Measuring SurfaGe- Tension and Angles of Contact. 143 



I have, therefore, in what has preceded shown that it is 

 possible that the light produced by the vapour rising from 

 the mercury arc is due to the recombination of ions. If it 

 is possible to hold this view, it is more reasonable to do so 

 than to adopt the seemingly impossible assumption that the 

 positive ions remain luminous for *001 sec. Strutt has shown 

 even more clearly than was shown in my previous article 

 the difficulty of such an assumption, and while it may be 

 possible to imagine some mechanism of the atom which would 

 allow this to occur, any reasonable explanation of the phe- 

 nomena is preferable. I believe, therefore, that in the case 

 of mercury vapours the assumption that light is produced 

 by recombination is to be preferred. 



Colgate University, U.S. A, 

 August 1915. 



XX. On a Method of measuring Surface-Tension and Angles 

 of Contact. By Prof. A. Anderson, M.A., and Mr. J. E. 

 Bowen, M.A* 



THE following paper gives a method, with some pre- 

 liminary results, of measuring the surface-tensions of 

 liquids and their angles of contact with glass which we 

 have been unable to find described elsewhere. The method 

 depends on the measurement by optical means of the radius 

 of curvature of the meniscus in a tube immersed in a vessel 

 of the liquid and the height of the centre above (or below) 

 the surface of the liquid outside. It is thus freed from any 

 correction due to the mass of the meniscus and any un- 

 certainty as to whether the liquid actually wets the glass. 

 The curve connecting the radius of curvature thus deter- 

 mined and the radius of the tube at the top of the meniscus, 

 for a number of tubes of different diameters, enables the 

 angle of contact to be determined. 



The method will easily be understood by referring to fig. 1. 

 A is a glass vessel with plane sides and base, is a mark on 

 the base, and the tube B is placed vertically so that is 

 on its axis produced. This was done, the tube being held 

 in a clip attached to a ball-and-socket joint, by the aid of the 

 microscope M, using a low-power objective. The micro- 

 scope was one of the Bailey & Smith design, and moved 

 horizontally and vertically, being provided with verniers 

 reading to ^ mm. The microscope is now focussed on 



* Communicated by the Authors. 



