74 PHYSICAL PROPERTIES AND COMPOSITION 



of the surface is diminished by z-rrrh (r is the radius in 

 centimetres) ; if y is the work done on diminishing the 

 area by one square cm., the so-called surface energy, the 

 total work done is 



in gram-centimetres. 



This work is spent in raising a column of liquid of 

 weight 7TT 2 hD (where D is the density referred to water) 

 through a height h cm. requiring an amount of work 



so that 



2-nrhy = 7>r 2 h 2 D, 

 whence 



y =. \ rhD in gram-centimetres. 



Taking into account the pressure of vapour the work 

 gained by the diminution in surface consists of two parts, 

 due to the change in surface of the liquid and vapour, and 

 obviously of opposite signs, so that the total amount is 



in which, at the critical point, 



yi = ?v ; 



we may, however, retain the simple value y provided we 

 understand by it the total work gained in the reduction of 

 surface of both the liquid and vapour. 



In the same way the total work done in raising the level 

 of the liquid in the capillary is modified, and if the densities 

 of the liquid and vapour referred to water are D L and D^ to 



7rr 2 A 2 (A-r), whence -y = lrh(D L -D<). 



Influence of temperature. The capillary rise obviously 

 becomes zero at the critical temperature, on account of the 

 identity between liquid and vapour ; so that it must 

 eventually fall with increase of temperature. The fall is 

 found to be approximately proportional to the rise of 

 temperature, as appears from the following table for 

 ether : 



