508 Mr. E. S. Johonhott on the Thickness of 



While the thickness of the first black film always diminishes 

 with a rise in temperature, the thicknesses corresponding to 

 any temperature are widely different for films from different 



Fig. 4. 



solutions similarly prepared, and even for different sets of 

 films from the same solution. This is shown in the following 

 tables, and may be partially due to irregularities in tempera- 

 ture and rate of thinning, but more probably to the difference 

 in exposure and surface conditions of the solution. 



The thinning of the sealed film because of a rise in tem- 

 perature has an interesting bearing on a demonstration by 

 Warburg*, that the thickness of a very thin film must 



* Wied. Ann. vol. xxviii. (1886), p. 399. 



