284 



NA TURE 



[July 17, 1S90 



magnet. The result is very distinct, and contrasts strongly with 

 that observed by Plateau. Instead of the surface enclosed by the 

 ring being carried round with it in its rotation, not the smallest 

 movement can be perceived, except perhaps in the immediate 

 neighbourhood of the wire itself. It is clear that an ordinary 

 water surface does not appreciably resist shearing. 



A very slight modification of the apparatus restores the 

 similarity to that of Plateau. This consists merely in the addi- 

 tion to the ring of a material diameter of the same brass wire, 

 CD (Fig. 2). If the experiment be repeated, the sulphur in- 



J'lci 2 



<licates that the whole water surface included within the semi- 

 circles 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 re- 

 sists 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 introduce any variation in the 

 density of the contamination, and therefore does not bring 

 Marangoni's principle into play. Under these circumstances 

 there is no resistance. 



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

 nomenclature. The addition of a Httle 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 saponine 

 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 well- 

 known experiment in which a saponine babble 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 re- 

 cognizable 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 

 exhausted 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 saponine solution is to a great extent released when 

 the surface is contaminated by grease from the finger or by a 

 minute drop of petroleum. 



To return to the case of water, it is a question of the utmost 

 importance to decide whether the superficial viscosity of even 

 distilled water is, or is not, due to contamination with a film of 



foreign matter capable of lowering the tension. The experiments 

 of Oberbeck would appear to render the former alternative very 

 improbable ; but, on the other hand, if the existence of the film be 

 once admitted, the observed facts can be very readily explained. 

 The question is thus reduced to this : Can we believe that the 

 water surface in Plateau's apparatus is almost of necessity con- 

 taminated with a greasy film ? The argument which originally 

 weighed most with me in favour of the affirmative answer is 

 derived from the experiments of Quincke upon mercury. It is 

 known that, contrary to all analogy, a drop of water does not 

 ordinarily spread upon the surface of mercury. This is certainly 

 due to contamination with a greasy film ; for Prof. Quincke 

 {Poggendorffs Annalen, vol. cxxxix., 1870, p. 66) found that 

 it was possible so to prepare mercury that water would spread 

 upon it. But the precautions required are so elaborate that 

 probably no one outside Prof. Quincke's laboratory has ever 

 witnessed what must nevertheless be regarded as the normal 

 behaviour of these two bodies in presence of one another. The 

 bedring of this upon the question under discussion is obvious. 

 If it be so difficult to obtain a mercury surface which shall stand 

 one test of purity, why may it not be equally difficult to prepare 

 a water surface competent to pass another ? 



The method by which I have succeeded in proving that 

 Plateau's superficial viscosity is really due to contamination con- 

 sists in the preparation of a pure surface exhibiting quite different 

 phenomena ; and it was suggested to me by an experiment of 

 Mr. Aitken {loc. cit., p. 69). This observer found that, if a 

 gentle stream of air be directed vertically downwards upon the 

 surface of water dusted over with fine powder, a place is cleared 

 round the point of impact. It may be added that on the cessa- 

 tion of the wind the dust returns, showing that the tension of 

 the bared spot exceeds that of the surrounding surface. 



The apparatus, shown in Figs. 3 and 4, is constructed of sheet 

 brass. The circular part, which may be called the well, has the 

 dimensions given by Plateau. The diameter is li cm., and the 

 depth 6 cm. The needle is 10 cm. long, 7 mm. in breadth at 

 the centre, and about 0*3 mm. thick. It is suspended at a 

 height of 2\ cm. above the bottom of the vessel. So far there 

 is nothing special ; but in connection with the well there is a 

 rectangular trough, or tail-piece, about 2\cva. broad and 20 cm. 

 long. Between the two parts a sliding door may be inserted, 

 by which the connection is cut off, and the circular periphery of 

 the well completed. The action of the apparatus depends upon 

 a stream of wind, supplied from an acoustic bellows, and dis- 

 charged from a glass nozzle, in a direction slightly downwards, 

 so as to strike the water surface in the tail-piece at a' point a 

 little beyond the door. The effect of the wind is to carry any 

 greasy film towards the far end, and thus to purify the near end 

 of the tail-piece. When the door is up, this effect influences 

 also the water surface in the well upon which the jet does not 

 operate directly. For, if the tension there be sensibly less than 

 that of the neighbouring surface in the tail-piece, an outward 

 flow is generated, and persists as long as the difference of 

 tensions is sensible. The movements of the surface are easily 

 watched if a little sulphur be dusted over ; whentthe water in the 

 well has been so far cleansed that but little further movement is 

 visible, the experiment may be repeated without changing the 

 water by contaminating the surface with a little grease from the 

 finger or otherwise. In this way the surface may be freed from 

 an insoluble contam.ination any number of times, the accumula- 

 tion of impurity at the far end of the tail-piece not interfering 

 with the cleanness of the surface in the well. 



Another device that I have usually employed facilitates, or at 

 any rate hastens, the cleansing process. When the operation is 

 nearly complete, the movement of the surface becomes sluggish 

 on account of the approximate balance of tensions. At this 

 stage the movement may be revived, and the purification acceler- 

 ated, by the application of heat to the bottom of the well at the 

 part furthest removed from the tail-piece. It may, perhaps, be 

 thought that convection currents might be substituted altogether 

 for wind ; but in my experience it is not so. Until a high degree 

 of purity is attained, the operation of convection currents does 

 not extend to the surface, being resisted by the film according to 

 Marangoni's principle. 



When the apparatus was designed, it was hoped that the door 

 could be made a sufficiently good fit to prevent the return of the 

 greasy film into the well ; but experience showed that this could 

 not be relied upon. It was thus necessary to maintain the wind 

 during the whole time of observation. The door was, however, 

 I useful in intercepting mechanical disturbance. 



NO. 108 1, VOL. 42] 



