132 Lord Rayleigh. [June ^ 



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 im- 

 portance 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 contaminated with a greasy film ? The argu- 

 ment which originally weighed most with me, in favour of the affir- 

 mative 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 Professor 

 Quincke* fouud 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 Professor Quincke 's laboratory has ever 

 witnessed what must nevertheless be regarded as the normal be- 

 haviour of these two bodies in presence of one another. The bearing 

 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 suriace 

 competent to pass another ? 



The method by which I have succeeded in proving that Plateau's 

 superficial viscosity is really due to contamination consists in the 

 preparation of a pure surface exhibiting quite different phenomena ; 

 and it was suggested to me by an experiment of Mr. Aitken.f 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 cessation 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, 4, is constructed of sheet brass. 

 The circular part, which may be called the well, has the dimensions 

 given by Plateau. The diameter is 11 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 

 ot the vessel. 89 far there is nothing special ; but in connexion 

 with the well there is a rectangular trough, or tail-piece, about 2| cm. 

 broad and 20 cm. long. Between the two parts a sliding door may 



* Poggenrlorff's ' Annalen,' vol. 139, 1870, p. 66. 

 f Loc. cit,, p. 69. 



