464 



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



= 0-0203. 



K. 



0-0156 

 0-0195 

 0-0200 

 0-0185 

 0-0180 

 0-0184 

 0-0190 



Table IXa. 

 N/200 KBr. K = 0-0170. 



Mean... K' = 0-0189. 



Mean... K' = 0-01603. 



In this case a reduction of only 5 per cent, in the velocity has taken 

 place. 



These measurements show that the rate at which a plate develops is 

 diminished with time of keeping. Apparently a slow irreversible 

 change takes place in the film, which is accelerated by heat and 

 presence of gases, etc. If we consider the expression for the velocity 



K = . a . , it seems probable that the alteration is due to an effect on 

 o 



A, the diffusivity coefficient. It is quite different from that produced 

 by so-called " hardening " agents, which do not affect the development 

 velocity. 



Only the absolute value of the velocity, and not the velocity-function, 

 is altered. 



The Diffusion Path, 8. In his work on heterogeneous reactions, Bruner 

 measured the thickness of the diffusion layer by electro-chemical 

 reactions, and found, for example, for rate of solution in water of benzoic 

 acid, 0*03 mm. In general, the layer was of this order, but was lessened 

 as the rate of stirring increased. 



In the photo film the diffusion path must be equal to the depth 

 of the latent image plus a small layer on the gelatine surface. The 

 former is not alterable by stirring, and the subsequent microscopic 

 measurements show that it is of about the same order as Bruner found, 

 viz., about 0'02 to 0'03 mm. The outer layer is alterable by rotation, 

 but is probably not very large, as increasing the rotation had but 

 a slight effect on the velocity. Unfortunately, very high velocities of 

 stirring increased the fog materially, and also produced uneven marking. 

 A rate of about 30 revolutions per minute was found to ensure homo- 

 geneity of the solution on the bounding surface. 



It should be noticed that the whole mass and surface of the develop- 

 able image in the case of development lie in the short diffusion layer in 



