Manchester Memoirs, Vol. Ixv. (1921), No. \ 



IV. — Studies in Capillarity. 



I. Some General Considerations and a Discussion of Methods 

 for the Measurement of Interfacial Tensions. 



By Allan Ferguson, M.A., D.Sc, Department of Physics, 

 Manchester College of Technology. 



{Read February nth, 1921, at a meeting held jointly with The Faraday 



Society.) 



The series of experimental studies in capillarity to which this paper 

 forms an introduction owes its immediate origin to the pressing necessity 

 for reliable values for the surface tensions at liquid-gas surfaces, at solid- 

 liquid surfaces, and at solid-gas surfaces. An equal need exists for a 

 knowledge of the temperature coefficients of these quantities, and of the 

 contact-angles of liquids with solids, a need which has grown with, and is 

 further emphasised by the remarkable development during the last genera- 

 tion of colloid chemistry and physics. 



The value assigned to a surface tension varies very greatly (see p. 6) 

 with the experimental method employed, and while it is true that much 

 excellent experimental work has been done, it is equally true that much 

 of this work has suffered from a lack of co-ordination, and from a want of 

 appreciation of the dynamical and mathematical principles involved. In- 

 deed a large part of the recent work on surface tension is characterised by 

 an accuracy of experiment which verges on the meticulous, accompanied 

 by vague dynamical argumentation in which writers have been saved from 

 serious mistakes of the hundred-per-cent. order only by a providential can- 

 cellation of errors. 



Thus, in a recent study l of the surface tensions of water and of benzene 

 in which the experimental work is of a very high order, the writers point 

 out that the values of the surface tension of water as measured by the 

 capillary-rise and the ripple method differ by as much as 4 per cent. — the 

 deviation being due mainly to the fact that " the theoretical basis of some 

 of the methods is not sufficiently exact ". 



Nevertheless, the same writers, in discussing the drop-weight method, 

 define an "ideal" drop as one whose weight is given by the equation 



W = Mg = 27T/-T (i) 



"the weight which the drop from any tip would have if Tate's law were 

 valid". Tate, 2 I fear, never promulgated any such law, but contented 

 himself with a simple statement of the proportionality between the weight 

 of the drop and the radius of the tip from which it falls. 



Equation (i), in fact, outrages elementary mechanical principles. It 



i your. Amer. Chem. Soc, 41, 499 (1919). 2 Tate, Phil. Mag., 27, 176 (1864). 



May 17th, 1921. 



