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



From these and other measurements it was calculated that from this 

 cause alone the probable error on a single developed density is 1 per cent.* 

 The authors have since constructed an instrument for the more exact 

 coating of plates. 



Method of Development and Developer. The thermostat and plate- 

 holders have already been described. Previous to development, the 

 plates were soaked for 2 to 3 minutes in a tube of distilled water in the 

 thermostat. Development was stopped by plunging the plates in 

 cold water, they were washed, fixed in pure hypo, washed, cleared in 

 dilute HC1, rinsed, wiped with cotton wool, and dried at a uniform 

 temperature. 



The developer first used was ferrous oxalate. A physico-chemical 

 investigation had shown that the iron was present as a complex anionf 

 Fe(C20 4 ) 2 , which is stable in the presence of excess of free oxalate ions, 

 according to the equilibrium 



4)2 ^ FeC 2 4 -f C 24 . 



When the ferrous oxalate is present in solid form, the constant for 

 the above equilibrium is 0'8 at 20 C. The standard solutions employed 

 were 



A. FeS0 4 ...... 278 grammes per litre, molecular, 



B. K 2 C 2 4 ...... 184 



checked by titration with standard KMn0 4 . 

 .The developer was always slightly acidified with pure H 2 S0 4 . 

 The Velocity of Development. This was studied by measuring the 

 gr.owth of the density, i.e., the mass of silver, with the time of reaction, 

 in full analogy with other kinetic investigations. The following facts 

 were experimentally proved : 



; (a) The silver deposited increases rapidly at first, then more slowly, 

 and finally tends to a limit. 

 . (b) This limit depends only on the exposure. | 



(c) The velocity depends upon the concentration of the reducer. 



(d) A soluble bromide reduces the velocity, but the " slowing off " 

 with time is not so pronounced. 



Temperature was always 20 '0 C., unless otherwise stated. 



Table I for a, b, and c. 



Developer A ...... 5 c.c. FeS0 4 , M/l. Exposure 10 CMS. 



45 K 2 C 2 4 , M/l. 

 B ...... 2 FeSO 4 , M/l. Exposure 50 CMS. 



48 K 2 C 2 4 , M/l. 



* Loc. cit., " Instruments for Sensitometry," ' P. J.,' 1904, No. 7. 



t ' Proc. of Chern. Soc.,' vol. 21, No. 288, p. 10. 



t Apparent exceptions to this law will be discussed in subsequent work. 



