514 Mr. E. S. Johonnott on the Thickness of 



become b 2 films, which shows the effect of sealing on the 

 character of the films. But on slightly warming the casing 

 all suddenly became b 2 films at a thickness of 6 fifi. 



Observation V. — This was taken Oct. 22nd, with the same 

 sealing as in IV. The greater thickness is evidently due to 

 the atmosphere of the films having been sealed for two days. 



Observation VI. — This was taken Oct. 24th, two days later, 

 with the same sealing, but at a temperature several degrees 

 lower, and gives practically the same thicknesses. 



Observation VII. — This was taken Oct. 25th at a tempe- 

 rature about 10° higher. The fall in thickness may have 

 been partially due to a leaking-in of the air. However, this 

 is a fair example of all observations on pure oleate solutions 

 at high temperature. 



Observation VIII. — This was taken Oct. 27th, with a low 

 and falling temperature, and shows a great thickness for over 

 six hours. The thicknesses are about the same as in VI. 



Observation IX.- — This was taken Oct. 28th after one side 

 of the casing had been unsealed for 16 hours. The fall in 

 thickness even at this low temperature corresponds closely 

 with that of Obs. II., at a higher temperature, taken ten days 

 previously under similar conditions. All the films broke 

 readily into the & 2 films as soon as the lights L and S (Hg. 1) 

 were left burning. 



Observation X. — This was taken some time later after 

 20 per cent, glycerine had been added to the solution, and 

 even with the most exposed atmosphere shows a great 

 thickness. No 5 2 films appeared. The casing was then 

 sealed, but the solution had become so contaminated that the 

 films broke too readily to permit of any trustworthy readings 

 being made. 



Solutions of Different Compositions. — No difference could 

 be observed between the pure oleate-of-soda and oleate-of- 

 potash solutions. The second black film formed under the 

 same conditions, and with the same characteristics, with both 

 solutions. However, with the addition of either glycerine 

 or potassium nitrate the character of the black film was com- 

 pletely changed. When freely exposed or heated, both would 

 show slowly moving eddies of darker regions. When potassium 

 nitrate was added these eddies broke up into tiny flecks moving 

 rapidly about in the film ; but never formed in clusters on the 

 frames, as was the case with the pure oleate solutions. While 

 the effect of the glycerine was to increase the thickness of the 

 black film, that of the potassium nitrate was to diminish the 

 range. With an oleate-of-soda solution to which 5 per cent, 

 potassium nitrate had been added, the maximum mean thick- 



