634: Dr. Allan Ferguson on the Drop- Weight 



this can only be satisfactorily done by an appeal to 

 experiment. 



In commenting, in a recent paper*, upon the remarks 

 which I made in the article cited, Mr. Morgan says, ". . . . until 

 the effect of the angle of contact is studied sufficiently to show 

 tliat it does have any effect upon capillary rise upon which 

 there is at present no general agreement . . . ." I think it 

 may be taken as quite certain that the contact-angle has a 

 marked effect upon capillary rise — the theoretical deductions 

 are perfectly clear on this point; and what few experiments 

 have been made are quite in agreement with the theoretical 

 conclusions. What is doubtful is, not the effect of the 

 contact-angle upon capillary rise, but the actual value of 

 the contact-angle for most organic liquids, and experimental 

 evidence on this point is much to be desired f. 



What evidence we have points very clearly to the existence 

 of a finite contact-angle in some common and easily purified 

 organic liquids, notably ether and oil of turpentine. MagieJ 

 has measured the total depths of large bubbles of air formed 

 inside such liquids, and has thus obtained values of a 2 



(2T\ 

 a 2 = — j, which depend on the angle of contact. He has 



also measured, for the same bubble, the vertical distance 

 from its vertex to its plane of greatest horizontal section, 

 thus obtaining a value for a 2 which is independent of the 

 contact-angle. If, assuming a zero contact-angle in the first 

 case, the two values for a 2 thus found be in agreement, the 

 assumption is clearly justified ; if the results disagree, an 

 approximate value of the contact-angle may be calculated 

 in terms of the disagreement. Theoretically and practically 

 the method is quite sound — its chief disadvantages being the 

 smallness of the observed quantities, and the difficulty of 

 accurately estimating the point on the contour of the bubble 

 at which the tangent is vertical §. In this way it has been 



* Journ. Amer. Chem. Soc, June 1915, p. 1461. 



t I hope, in the near future, to be able to give figures for the contact- 

 angles of various organic liquids obtained by several independent methods, 

 aud notably by comparing the values of T obtained by the capillary-rise 

 method with those deduced from Jaeger's method. 



X Phil. Mag. Aug. 1888. 



§ Magie's numbers do not give the absolute values of the surface- 

 tensions of the liquids used with any high order of accuracy — the object 

 of his investigation being to examine the evidence for the existence of 

 a finite contact-angle, and not to determine surface-tensions absolutely, 

 the liquids, as he states, were not specially purified. Nevertheless, with 

 one exception, the values are in fair agreement with those obtained by 

 other methods, and in every case are quite self-consistent. That exception 



