July 17, 1890] 



NATURE 



283 



the liquids of Plateau's second category, alcohol, ether, oil of 

 turpentine, &c., the time on the surface was about lialf oi 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 

 resistance. 



These experiments of Plateau undoubtedly establish a special 

 property of the surface of liquids of the first and third cate- 

 gories ; 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 super- 

 ficial viscosity may be explained as due to the operation of 

 causes already recognized. In the case of water and other 

 liquids of the first category, he regards the resistance experienced 

 by the needle as mainly the result of the deformation of the 

 meniscuses developed at the contacts on the two sides with the 

 liquid surface. This view does not appear to me to be sound : 

 for a deformation of a meni:cus due to inertia would not involve 

 any dissipation of energy, nor permanent resistance to the 

 movement. But the second suggestion of Marangoni is of great 

 importance. 



On various grounds the Italian physicist concludes that 

 "many liquids, and especially those of Plateau's third category, 

 are covered with a superficial pellicle ; and that it is to this 

 pellicle that they owe their great superficial viscosity." After 

 the observations of Dupre- and myself,* supported as they are 

 by the theory of Prof. Willard Gibbs,'* the existence of the 

 superficial pellicle cannot be doubted ; and its mode of action is 

 thus explained by Marangoni^: — "The surface of a liquid, 

 covered by a pellicle, possesses two superficial tensions ; the 

 first, which is the weaker and in constant action, is due to the 

 pellicle ; the second is in the latent state, and comes into 

 operation only when the pellicle is ruptured. Since the latter 

 tension exceeds the former, it follows that any force which tends 

 to rupture the superficial pellicle upon a liquid encounters a 

 resistance which increases with the difference of tensions 

 between the liquid and the pellicle." In Plateau's experiment 

 the advancing edge of the needle tends to concentrate the super- 

 ficial contamination, and the retreating edge to attenuate it ; the 

 tension in front is thus inferior to the tension behind, and a force 

 is called into operation tending to check the vibration. On a 

 pure surface it is evident that nothing of this sort can occur, 

 unless it be in a very subordinate degree, ai the result of 

 difference of temperature. 



This is an important distinction, discussel by Willard Gibbs, 

 according as the contaminati ^n, to which is due the lowering of 

 tension, is merely accidentally present upon the surface, or is 

 derived from the body of the liquid under the normal operation 

 of chemical and capillary forces. In the latter case, that, for 

 example, of solutions of soap and of camphor, the changes of 

 tension which follow an extension or contraction of the surface 

 may be of very brief duration. After a time, dependent largely 

 upon the amount of contaminating substance present in the 

 body of the liquid, equilibrium is restored, and the normal 

 tension is recovered. On the other hand, in the case of a 

 surface of water contaminated with a film of 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 efTectof 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, 

 arid 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 



' Nuovo Citttento, Ser. 2, vols, v.-vi., April 187a : Ser. 3, vol. iii., 1878. 



^ "Thtforie M^canique de la Chaleur," Paris, 1869, p. 377. 



3 "On the Tension of Recently Formed Liquid Surlaces," Roy. Soc. 

 Proc, vol. xlvii., 1890, p. 281 (stt/>ra) 



* Connecticut Acad. Irans., vol. iii., Part II., 1877-73. In my former 

 communication I overlooked Prof. Gibbs's very valuable discussion on this 

 subject. 



5 Nuoz'O Cimento, vols, v.-vi., 1871-72, p. 260 (May 1872). 



NO. 1 08 I, VOL. 42] 



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 pre- 

 cisely opposite application of Marangoni's principle, as in the 

 theories of Reynolds (Brit. Assoc. Rep,, 1880) and Aitken 

 (Edinburgh Roy. Soc. Proc, 1882-83, vol. xii. p. 56). Maran- 

 goni was, perhaps, insufficiently alive to the importance oi vary- 

 ing degrees of contamination. An ordinary water surface is 

 indeed more or less contaminated ; and on 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 dis- 

 appear. 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 contamina- 

 tion must, I suppose, be regarded as still undecided. Oberbeck, 

 who has experimented on the lines of Plateau, thus ."^ums up his 

 discussion : — " Wir miissen daher schliessen, entweder, dass der 

 freien Wasseroberflache ein recht bedeutender Oberflachen- 

 widerstand zukommt, oder dass eine reine Wasseroberflache in 

 Beriihrung mit der Luft iiberhaupt nicht existirt" {Wudemanns 

 Annalen, vol. xi. 1880, p. 650). 



Postponing for the moment the question of the origin of 

 "superficial viscosity," let us consider its character. A liquid 

 surface is capable of two kinds of deformation, dilatation (posi- 

 tive or negative) and shearing ; and the question at once presents 

 itself, Is it the former or the latter which evokes the special 

 resistance ? Towards the answer of this question Marangoni 

 himself made an important contribution in the earlier of the 

 memoirs cited. He found (p. 245) that the substitution for the 

 elongated needle of Plateau of a circular disk of thin brass turn- 

 ing upon its centre almost obliterated the distinction between 

 liquids of the two first categories. The ratio of the superficial 

 to the internal viscosity was now even greater for ether than for 

 water. From this we may infer that the special superficial 

 viscosity of water is not called into play by the motions of the 

 surface due to the rotation of the disk, which are obviously of 

 the nature of shearing. 



A varied form of this experiment is still more significant. I 

 have reduced the metal in contact with the water surface to a 

 simple (2") ring, acbd, of thin brass wire (Fig. 1). This is 



rrq /, 



supported by a fine silk fibre, so that it may turn freely about its 

 centre. To give a definite set, and to facilitate forced displace- 

 ments, a magnetized sewing needle, NS, is attached with the aid 

 of wax. In order to make an experiment, the ring is adjusted to 

 the surface of water contained in a shallow ve.«sel. When all is 

 at rest, the surface is dusted over with a little fine sulphur, ^ and 

 the suspended system is suddenly set into rotation by an external 



' Sulphur seems to be on the whole the best material, although it certainly 

 communicates some impurity to the surface. Freshly heated pumice or 

 wood-ashes sink immediately ; and probably all powders really free from, 

 grease would behave in like 1 



