386 Lord Rayleigh on the Tension of Water 



From these data the density of dry air at 747*97 millim., 

 and at the temperature 15*70, is found to be 0-0012046. The 

 last figure is doubtful, as being beyond the limit of accuracy 

 with which the volume is known. 



If, for confirmation of this result, reference is made to 

 accepted results from the Tabellen of Landholt and Bornstein, 

 the density of dry air under the same conditions of pressure 

 and temperature is deduced as 0*0012033. From Bieder- 

 mann's Chemiker-Kalender (1888) is deduced the number 

 0*0012052. The mean of these two closely agrees with the 

 experimental result, absolutely indeed to the fourth significant 

 figure, the last reliable one in the determination. 



In conclusion, I may point out that this method leaves the 

 gas finally in a vessel, which, by the addition of a small side 

 way to the connexion b, may be made a most convenient one 

 for the subsequent transference of the gas into any apparatus 

 for its analysis. 



The present method may, as in the case of Regnault's, be 

 by obvious procedures applied to the determination of the 

 variation of the density with change of temperature, or 

 change of pressure. And its application to the determination 

 of vapour densities suggests itself. In this case the tempera- 

 ture of the volumenometer would, perhaps, best be adjusted 

 by the use of a vapour in the jacket, and an arrangement for 

 controlling its pressure. 



XLIII. On the Tendon of Water Surfaces, Clean and Conta- 

 minated, investigated by the Method of Ripples. By Lord 

 Rayleigh, Sec. R.S.* 



"l^TUMEROUS and varied phenomena prove that the ten- 

 -i-1 sion of a water surface is lowered by the presence of 

 even a trace of grease. In the case of olive-oil, a film whose 

 calculated thickness is as low as 2 micro-millimetres is sufficient 

 to entirely alter the properties of the surface in relation to 

 fragments of camphor floating thereupon. It seemed to me 

 of importance for the theory of capillarity to ascertain with 

 some approach to precision the tensions of greasy surfaces ; 

 and in a recent paper f I gave some results applicable to the 

 comparison of a clean surface with one just greasy enough to 

 stop the camphor movements and also with one saturated with 

 olive-oil. The method employed was that depending upon the 



* Read September 6 before Section A of the British Association at 

 Leeds. Communicated by the Author. 



t Proc. Roy. Soc. March 1890, vol. xlvii. p. 367. 



