On the Superficial Viscotity of Water. 131 



A very slight modification of the apparatus restores the similarity 

 to that of Plateau. This consists merely in the addition to the ring 

 of a material diameter of the same brass wire, CD, fig. 2. If the ex- 

 periment be repeated, the sulphur indicates that the whole water 

 surface included within the semicircles now shares in the motion. 

 In general terms the surface may be said to be carried round with 

 the ring, although the motion is not that of a rigid body. 



Experiments of this kind prove that what a water surface resists is 

 not shearing, but local expansions and contractions of area, even 

 under the condition that the total area shall remain unchanged. And 

 this is precisely what should be expected, if the cause of the viscosity 

 were a surface contamination. A shearing movement does not intro- 

 duce any variation in the density of the contamination, and therefore 

 does not bring Marangoni's principle into play. Under these circum- 

 stances there is no resistance. 



It remains to consider liquids of the third category in Plateau's 

 nomenclature. The addition of a little oleate of soda does not alter 

 the behaviour of water, at least if the surface be tolerably fresh. On 

 the other hand, a very small quantity of sapouine suffices to render 

 the surface almost rigid. In the experiment with the simple ring 

 the whole interior surface is carried round as if rigidly attached. 

 A similar effect is produced by gelatine, though in a less marked 

 degree. 



In the case of saponine, therefore, it must be fully admitted that 

 there is a superficial viscosity not to be accounted for on Marangoni's 

 principle by the tendency of contamination to spread itself uniformly. 

 It seems not improbable that the pellicle formed upon the surface 

 may have the properties of a solid, rather than of a liquid. However, 

 this may be, the fact is certain that a contracting saponine surface 

 has no definite tension alike in all directions. A sufficient proof is to 

 be found in the weH known experiment in which a saponine bubble 

 becomes wrinkled when the internal air is removed. 



The quasi-solid pellicle on the surface of saponine would be of 

 extreme thinness, and, even if it exist, could hardly be recognisable 

 by ordinary methods of examination. It would moreover be capable 

 of re-absorption into the body of liquid if unduly concentrated by 

 contraction of surface, differing in this respect from the gross, and 

 undoubtedly solid, pellicles which form on the surface of hard water 

 on exposure to the atmosphere. 



Two further observations relative to saponine may here find a 

 place. The wrinkling of a bubble when the contained gas is ex- 

 hausted occurs also in an atmosphere (of coal gas) from which 

 oxygen and carbonic acid are excluded. 



In Plateau's experiment a needle which is held stiffly upon the 

 surface of a saponiue solution is to a great extent released when the 



