the Black Spot in Liquid Films. 509 



diminish with an increase in the pressure of its surrounding 

 saturated vapour. It was assumed, in accordance with Lord 

 Kelvin's* surface-energy principle, that the thickness was so 

 small that the surface-tension diminished with the thickness. 

 As the pressure of the saturated vapour of a liquid increases 

 with the temperature, it would be expected, from this principle, 

 that the film would thin with an increase in the temperature. 

 Whether the thinning is due to this cause or to a loss of water 

 by evaporation from the surface of the films, such as Reinold 

 and Ruckerf have shown takes place at the surface of coloured 

 films, seems to be an open question. 



The remaining results are given in the following tables. 

 The numbers in the first column indicate the age of the films 

 in hours and minutes ; the second, the number of films ; the 

 third, the mean thickness in fifi ; the fourth, the deflexion in 

 fringes of sodium or white light ; the fifth, the temperature 

 inside of the casing ; while the character of the films is given 

 in the last column. The black film first formed is designated 

 by b x and the next by b 2 . 



Table I. — This was taken September 9th with a 1 : 40 oleate 

 of soda solution that had been sealed two hours. The films 

 were all thinned to the b 2 films twice by warming a brass 

 plate P (fig. 2), placed above the casing. From inspection of 

 the table it may be seen that the thickness fell in 1 hour to 

 15 /z/z. So long as the temperature was 24°°6 it remained 

 constant, and no b 2 films formed. The plate P was then 

 gently warmed, and as a result the b 2 films formed suddenly 

 on all the frames, while the mean thickness dropped to 6 '3 fjb/jb. 

 The plate was then removed, and the films all returned to the 

 first black within 10 minutes. The operation was repeated 

 two hours later with the same results. It may be observed 

 that the mean thickness, just before all had begun to thin to 

 b 2 and just as all had returned to b l films, was about 15 /jl/ju, 

 while the thickness of the b 2 films was about 6*5 fifi. 



Table II. — This was taken September 15th, with a 1 : 70 

 solution of oleate of soda that had been sealed 24 hours. 

 The deflexion began at slightly over two fringes, corresponding 

 to a thickness of 33 ft/z, fell to 20 fifj, in 3 hours, and to 15 fifi 

 in 20 days. The room was left undisturbed for two weeks. 

 The reading was then (412th hour), as usual, taken before 

 the number of broken films was counted, and gave practically 

 the same mean thickness. Two days later the brass plate was 

 warmed, causing the first b 2 films to appear. The resulting 



* Constitution of Matter, p. 179. 



+ Phil. Trans, vol. clxxii. (1881) p. 486. 



