluoride. The use of this salt was suggested by Peters' 

 work on the reduction potential of ferrous salts. Luther explains the 

 fact that ordinary ferrous salts Fe"R as FeSO 4 do not develop as 

 due to the reversal of the reaction by ferric ions. But Peters has 

 shown that the E.M.F. of a ferrous chain is very much increased by 

 the addition of a soluble fluoride, owing to the fact that Fe'"Fl 3 is but 

 slightly dissociated, and further that a complex ferrid-fluoridion is 

 formed, both causes tending to remove ferric ions. We found that 

 an N/10 solution of FeSO4 in excess of KF developed but slowly. 

 Prolonged development removed the film from the plate, so that 

 celluloid films had to be used. 



The developer was made by mixing FeSO 4 and NaF solutions, as 

 the sodium double salt seems more soluble than the potassium one. 

 The solutions were nearly colourless. 



. Table XIV. Ferrous Oxalate, N/10. 

 Temp. 13 C. 



Time. 

 3'0 mins. 

 6-0 



8-0 

 oo 



Time. 

 2'0 mins. 

 4-0 

 7-0 



8-0 



00 



y. 



0-64 

 1-13 

 1-39 

 2-00 



0-056 

 0-060 

 0-064 



K = 0-060 



Temp. 20 C. 



y- 



0-49 

 1-01 

 1-35 

 1-49 

 2-00 



-7 



0-062 

 0-076 

 0-071 

 0-074 



K = 0-071 



7^ = 2-00, mean value found by Filon's expression. 



I 



The experiments with ferrous fluoride showed that the law of the 

 constancy of inertia with time of development held here. For brevity, 

 only the 7 values are given. 



