Capillary Surface inside a Tube of Small Radius. 137 



experiments surface-tensions and their temperature-coefficients 

 were measured by the capillary-rise method. It is of primary 

 importance to be perfectly sure that, when a liquid is in 

 contact with its vapour (as was the case in these experiments) 

 the contact-angle with glass is zero. The experimental 

 test made to verify this condition consisted in forming a 

 bubble of the vapour of the liquid inside a capillary tube 

 (*65 mm. in diameter) containing the liquid under examina- 

 tion. The two menisci so formed — which were, of course, 

 concave to each other — were then caused to approach as 

 nearly as possible. It was argued that, were the contact- 

 angle zero, the capillary surface so formed would be spherical; 

 if the contact-angle were finite and acute, the capillary 

 surface would be lenticular. Direct measurement failed to 

 show any deviation from the spherical form, and it was 

 therefore assumed that the contact-angle was zero. 



But even assuming that the capillary surface in such a 

 tube is a segment of a sphere — which is not the case — it 

 seems as if the method would fail to show the existence 

 of contact-angles of a few degree^. Thus if we assume 

 a contact-angle of 8° for which cos i is *99, this would 

 involve an error of 1 per cent, in the value of the surface- 

 tension, as determined by the capillary-rise method. The 

 radius of the capillary tube (BA in fig. 1) being '033 cm., 

 the value of OB would be '029 cm., the difference between 

 the two values being 4-hundredths of a millimetre — a suf- 

 ficiently small quantity on which to base so wide a general- 

 ization. 



But, assuming that quantities of this order could be ac- 

 curately estimated, the deviation from sphericity of the 

 capillary surface should have been appreciable. Thus, to 

 take the case of ethyl alcohol, for which the contact-angle 

 with glass is almost certainly zero, the value of BA being 

 •033 cm., the value of OB would be, from equation (x.), 

 about *031 cm., the difference therefore being about 

 2 hundredths of a millimetre*. 



Taking into account the difficulties of measurement, the 

 test cannot be said to establish very satisfactorily the existence 

 of zero contact-angles, and serves further to emphasize the 

 fact that, in spite of its wide usage, the capillary-rise method 

 is not very trustworthy for the determination of capillary 

 constants. 



* For a discussion of certain other points connected with these 

 measurements, see Feustel, /. c. 



