404 Mr. A. Ferguson on the Surface-tensions of 



the writer is acquainted are those o£ Forch and of Bonicke, 

 which were carried out by the capillar y-rise method*. 



In view of the fact that this method, despite its wide use,, 

 is by no means an ideal one for the determination of capillary 

 constants, it seemed very desirable to institute experiments 

 to investigate the point by a quite independent method. 



The method chosen was that which consisted in deter- 

 mining the surface-tension of the liquid under experiment 

 by observations of the maximum pressure required to release 

 a bubble of gas from the end of a vertical capillary tube 

 plunged below the surface of the liquid — usually known as 

 Jaeger's method. 



In a recent communication j" the writer has developed a 

 formula which expresses, in convenient form, the surface- 

 tension in terms of certain easily observed quantities. This 

 equation is 



where r is the radius of the capillary employed, h ± is the 

 maximum difference in level observed in the manometer, 

 pi the density of the liquid in the manometer, p' the density 

 of the liquid under experiment, h' the vertical depth of the 

 end of the capillary below the free horizontal surface of 



T 



the liquid, and a 2 , as usual, is identical with — -, . 



Writing (i.) in the form 9P 



a==B + -12a~> (»■> 



where B is therefore a known quantity, we see that to a first 

 approximation a 2 = B; and, therefore, more closely 



12 VB ^ ; 



which determines a 2 in terms of known quantities. 



It is convenient here briefly to point out the chief advan- 

 tages of the method. In the first place, it is independent of 

 any knowledge of the contact-angle between the liquid and 

 the tube ; Secondly, it is on two counts more sensitive than 

 the capillary-tube method. Neglecting minor corrections, 

 and assuming h' to be zero, equation (i.) becomes V ia&jzau 



2a 2 =rh u (iv.) 



* Winkelmann, Handbuch d. P/iysik, ed. 2, Bd. i. p. 1173 

 t Phil. Mag. July 1914. * 



