288 The Bight Hon. Lord Bayleigh [April 3, 



WEEKLY EVENING MEETING, 

 Friday, April 3, 1903. 



His Grace the Duke of Northumbebland, E.G. D.C.L. LL.D. 

 r.K.S., President, in the Chair. 



The Right Hon. Lord Rayleigh, O.M. 

 M.A. D.C.L. LL.D. Sc.D. F.R.S. 



PROFESSOE OF NATUEAL PHILOSOPHY, E.I. 



Drops and Surface Tension. 



Lord Rayleigh introduced his subject by showing the well-known 

 experiment of water rising inside a capillary tube to a higher level 

 than that at which it stood outside, and explained the phenomenon 

 as a compromise between the tendency of the water to come in con- 

 tact with the glass, and thus creep upwards, and the tendency of 

 gravity to pull it downwards. Water was a liquid which tended to 

 wet glass, but if one, such as mercury, which did not have that ten- 

 dency, were employed, the opposite effect was to be seen, and the 

 liquid did not rise inside the tube so high as it did outside. 



Lord Rayleigh then illustrated the effect of capillary attractioE, or 

 surface tension, in determining the formation of drops, and mentioned 

 the part it played in soap-films. He next discussed some interesting 

 phenomena depending on the contact of materials with water not 

 perfectly pure. For example, fragments of camphor dropped on 

 perfectly clean water immediately were set in rapid rotation. But 

 if the surface were at all greasy, even to the extent that could be 

 produced by dipping the finger in the water for a few seconds, the 

 rotation stopped, to begin again if the greasiness were removed. He 

 had calculated that a thickness of oil amounting to two-millionths 

 of a millimetre was sufficient to stop the rotation from taking place. 



Extremely small amounts of grease had no effect on the surface 

 tension ; the first degrees of contamination produced no alteration at 

 all, and it was only after a certain quantity of grease had been added 

 that the alteration was noticeable, though it then increased very 

 rapidly. About half the amount of oil necessary to stop the camphor 

 rotating was required to affect the surface tension. But one-millionth 

 of a millimetre we might suppose to be about the diameter of an oil- 

 molecule; hence, short of the point where the surface tension altered, 

 there was only a single layer of oil-molecules on the water. 



Why the surface tension was altered by a greater number might 

 be indicated by an analogy. If a few marbles were floating sparsely 

 on mercury, tbey did not offer any particular resistance if one pushed 



