1904.] The Chemical Dynamics of Photographic Development. 453 



The extent to which the exponential law holds for gelatine emul- 

 sions is of great practical importance, as it provides a rapid and easy 

 method of measuring the amount of substance present. The following 

 experiments indicate that it holds within certain limits. 



Table calculated from Hurter and Driffield. 

 Plate - AgBr per 100 cms. 2 Density. 



1 .................. 0-016 gramme 0-240 



2 .................. 0-031 ; ' Q-520 



3 .................. 0-062 1-000 



4 .................. 0-124 2-900* 



The writers converted silver into AgBr and measured the density in 

 each case. Considering how high the figures are, the results are 

 satisfactory. 



Series I. 



Series II. -, .. 



Density of silver ......... 1-009 "I -or A r> 



AgBr ......... 2 . 026 |P, for AgBr = 0-0393. 



Comparing these with Hurter and Driffield's numbers, it is obvious that 

 the physical state of the haloid has tremendous influence on the opacity. 

 Considering the importance of this quantity for sensitometry, the 

 constant P seems worthy of investigation in connection with emulsifica- 

 tion. 



The investigation shows that the density can be accepted as a measure 

 of the mass of silver reduced. 



Errors due to Plates. Unevenness in the coating of the plates intro- 

 duces a large source of errors. In these researches, Wratten ordinary 

 emulsion specially coated on patent plate glass was used, the edges 

 being rejected. We have to thank Mr. Wratten for the care taken 

 and his readiness in coating experimental plates. Nevertheless, as the 

 following figures show, the error from this source is considerable. 

 Plates similarly exposed and developed, resulted as below : 



a. b. c. 



1 ......... 1-364 1-336 1-403 



2 ......... 1-329 1-342 1-354 



3 ......... 1-289 1-321 1-247 



4 ......... 1-287 1-349 1-280 



5 . 1-337 1-375 1-340 



Mean 1-3212 1-345 1'325 



Greatest dev. 3 '2 per cent. 2 -2 per cent. 6-0 per cent. 

 * Probably due to "scatter" error. 



