Spread of Liquids on Solid Bodies. 419 



turpentine drove the cotton-seed oil from the surface of glass. 

 Briicke's theory of the diffusion of liquids at the surface of 

 solid bodies satisfies every postulate when once the spread 

 of one of the liquids upon the surface of the solid has been 

 explained. 



As already mentioned above, a liquid 2 with a small capil- 

 lary tension a 12 at the boundary of a solid body 1, must drive 

 away a liquid 3 possessing a greater capillary tension a 13 at 

 the boundary of the same solid body, provided access of the air 

 is prevented and that the liquids 2 and 3 are miscible in all 

 proportions. In this case a 23 = 0, and equation (6 a) is im- 

 mediately fulfilled. 



Liquids which are miscible in all proportions may also be 

 arranged in relation to any given solid body 1 in a determi- 

 nate series according to the magnitude of the capillary con- 

 stant or surface-tension of the common boundary of the solid 

 body and of the liquid in question. Each liquid will be driven 

 away by one standing lower in this series. But the series 

 changes with the nature of the solid body. 



I have detailed above (§9, conclusion) the difficulties which 

 beset an exact determination of the magnitudes a 12 , a 13 , &c. 



In view of the uncertainty of inferring the surface-tension 

 of the boundary of liquids and solids from observations on 

 flat drops, the method of observing directly the spread of a 

 liquid on the surface of a solid body when air is excluded 

 deserves at least the preference. 



The method employed by me in the case of olive-oil and 

 turpentine cannot unfortunately be applied to all liquids, and 

 fails where no diminution of the capillary height of ascent 

 can be observed, and when the liquid with the greater capil- 

 lary constant of the free surface is such that it spreads upon 

 the surface of the solid. This is, for example, the case with 

 water and alcohol upon the surface of glass. 



The influence upon the diffusing liquids of the dependence 

 of diffusion upon the nature of the solid body is beautifully 

 shown in the case of alcohol and water which are separated 

 by a piece of pig's bladder or by india-rubber tissue. In the 

 first case the volume of the water diminishes ; in the second it 

 increases. 



Here Briicke * has already shown, by experiment, that on 

 the exclusion of air the water overspreads the surface of the 

 bladder, and the alcohol invests the surface of the indiarubber 

 tissue — that therefore in the first case the water can flow to 

 the alcohol, in the second the alcohol can flow to the water. 



* Pogg. Ann. lviii. p. 87 (1843). 

 2E 2 



