1904.J The Chemical Dynamics of Photographic Development. 469 



Table XIII. 



The velocity of development is proportional to the concentration of the ferrous 

 oxalate. 



The magnitude y^, i.e., the limiting y of the plate, is of great 

 importance. It expresses numerically the ultimate density-giving 

 powers of plates, and gives a numerical measure of the quality that 

 has hitherto been expressed by such phrases as " contrasty plates," 

 soft plates, " flaue platten," and so forth, y^ is proportional to the 

 photometric constant of the reduced silver, and so varies slightly with 

 the developer, but more so with the emulsification of the haloid. 



The practical estimation of y^ and D^ is in many cases beset with 

 much experimental difficulty. If y x is very high, the resultant 

 densities are hard to measure, while many commercial plates, especially 

 rapid ones, give much fog on prolonged development. Since the 

 equilibrium is only theoretically reached after infinite time, only a 

 close approximation is reached by very long development. The 

 method given by Ostwald* for evaluating the end-result was employed 

 to check the results. 



The following method of obtaining y x and K from simultaneous 

 values of y and t was pointed out by Dr. L. N. G. Filon : 



Writing the equation -log e ^ = Kintheformy 

 and taking yi and y 2 so that t 2 = %, we get 



y w (1 - - 



putting e~ Kt i = x we have from (a) and (b) 



7i - l ~ x l 



^2 ~ (l- 2 ) 1+fc 1+e- 



and 



KC, 



T9 y~\ vt 



. /A /l /j K.C-J 



7i 



72-71 



_7i = K 

 72-71 

 Lehrbuch,' 2te ^uflage. 2 Th., 2 Bd., p. 210. 



