94 



Lord Bayleigh 



[March 28, 



distance between corresponding points A, B, Fig. 3, is a measure of 

 the capillary tension to which these vibrations of the elliptical section 

 about the circular form are due. A measure, then, of the wave- 

 length of the recurrent pattern formed by the liquid gives us infor- 

 mation as to the tension immediately after escape ; and if we wish to 

 compare the tensions of various liquids, all we have to do is to fill a 

 vessel alternately with one liquid and another, and compare the wave- 

 lengths in the various cases. The jet issues from a flask, to which is 

 attached below a tubular prolongation ; the aperture is made small in 

 order that we may be able to deal with small quantities of liquid. 

 You now see the jet upon the screen (Fig. 3), it issues from the 



Fig. 3 



orifice ; it oscillates, and we can get a comparative measure of the 

 tension by observing the distance between corresponding points 

 (A, B). 



If we were now to take out the water, and substitute for it a 

 moderately strong solution of soap or saponine, we should find but 

 little difference, showing that in the first moments the tension of soapy 

 water is not very diiferent from that of pure water. It will be more 

 interesting to exhibit a case in which a change occurs. I therefore 

 introduce another liquid, water containing 10 per cent, of alcohol, 

 and you see that the wave-length is different from before. So this 

 method gives us a means of investigating the tensions of surfaces 

 immediately after their formation. If we calculate by known methods 

 how long the surface has been formed before it gets to the point B, 

 at which the measurement is concluded, we shall find that it does not 

 exceed y^ of a second. 



Another important property of contaminated surfaces is what 

 Plateau and others have described as superficial viscosity. There 

 are cases in which the surfaces of liquids — of distilled water, for 

 example — seem to exhibit a special viscosity, quite distinct from the 

 ordinary interior viscosity, which is the predominant factor in deter- 

 mining the rate of flow through long narrow tubes. Plateau's 

 experiment was to immerse a magnetised compass needle in water ; 

 the needle turns, as usual, upon a point, and the water is contained in 

 a cylindrical vessel, not much larger than the free rotation of the 



