CAPILLARY ACTION. 



.. 



i- 



, when p i it* density and the intensity of gravity. 

 Equating thia force with the resultant of the tension 



ZTcoaa 



" = ' 



Heooe the mean height to which the fluid rises is inversely as the radius of 

 the tube. For water in a clean glass tube the angle of contact is zero, and 



ZT 

 n = . 



For mercury in a glass tube the angle of contact is 128' 52', the cosine of 

 which is negative. Hence when a glass tube is dipped into a vessel of mercury, 

 the mercury within the tube stands at a lower level than outside it. 



RISE OF A LIQUID BETWEEN Two PLATES. 



When two parallel plates are placed vertically in a liquid the liquid rises 

 between them. If we now suppose fig. 5 to represent a vertical section per- 

 pendicular to the plates, we may calculate the rise of the liquid. Let / be 

 the breadth of the plates measured perpendicularly to the plane of the paper, 

 then the length of the line which bounds the wet and the dry parts of the 

 plates inside is / for each surface, and on this the tension T acts at an angle 

 a to the- vertical. Hence the resultant of the surface-tension is ZITcosa. If 

 the distance between the inner surfaces of the plates is a, and if the mean 

 height of the film of fluid which rises between them is h, the weight of fluid 

 raised is pyhla. Equating the forces 



, ZTcosa 



whence h = - 



pga 



This expression is the same as that for the rise of a liquid in a tube, except 

 that instead of r, the radius of the tube, we have a the distance of the plates. 



