1890.] On the Superficial Viscosity of Water. 127 



IV. " On the Superficial Viscosity of Water." By LORD 

 RAYLEIGH, Sec. R.S. Received May 15, 1890. 



The idea that liquids are endowed with a viscosity peculiar to the 

 surface is to be found in the writings of Descartes and Rumford ; but 

 it is to Plateau that its general acceptance is due. His observations 

 related to the behaviour of a compass needle, turning freely upon a 

 point, and mounted in the centre of a cylindrical glass vessel of 

 diameter not much more than sufficient to allow freedom of move- 

 ment. By means of an external magnet the needle was deflected 90 

 from the magnetic meridian. When all had come to rest the 

 magnet wap suddenly removed, and the time occupied by the needle 

 in recovering its position of equilibrium, or rather in traversing an 

 arc of 85, was noted. The circumstances were varied in two ways : 

 first, by a change of liquid, e.g., from water to alcohol ; and, secondly, 

 by an alteration in the level of the liquid relatively to the needle. 

 With each liquid observations were made, both when the needle 

 rested on the surface, so as to be wetted only on the under side, and 

 also when wholly immersed to a moderate depth. A comparison of 

 the times required in the two cases revealed a remarkable depend- 

 ence upon the nature of the liquid. With water, and most aqueous 

 solutions, the time required upon the surface was about double of that 

 in the interior ; whereas, with liquids of Plateau's second category, 

 alcohol, ether, oil of turpentine, &c., the time on the surface was 

 about half of the time in the interior. Of liquids in the third 

 category (from which bubbles may be blown), a solution of soap 

 behaved in much the same manner as the distilled water of the first 

 category. On the other hand, solutions of albumen, and notably of 

 saponine, exercised at their surfaces an altogether abnormal resist- 

 ance. 



These experiments of Plateau undoubtedly establish a special pro- 

 perty of the surfaces of liquids of the first and third categories ; but 

 the question remains open whether the peculiar action upon the 

 needle is to be attributed to a viscosity in any way analogous to the 

 ordinary internal viscosity which governs the flow through capillary 

 tubes. 



In two remarkable papers,* Marangoni attempts the solution of 

 this problem, and arrives at the conclusion that Plateau's superficial 

 viscosity may be explained as due to the operation of causes already 

 recognised. In the case of water and other liquids of the first 

 category, he regards the resistance experienced by the needle as 



* ' Nuovo Cimento,' Ser. 2, vol. 6-6, Apr., 1872 ; ' Nuovo Cimento,' Ser. 3, vol. 3, 

 1878. 



