Surfaces, Clean and Contaminated. 387 



rise of liquid between parallel plates of glass ; and I was not 

 satisfied with it, not merely on account of the roughness of 

 the measurement, but also because the interpretation of the 

 result depends upon the assumption that the angle of contact 

 with the glass is zero. In the opinion of Prof. Quincke, whose 

 widely extended researches in this field give great weight to 

 his authority, this assumption is incorrect even in the case of 

 pure liquids, and, as it seemed to me, is still less to be trusted 

 in its application to contaminated surfaces, the behaviour of 

 which is still in many respects obscure. I was thus desirous 

 of checking my results by a method independent of the 

 presence of a solid body. 



The solution of the problem was evidently to be found in 

 the observation of ripples, as proposed by Prof. Tait, upon the 

 basis of Sir W. Thomson's theory. Thomson has shown that 

 when the wave-length is small, the vibration depends princi- 

 pally upon capillary tension ; so that a knowledge of corre- 

 sponding wave-lengths and periods will lead to a tolerably 

 accurate estimate of tension. 



Besides some early observations of my own *, made for the 

 most part for another purpose, I had before me the work of 

 Matthiessen f , who has compared Thomson's formulae with 

 observation over a wide range of wave-length. The results 

 are calculated on the basis of an assumed surface-tension, and 

 are exhibited as a comparison of calculated and observed wave- 

 lengths. On the whole the agreement is fair ; but the accuracy 

 attained seemed to be insufficient for the purpose which I had 

 in view. As will presently appear, an error in the wave- 

 length is multiplied about three times in the tension deduced 

 from it, so that, in a reversal of Matthiessen's calculations, the 

 errors would appear much magnified. 



Quite recently Mr. Michie Smith has published an account 

 of experiments made by Thomson's method for the determi- 

 nation of the tension of mercury. Some anomalies were 

 met with ; and it seems not improbable that the vibrations 

 observed were in some cases an octave below those of the 

 vibrating source %. 



When it is remembered that Thomson's theory is one of 

 infinitely small vibrations, it will be seen that for my purpose 

 it was necessary above all things that the amplitude of vibra- 

 tion should be very moderate. The sub-octave vibrations of 



* " On the Crispations of Fluid resting upon a Vibrating Support," 

 Phil. Mag. July 1883. 



t Wied. Ann. xxxviii. p. 118 (1889). 



% Faraday, Phil. Trans. 1831. See also Rayleigh, Phil, Mag. April 

 and July 1883. 



