Measuring the Surface- Tension of Liquids. 389 



zero) as compared with 71*0 by the common method at the 

 same temperature. 



Wilhelmy* weighed a rectangular plate of known dimen- 

 sions and specific gravity when dipped a determinate distance 

 into the liquid. He made a large number of determinations 

 which agreed closely for any one plate, but varied greatly 

 for plates of different materials in the same liquid. Using a 

 glass plate in water at 0° C. he found T = 77*9 dynes, assuming 

 that the angle of contact is zero. 



Waterstonf andWorthingtonJ modified Wilhelmy's method, 

 touching only the lower edge of the plate to the liquid surface. 



Sentis§ measured with a spherometer the height of a 

 weighed and measured rectangular iron plate floating on 

 mercury, eliminated the irregularity due to the corners by 

 means of a second plate of the same thickness and density, 

 and deduced the value of the surface-tension of the mercury. 



Lenard || arranged a regular succession of falling drops so 

 as to be seen by intermittent light ; measured the extent and 

 time of their oscillations ; and from these data calculated the 

 surface-tension. For water he found T = 70*3 dynes at 16° C. 

 (72*6 at zero), the mean of a considerable number of deter- 

 minations. With reasonable care in this method a clean 

 surface is ensured, and no question of contact-angle interferes 

 with confidence in the results. 



RayleighH measured the wave-length of the transverse 

 vibrations of water-jets issuing under constant pressure from 

 elliptical orifices. The results, which are not closely con- 

 cordant, are in harmony with the supposition that the surface- 

 tension is about 76 dynes. 



Bayleigh'*"* also measured the wave-length of ripples and 

 calculated the surface-tension by Thomson's formula. Par- 

 ticular attention was given to the purity of the water-surface, 

 and the results appear to be free from constant errors. 

 Individual determinations differ by 2 dynes in a few cases. 

 The mean of all, for both distilled water and tap water, is 

 T= 73*9 dynes at 18° C, corresponding to 75*4 at zero, which 

 happens to be exactly the mean of all the results quoted for 

 capillary tubes. 



The correctness of the assumption formerly made, that when 

 a liquid wets a solid the angle between the two surfaces may 

 be considered zero, is now pretty well established. 



* Pogg. Ann. cxix. p. 186 (1863). 



t Phil. Mag. Jan. 1858, p. 4. t Ibid. Tan. 188-5, p. 43. 



§ Journ. de Phys. ix. p. 384 (1890) ; or Phil. Mag. Dec. 1891, p. 564. 



|| Wied. Ann. xxx. p. 209 (1887). 



1] Proc. Roy. Soc. xxix. p. 71 (1879). 



** Phil. Mag. Oct. 1890, p. 386. 



