' 



CAPILLARY ACTION. 



w hich it has in a large mass. Thus a drop of train oil will 



OTer Uw surface of the sea till it shews the colours of tliin plates. 



Thaw rapidly dwcend in Newton's scale and at last disappear, shewing that 

 the thidtMM of the film is less than the tenth part of the length of a wave 

 nf light But even when thus attenuated, the film may be proved to be pre- 

 iwnt, since the surface-tension of the liquid is considerably less than that of 

 pore wmter. This may be shewn by placing another drop of oil on the surface. 

 Thk drop will not spread out like the first drop, but will take the form of 

 a flat lens with a distinct circular edge, shewing that the surface-tension of 

 what is still apparently pure water is now less than the sum of the tensions 

 nf the surfaces separating oil from air and water. 



The spreading of drops on the surface of a liquid has formed the subject 

 of a very extensive series of experiments by Mr Tomlinson. M. Van der 

 Mensbragghe has also written a very complete memoir on this subject*. 



When a solid body is in contact with two fluids, the surface 

 of the solid cannot alter its form, but the angle at which the 



\> I surface of contact of the two fluids meets the surface of the solid 

 s^ depends on the values of the three surface-tensions. If a and l> 

 are the two fluids and c the solid then the equilibrium of the 

 tensions at the point depends only on that of thin components 

 parallel to the surface, because the surface-tensions normal to the 

 surface are balanced by the resistance of the solid. Hence if the 

 angle ROQ (fig. 4) at which the surface of contact OP meets the solid is 

 denoted by a, 



^-2^-2 



whence 



-'oft 



As an experiment on the angle of contact only gives us the difference of the 

 surface-tensions at the solid surface, we cannot determine their actual value. 

 It is theoretically probable that they are often negative, and may be called 

 surface-pressures. 



The constancy of the angle of contact between the surface of a fluid and 

 a solid was first pointed out by Dr Young, who states that the angle of 

 contact between mercury and glass is about 140. Quincke makes it 128 52'. 



* Sur la Tension StiperficieUe des lAquldes, Bruxclles, 1873. 



