1890.J On the Superficial Viscosity of Water. 129 



insoluble grease, the changes of tension which accompany changes of 

 area are of a permanent character. 



It is not perfectly clear how far Marangoni regarded his principle 

 of surface elasticity as applicable to the explanation of Plateau's 

 observations upon distilled water ; but, at any rate, he applied it to 

 the analogous problem of the effect of oil in calming ripples. It is 

 unfortunate that this attempt at the solution of a long-standing riddle 

 cannot be regarded as successful. He treats the surface of the sea 

 in its normal condition as contaminated, and therefore elastic, and 

 he supposes that, upon an elastic surface, the wind will operate 

 efficiently. When oil is scattered upon the sea, a non-elastic surface 

 of oil is substituted for the elastic surface of the sea, and upon this 

 the wind acts too locally to generate waves. It is doubtless true that 

 an excess of oil may render a water surface again inelastic ; but I 

 conceive that the real explanation of the phenomenon is to be found 

 by a precisely opposite application of Marangoni's principle, as in the 

 theories of Reynolds* and Aitken.f Marangoni was, perhaps, in- 

 sufficiently alive to the importance of varying degrees of contamina- 

 tion. An ordinary water surface is indeed more or less contaminated ; 

 and 011 that account is the less, and not the more, easily agitated by 

 wind. The effect of a special oiling is, in general, to increase the 

 contamination and the elasticity dependent thereupon, and stops short 

 of the point at which, on account of saturation, elasticity would again 

 disappear. The more elastic surface refuses to submit itself to the 

 local variations of area required for the transmission of waves in a 

 normal manner. It behaves rather as a flexible but inextensible 

 membrane would do, and, by its drag upon the water underneath, 

 hampers the free production and propagation of waves. 



The question whether the effects observed by Plateau upon the 

 surface of distilled water are, or are not, due to contamination must, 

 I suppose, be regarded as still undecided. Oberbeck, who has ex- 

 perimented on the lines of Plateau, thus sums up his discussion : 

 " Wir miissen daher schliessen, entweder, dass der freien Wasser- 

 oberflache ein recht bedeutender Oberflachenwiderstand znkommt, 

 oder dass eine reine Wasseroberflache in Beriihrung mit der Luft 

 uberhaupt nicht existirt."J 



Postponing for the moment the question of the origin of " super- 

 ficial viscosity," let us consider its character. A liquid surface is 

 capable of two kinds of deformation, dilatation (positive or negative) 

 and shearing ; and the question at once presents itself, is it the 

 former or the latter which evokes the special resistance ? Towards 



* ' Brit. A88oc. Rep.,' 1880. 



t ' Edinburgh Roy. Soc. Proc.,' 1882-83, vol. 12, p. 56. 



J Wiedemann's 'Annalen,' vol. 11, 1880, p. 650. 



