March 23, 1922^ 



NA TURE 



377 



de Morveau, for example, in 1773, in his examination 

 of the nature of chemical affinity, attempted to deter- 

 mine the relative affinities of a variety of substances 

 from the force required to detach small plates of glass 

 from their surfaces. ^ The experimental method was 

 investigated mathematically by Laplace and Dupre, and 

 used widely as a means of measuring surface tension. 



Though the range of the force of cohesion was re- 

 ( ognised as being insensible by these earlier writers, 

 nowhere, so far as I know, did they draw the conclusion 

 that the surface layer of a fluid must be the seat of 

 special forces, though a strong hint appears in Newton's 

 comments upon Hauksbee's experiments. The enuncia- 

 tion of the secondary principle of surface tension was 

 reserved for Segner.^ Segner appears to have had 

 little or no acquaintance with other work on the subject; 

 he refers only to Clairaut (" Figure de la Terre," 1743), 

 whose book, however, he could not obtain (" quesitum 

 nancisci non potui ") : " Cupiebam autem inspicere, 

 propter articulos quasi episodicos . . . rotunditatem 

 guttarum . . . elevationemque et depressionem flui- 

 dorum in tubis capillaribus, spectantes. Ea ergo qualia 

 sint, quantumquecum meis consentiant, dicere nequeo." 

 The subject matter of Segner's paper is, in the first 

 instance, the equilibrium of drops of fluid ; the equili- 

 brium in tubes is treated from the point of view of the 

 curvature of the free surfaces. The important theorems 

 are Nos. 2 and 3, which assert that if in any drop 

 the volume be divided into a shell, the thickness of 

 which is that of the range of the force of attraction, and 

 an interior mass, the forces on any particles in the 

 latter contribute nothing to determining the form of 

 the drop, but only those forces on any particles in the 

 surface shell which can be resolved along the normal 

 to the surface and in the tangent plane. In his calcula- 

 tions of the effects of the surface tension so produced 

 Segner made the mistake, afterwards corrected by 

 Laplace, of taking account only of the curvature of a 

 meridian section of the drop, neglecting the effect of 

 the curvature in a plane at right angles to this section. 

 To Segner, however, belongs the credit of being the first 

 to deduce the phenomena of capillarity from the 

 surface tension. 



The existence of a surface tension was demonstrated 

 objectively when Leidenfrost showed, in 1756,^ that a 

 soap bubble tends to contract. In 1787 Monge^ ap- 

 plied the principle to explain the apparent attractions 

 and repulsions between bodies floating on a liquid. 



Reference is made by Leslie (see later) to experiments 

 on the subject made in Holland by Musschenbroek. 

 I have not succeeded in tracing these. The only 

 reference in his " Cours de Physique " of 1769 is to the 

 experiments of Hauksbee, and theory is limited to the 

 statement that " I'explication se presente naturelle- 

 ment a I'esprit " ! ^ 



Leslie, in a curiously polemical and pedantic paper ,^ 

 attempts to replace Jurin's " explication " of the rise 

 in capillary tubes, which " is almost universally 



' He used " la mdthode du Docteur Taylor [Brooke Taylor] . . . qm, 

 . II le choix des mati^res employees, peut servir ^ faire cormoltre que 

 .'.iltrflction que les Chymistes notnment affinite a necessairement quelque 

 part 4 cette adh&ion," Jour, de Pkysujue, vol. i, p. 172, 1773. 



' " De Figuris Superficienim Fluidarum," Comm. See. Reg. Sci. Gottin- 

 gensis, vol. i, p. 301, 1751. 



' " De aquae communis nonnullis qualitatibus tractatus." (Duisburg.) 



* Mdmoires de VAcad. des Sciences, p. 506, 1787. 



' Pencilled on the margin of my MS. is the note " Not altogether just." 

 At this distance of time I cannot elucidate the remark. 



• Tilloch's Phil. Mag., vol. 14, p. 193, 1802. 



NO. 2734, VOL. 109] 



adopted. It is repeated in all the elementary books 

 of natural philosophy." The attraction of the glass, 

 everywhere normal to the surface and of narrow range, 

 gives rise to an increase in pressure in the layer of water 

 next to the surface of the glass. The result of this 

 pressure is that a drop of water tends to spread out over 

 the surface of the glass and consequently to mount 

 upwards in a tube. " But why should the mere 

 tendency of the water to the surface of the glass 

 occasion a dispersive motion } The reason is that 

 the external particles could not approach without 

 spreading themselves and extending the film : and 

 analogy will instruct us, that the attraction of water 

 to glass must increase in proportion to the proximity 

 of its approach." The hquid film flows up the walls 

 of the tube, carrying with it water which adheres to it, 

 and equilibrium is reached when the weight of the 

 column balances the force by which the film spreads 

 itself over the glass. " This explanation of the action 

 of the solid is equivalent to that by which Gauss 

 afterwards supplied the defect of the theory of Laplace, 

 except that, not being expressed in terms of mathe- 

 matical symbols, it does not indicate the mathematical 

 relations between attraction of individual particles and 

 the final result." ^ Maxwell gives to LesHe the credit 

 of being the first to explain correctly the rise of fluid 

 in a capillary tube. " He [Leslie] does not, like the 

 earlier speculators, suppose this attraction [of the 

 solid] to act in an upward direction so as to support 

 the fluid directly." Yet a few pages further on Maxwell 

 himself speaks of the tension of the solid as though it 

 intervened actively as an upward pull ! 



On few subjects has more been written than on 

 capillarity, and yet the exact way in which the 

 attractive forces act in causing the rise of fluid in 

 capillary tubes and the spreading of fluids over solid 

 or fluid surfaces is still obscure. LesHe's account is 

 probably the best, and if true it carries an important 

 corollary — namely, that the layer of fluid attracted 

 by the glass is at least two molecules in depth. Recent 

 writers, if I understand them rightly, would restrict 

 the influence to a layer only one molecule deep. 



Leslie's paper is original and powerful, and even 

 now very little out of date. It includes many observa- 

 tions which are still of great interest ; of these the 

 only one I have space to mention is the discovery of the 

 fact that the " assimilation " of fluid by porous bodies 

 is accompanied by a rise of temperature. He was, I 

 believe, the first to detect this fact. 



In the early years of the nineteenth century the 

 subject received attention at the hands of two remark- 

 able men — Dr. Thomas Young and the Marquis de 

 Laplace. Their methods were entirely dissimilar. 

 Young founded his theory on the principles of surface 

 tension, or " superficial cohesion," as he calls it. 

 " Since the time of Segner," he says, " little has been 

 done in investigating accurately and in detail the 

 various consequences of the principle." He begins by 

 making two assumptions — the first, which he attributes 

 to Monge " and others," that the cohesive attraction 

 of the superficial particles causes the free surface of 

 fluids to " be formed into curves of the nature of 

 linteariae which are supposed to be the results of a 

 uniform tension of a substance"; and the second, 

 " which appears to be new," that the angle of contact 



' Clerk Maxwell, art. " Capillary Action," " Encyc. Brit.," 9th edition. 



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