370 



considered as flat; tlio i-adiiis of' ilie iiari-ow ttibe at the to() wliicli 

 was seiisililj flat was 1.090 mm. Tlirongli a rubber tube llie wide 

 tube was connected with an adjustable funnel filled with mercury ; 

 by slowly raising the funnel the moments are easily marked at 

 which first k and then // attain a maximum '). 



The maximum-values of h and 7/ were found to be to a high 

 degree dependent on the slowness with which the drop was being 

 formed'. From these experiments therefore a definite value for the 

 surface tension mercury-air did not follow. As an instance for a 

 drop which had been exposed to the air for a very long rime 

 /i„j =z 0.490 (at /'=:J7°.3) was found, which l)y means of eq. (4) 

 fi, — f/, =z 13.55 g = 981) leads to 



o — 0,355 (1 +0,074-0,006) =: 379, 

 wheieas immediately after the formation of a new drop the observ- 

 ation gave A = 0.592, whence ^> = 454. 



Similarly an experiment where the meiciiry lan over about 1 min- 

 ute after the surface being renewed gave //,„== 0.708 (at 18°. 2 C) 

 whence (eq. 11) 



a =51 3(1 -0,102 -0,004) = 459, 

 whereas for a drop which did not flow ovei' till after half an houi- 

 Hm = 0,Q5d, c. e. '7 = 423; after some hours these values had even 

 gone down to //,„ = 0,619, n = 393. ') 



') The maximum of // rati he very easily observed by using a micrometer with 

 moveable cross-wire ; the fixed horizontal wire f is set on the meniscua in tlie 

 wide tube, the moveable wire 11, ;ilso horizontal, on the meniscus in the narrow 

 tube. The funnel is (irst moved up until the mercury protrudes above the narrow 

 tubf as an almost hemispherical meniscus: by now raising it very slowly or by 

 adding mercury a drop at the time, so that 1 and 11 rise slowly, the distance 

 I — II is seen to increase slowly and attain a greatest value. After that I and II 

 continue to rise, but the distance I-U now diminishes At the same time the 

 drop above the narrow tube is seen to bulge out more and more, to exceed 

 distinctly tiie half-sphere and finally fairly suddenly to swell and tlow over the 

 edge of the tube ; at that moment tlie level 1 falls very rapidly, so that H has 

 gone through a maximum. 



2) This diminution of the surface tension of mercury which is exposed to the 

 air was first observed by Quincke (Pogg. Ann. 1 (1858) p. 105). Similarly Grünmach 

 (Ann. d. Phys , ('4) 28 (1909) p. 247 ; method of capillary waves) found a much higher 

 value of T for fresh surfaces ('7=491,2 at about 18°) tlian for surfaces which 

 had been exposed to the air for half an hour (t = 405,0). See also VVinkelmann, 

 loc cit., p. 1 1G8. 



Volatile vapours in tlie air also appear to lower the surface tension of mercury 

 very considerably; it was found in the experiments of § 6 that it was sutficient 

 to bring a piece of blotting paper soaked in benzene or alcohol near a practically 

 hemispherical drop in order to make it flow over at once. 



