1904.] The Chemical Dynamics of Photographic Development. 465 



the gelatine. How far this modifies the Nernst theory for the reaction- 

 velocity will be considered later. 



A slight periodicity in the rate of development was sometimes noted, 

 see curve*. 



'5 



Periodicity in development 



o 10 20 30 

 Time in mins 



50 60 70 80 



30 100 



Note on Fog. It is usual in density measurements to subtract the so 

 called " fog " from the total density. This is the density due to inherent 

 fog in the plate, extraneous light, etc. Usually a separate reading is 

 made of the so-called fog strip, but, owing to the differential nature of 

 our instrument, the " fog " reading was automatically subtracted, and 

 taken from the same portion of emulsion as the exposure. 



But the formula - log =- 5 = K shows that there is a theoretical 



^00 ~ ^ 



error in assuming the fog in the unexposed strip is equal to that in the 

 exposed strip. 



For let A + B = C be total mass AgBr, 



where A = AgBr changed by light, B = AgBr unchanged. 



Then rate of fogging = (dD/dt) C = K (A + B) in fog strip, and equals 

 (dD/dt) B = KB in exposed film. 



Obviously, fog increases faster in unexposed film than in the exposed ; 

 it is desirable, therefore, for photo-chemical investigations, to have an 

 emulsion which does not give fog-density higher than (H5 to 0'2, even 

 on infinite development, and in most of this work this was fulfilled. 



Law of Constant Density Ratios. Another deduction from the develop- 

 ment formula is the law of constant density ratios. If a series of 

 increasing exposures are given to a plate, as every density increases 



* Cf. Ostwald, ' Tories, iiber Naturphil.,' Leipzig, 1902, pp. 274, 315, 362, ' Zeit. 

 Phys. Chem.,' 35, p. 33, 1900. G-. Bredig u. Weinmayer, ' Z. f. p. Chem.,' 42, 

 p. 600, " Periodic Contact Analysis." 



2 M 2 



