Lord Rayleigh : Investigations in Cipillarity. 331 



explained by its solubility. Hydrogen is, however, much 

 less viscous than other gases, and to this we may plausibly 

 attribute its activity in promoting union. A layer of hydro- 

 gen may be effectively squeezed out in a time that would be 

 insufficient in the case of air and oxygen. 



The Tension of Contaminated Water-Surfaces . 



In my experiments upon the superficial viscosity of water 

 (Proc. Roy. Soc. June 1890) I had occasion to notice that the 

 last traces of residual contamination had very little influence 

 upon the surface-tension, but that they became apparent when 

 compressed in front of the vibrating needle of Plateau's 

 apparatus. Subsequently I showed (Phil. Mag. vol. xxxiii., 

 p. 470, 1892) that according to Laplace's theory of Capillarity, 

 in which matter is regarded as continuous, the effect of a 

 thin surface-film in diminishing the tension of pure water 

 should be as the square of the thickness of the film. 



The tension of slightly contaminated surfaces was made 

 the subject of special experiments by Miss Pockels (' Nature,' 

 vol. xliii. p. 437, 1891), who concluded that a water-surface 

 can " exist in two sharply contrasted conditions ; the normal 

 condition, in which the displacement of the partition [altering 

 the density of the contamination] makes no impression upon 

 the tension, and the anomalous condition, in which every 

 increase or decrease alters the tension. ,} It is only since I 

 have myself made experiments upon the same lines that I 

 have appreciated the full significance of Miss Pockels' state- 

 ment. The conclusion that, judged by surface-tension, the 

 effect of contamination comes on suddenly, seems to be of 

 considerable importance, and I propose to illustrate it further 

 by actual curves embodying results recently obtained. 



The water is contained in a trough modelled after that of 

 Miss Pockels. It is of tin-plate, 70 cm. long, 10 cm. broad, 

 and 2 cm. deep, and it is filled nearly to the brim. The 

 partitions, by which the oil is confined, are made of strips of 

 glass resting upon the edge of the trough in such a manner that 

 their lower surfaces are wetted while the upper surfaces remain 

 dry. The strips may be 1^ cm. wide, and for convenience 

 of handling their length should exceed considerably the width 

 of the trough. I have found advantage in cementing (with 

 hard cement) slight webs of glass to the lower faces. The 

 length of these is a rough fit with the width of the trough, 

 enabling them to serve as guides preventing motion of the 

 strips parallel to their length. 



In order to observe the surface-tension Miss Pockels used 



