80C Professor Sir James Dewar [Jan. 21, 



ji rate of Ij cm. per hour for vertical fall of black boundary. The 

 black does not generally extend at a steady rate across a circular film. 

 Thus a film on a thin platinum wire ring 3 cm. in diameter, using; 

 IJ per cent potass oleate in 2b per cent glycerin, gave in successive 

 half-hours rates of 0'3, 0*23, 0-18 mm. per minute. This slowing 

 down is to be expected as the black approaches the lowest part, where 

 the ordinary drainage from the film accumulates to fcrm a permanent. 

 drop. It follows that a more uniform rate is obtained in a large 

 film than in a small one. Thus in a film on a rectangular frame 

 8 cm. long vertically by 2 cm. across, a uniform rate of U'iS mm. 

 per minute was registered through 5 cm. 



This last measure was obtained by a solution of 3^ per cent 

 potass oleate in 33 per cent glycerin in the evacuated tube of the- 

 form shown in Fig. 11. 



In this apparatus the film can be produced under such undisturbed 

 conditions that several grades of black frequently appear between the 

 ordinary deep velvety black and the silvery film which usually directly 

 follows it in order of thickness. As many as five distinct grades have 

 been seen, each perfectly defined in regular proportion by a sharp 

 horizontal boundary line. In the development of the film these 

 extend downwards at different rates, as shown in the accompanying: 

 graphs (Figs. 12, 13, 14.) 



The values of the observed rates in mm. per min. for a 5 per cent- 

 ammonium oleate solution in 50 per cent glycerin, in successive 

 periods of 20 min., were : — 



Comparable observations with a 3J per cent potass oleate in 

 33 per cent glycerin gave the following values. Here five grades 

 were detected : — 



Top. Uniform rate of 0*45 mm. per min. 



2nd 0-55 0-375 0-26 „ 



3rd 0-55 0-38 0-40 ,, 



4th 0-50 0-375 0-26 ., 



5th 0-40 0-28 0-20 „ 



Preceding these observations, the 4th and 5th grades had not ver}' 

 definitely separated. The values observed during^ this initial period 

 for the 2nd and 3rd grades were 1 • 3 mm. and 1 • mm. in succeeding- 

 10 min. : the two separating lines being only a few mm. apart and 

 parallel. The form of the graph was thus :— 



