CA PILL A RY A TTRA C TION. (A PP. C. ) 69 



- and being the curvatures in the principal sec- 



tions of the surface bounding liquid and vapour, 

 reckoned positive when concave towards the 

 vapour. 



In strictness, the value of a to be used in these 

 equations, (i) and (2), ought to be the mean 

 density of a vertical column of vapour, extending 

 through the height h from the plane of reference. 

 But in all cases to which we can practically apply 

 the formulae, according to present knowledge of 

 the properties of matter, the difference of densities 

 in this column is very small, and may be neglected. 

 Hence, if H denote the height of an imaginary 

 homogeneous fluid above the plane of reference, 

 which, if of the same density as the vapour at that 

 plane, would produce by its weight the actual 

 pressure &, we have 







= H- 



Hence by (i) and (2) 



For vapour of water at ordinary atmospheric 

 temperatures, H is about 1,300,000 centimetres. 

 Hence, in a capillary tube which would keep 

 water up to a height of 1 3 metres above the plane 

 level, the curved surface of the water is in equi- 

 librium with the vapour in contact with it, when 

 the pressure of the vapour is less by about 



