On an Optical Test for Angles oj Contact. 285 



in which 



ty\ 



, 8a 11 



• in -t- fir _i • ± _ 



H" 



W -*s "=; + ,- 7 - 



, ,„ 11 1 1 > 

 1 1 



The above series are suitable for short coils. (J) and (K) 

 are convergent for all lengths. (L) and (M) converge only 

 if b < 2a. 



XXXV. On an Optical Test for Angles of Contact. 

 By Prof. A. Anderson, M.A., and J. E. Bowen, M.A.* 



IN the February Number of the ' Philosophical Magazine ' 

 we gave a method of determining surface-tensions from 

 observations on the heights to which liquids rise in tubes, 

 which is free from corrections of any kind, and the accuracy 

 of which depends entirely on the capabilities of a measuring 

 microscope. We showed how, by plotting curves giving the 

 relation between the radius of the tube and the radius of 

 curvature at the centre of the meniscus, the angle of contact 

 could be deduced. The curves obtained for water and some 

 other liquids showed that — at least, very approximately — this 

 angle was zero. 



The following experiment was designed to test directly 

 whether any angle of contact differing from zero really exists 

 in the case of water and some other liquids which were 

 examined. If a rectangular piece of very thin glass be 

 dipped in a liquid and then held in a vertical plane with two 

 of its edges horizontal, there will be a long cylindrical drop 

 attached to it. Its cross-section or the appearance it presents 

 when looked at endwise is one of the forms represented in 

 the figure (fig. 1). 



* Communicated by the Authors. 



