1904] The Chemical Dynamics of Photographic Development. 473 



(2) From considerations of the growth of the density during develop- 

 ment, and of the theory of heterogeneous reactions, the equation 



- log ^ ^y = K, a constant, was shown to represent the reaction ; 



00 



this was experimentally confirmed. It is considered that the velocity 

 of development mainly depends upon the rate of diffusion to the 

 affected haloid. In agreement with this (a), the velocity as measured 

 by the constant above, is directly proportional to the concentration of the 

 reducer ; (b) is affected by the age and condition of the plate ; (c) is 

 reduced by the addition of soluble bromides, but not in the propor- 

 tion which would occur if the velocity of the chemical reaction 



Ag + R = Ag (met.) + R alone were measured. 



(3) The law of constant density -ratios due to varying exposures, 

 deduced from the above theory, is experimentally confirmed. 



(4) It is shown that the development-factor y in the sensitometry 

 equation D = y log E/i is governed by the same laws as a single 

 density. Methods for calculating and controlling y are given, by means 



of the equation log ^ = K. 

 * Tx-7 



(5) By means of this expression the velocities with ferrous oxalate, 



fluoride, and citrate were compared, the relative values of K being in 

 N/10 concentration 0'0487, 0'003, O'OOl ; the anomalies here and the* 

 theory of development are discussed. 



In conclusion, it is our pleasant duty to express our thanks to 

 Professor Sir William Ramsay, F.R.S., for continual interest and 

 advice throughout the research, and also to Dr. F. G. Donnan, and to- 

 Dr. M. W. Travers, F.R.S., for much advice and sympathy. 



