CAPILLARY ACTION. 549 



M. Duprd in his 5th, 6th, and 7th Memoirs on the Mechanical Theory 

 of Heat (Ann. de Chimie et de Physique, 1866 to 1868) has done much to- 

 wards applying the principles of thermodynamics to capillary phenomena, and 

 the experiments of his son are exceedingly ingenious and well devised, tracing 

 the influence of surface-tension in a great number of very different circumstances, 

 and deducing from independent methods the numerical value of the surface- 

 tension. The experimental evidence which M. Dupre" has obtained bearing on 

 the molecular structure of liquids must be very valuable, even if many of our 

 present opinions on this subject should turn out to be erroneous. 



M. Quincke* has made a most elaborate series of experiments on the tension 

 of the surfaces separating one liquid from another and from air. 



M. Liidtget has experimented on liquid films, and has shewn how a film 

 of a liquid of high surface-tension is replaced by a film of lower surface-tension. 

 He has also experimented on the effects of the thickness of the film, and has 

 come to the conclusion that the thinner a film is, the greater is its tension. 

 This result, however, has been tested by M. Van der Mensbrugghe, who finds 

 that the tension is the same for the same liquid whatever be the thickness, 

 as long as the film does not burst. The phenomena of very thin liquid films 

 deserve the most careful study, for it is in this way that we are most likely 

 to obtain evidence by which we may test the theories of the molecular structure 

 of liquids. 



Sir W. Thomson J has investigated the effect of the curvature of the 

 surface of a liquid on the thermal equilibrium between the liquid and the 

 vapour in contact with it. He has also calculated the effect of surface-tension 

 on the propagation of waves on the surface of a liquid, and has determined 

 the minimum velocity of a wave, and the velocity of the wind when it is just 

 sufficient to disturb the surface of still water . 



THEORY OF CAPILLARY ACTION. 



When two different fluids are placed in contact, they may either diffuse 

 into each other or remain separate. In some cases diffusion takes place to a 

 limited extent, after which the resulting mixtures do not mix with each other. 



* Pogg. Ann., cxxxix. (1870), p. 1. t Ibid. p. 620. 



J Proceedings R. S., Edinburgh, February 7, 1870. 

 Philosophical Magazine, November, 1871. 



