398 



Lord Rayleigh on the Tension of Water 



The tension of the surface when the camphor movements 

 are just stopping may thus be reckoned at 53*0 O.G.S., or 

 about 72 per cent, of that of a clean surface. There is some 

 reason to infer that the tension is the same whatever kind of 

 grease be used. In the last experiment the sample of oil was 

 one of which it was necessary to take decidedly more than 

 usual (in the ratio of about 3 : 2) in order to stop the motions. 

 This was proved by the balance in the manner described in a 

 former paper. 1 have other grounds for thinking it probable 

 that the tension does not depend upon the kind of oil, and 

 hope to investigate the matter further by a more appropriate 

 method. 



On several occasions the effect of large additions of oil was 

 tried. The limit did not appear to be very definite ; for a 

 second and even a third drop gave a sensible indication. The 

 results were June 30, 38*8 ; July 1, 40'3 ; July 7, 41*0 ; 

 July 8, 41-7 ; July 26, 38'9 ; mean 40*1. They relate to 

 olive-oil ; and it is possible that the largeness of the quantity 

 required to approach the limit depends upon the heterogeneous 

 character of the substance. 



Two observations were made of the effect of additions of 

 oleate of soda to distilled water. When the limit seemed 

 nearly attained final readings were taken with the results : 

 July 8, 25-3 ; July 11, 24-6 ; mean 25'0. It will be seen 

 that the tension is lowered very much further by soap than 

 by oil. 



The principal results of the present experiments may be 

 thus summarized. The tension of a water surface, reckoned 

 in C.G.S. measure, is in the various cases : — 





74-0 



53-0 



41-0 

 25-0 



100 



72 



54 

 34 



Greasy to the point where the 



camphor motions 1 









In the last column the tensions are exhibited as fractions 

 of that of a clean surface. 



Postscript, Sept. 19. 



It appeared probable that the tension of otherwise pure 

 water saturated with camphor would be the same as that of 

 greasy water upon which camphor fragments were just dead ; 



