I20 



NA TURE 



[June 1 8, 1874 



any solid framework which forms its boundary, is said to 

 have surface-tension. Surface-tension is measured by the 

 force acting on one millimetre of the boundary edge. In 

 the case of water at 20° C, the tension is, according to 

 M. Quincke, a force of 8'253 milligrammes weight per 

 millimetre. 



M. Plateau hardly enters into the theoretical deduction 

 of the surface-tension from hypotheses respecting the con- 

 stitution of bodies. We have therefore thought it desirable 

 to point out how the fact of surface-tension may be 

 deduced from the known fact that there is a difference in 

 energy between a liquid and its vapour, combined with the 

 hypothesis, that as a milligramme of the substance passes 

 from the state of a liquid within the liquid mass, to that 

 of a vapour outside it, the change of its energy takes 

 place, not instantaneously, but in a continuous manner. 



M. van der Waals, whose academic thesis, " Over de 

 Continuiteit van den Gas- en Vloeistoftoestand,"* is a most 

 valuable contribution to molecular physics, has attempted 

 to calculate approximately the thickness of the stratum 

 within which this continuous change of energy is ac- 

 complished, and finds it for water about o"oooooo3 

 millimetre. 



Whatever we may think of these calculations, it is at 

 least manifest that the only path in which we may hope to 

 arrive at a knowledge of the size of the molecules of 

 ordinary matter is to be traced among those phenomena 

 which come into prominence when the dimensions of 

 bodies are greatly reduced, as in the superficial layer of a 

 liquid. 



But it is in the experimental investigation of the effects 

 of surface-tension on the form of the surface of a liquid 

 that the value of M. Plateau's book is to be found. He 

 uses two distinct methods. In the first he prepares a 

 mixture of alcohol and water which has the same density 

 as olive oil, then introducing some oil into the mixture 

 and waiting till it has, by absorption of a small portion 

 of alcohol into itself, become accommodated to its posi- 

 tion, he obtains a mass of oil no longer under the action 

 of gravity, but subject only to the surface-tension of its 

 boundary. Its form is therefore, when undisturbed, 

 spherical, but by means of rings, disks, &c., of iron, he 

 draws out or compresses his mass of oil into a number 

 of difi'erent figures, the equiUbrium and stability of which 

 are here investigated, both experimentally and theoreti- 

 cally. 



The other method is the old one of blowing soap- 

 bubbles. M. Plateau, however, has improved the art, first 

 by finding out the best kind of soap and the best propor- 

 tion of water, and then by mixing his soapy water with 

 glycerine. Bubbles formed of this liquid will last for 

 hours, and even days. 



By forming a frame of iron wire and dipping it into 

 this liquid he forms a film, the figure of which is 

 that of the surface of minimum area which has the 

 frame for its boundary. This is the case when the 

 air is free on both sides of the film. If, however, the 

 portions of air on the two sides of the film are not in con- 

 tinuous communication, the film is no longer the surface 

 of absolute minimum area, but the surface which, with 

 the given boundarj-, and inclosing a given volume, has a 

 minimum area. 



M. Plateau has gone at great length into the interesting 



' Leiden, A. W. Sijthoff, 1873. 



but difficult question of the conditions of the persistence 

 of liquid films. He shows that the surface of certain 

 liquids has a species of viscosity distinct from the inte- 

 rior viscosity of the mass. This surface-viscosity is very 

 remarkable in a solution of saponinc. There can be 

 no doubt that a property of this kind plays an important 

 part in determining the persistence or collapse of liquid 

 films. M. Plateau, however, considers that one of the 

 agents of destruction is the surface-tension, and that the 

 persistence mainly depends on the degree in which the 1 

 surface-viscosity counteracts the surface-tension. It is I 

 plain, however, that it is rather the inequality of the ' 

 surface-tension than the surface-tension itself which acts 

 as a destroying force. 



It has not yet been experimentally ascertained whether 

 the tension varies according to the thickness of the film. 

 The variation of tension is certainly insensible in those 

 cases which have been observed. 



If, as the theory seems to indicate, the tension di- 

 minishes when the thickness of the film diminishes, the 

 film must be unstable, and its actual persistence would 

 be unaccountable. On the other hand, the theory has not 

 as yet been able to account for the tension increasing 

 as the thickness diminishes. 



One of the most remarkable phenomena of liquid films 

 is imdoubtedly the formation of the black spots, which 

 were described in 1672 by Hooke, under the name of 

 holts. 



Fusinieri has given a very exact account of this pheno- 

 menon as he observed it in a vertical film protected from 

 currents of air. As the film becomes thinner, owing to 

 the gradual descent of the liquid of which it is formed, 

 certain portions become thinner than the rest, and begin 

 to show the colours of thin plates. These little spots of 

 colour immediately begin to ascend, dragging after them 

 a sort of train like the tail of a tadpole. These tadpoles, 

 as Fusinieri calls them, soon begin to accumulate near 

 the top of the film, and to range themselves in horizontal 

 bands according to their colours, those which have the 

 colour corresponding to the smallest thickness ascending 

 highest. 



In this way the colours become arranged in horizontal 

 bands in beautiful gradation, exhibiting all the colours of 

 Newton's scale. When the frame of the film is made to 

 oscillate, these bands oscillate like the strata formed by 

 a series of liquids of different densities. This shows that 

 the film is subject to dynamical conditions similar to those 

 of such a liquid system. The liquid is subject to the con- 

 dition that the volume of each portion of it is invariable, 

 and the motion arises from the fact that by the descent 

 of the denser portions (which is necessarily accompanied 

 by the rise of the rarer portions) the gravitational potential 

 energy of the system is diminished. In the case of the 

 film, the condition which determines that the descent of 1 

 the thicker portions shall entail the rise of the thinner I 

 portions must be that each portion of the film ofters a 

 special resistance to an increase or diminution of area 

 This resistance probably forms a large part of the super- 

 ficial viscosity investigated by M. Plateau, which retards 

 the motion of his magnetic needle, and evidently is far 

 greater than the viscosity of figure, in virtue of which the 

 film resists a shearing motion. 



The coloured bands gradually descend from the top of 

 the film, presenting at first a continuous gradation of 



