;o POPULAR LECTURES AND ADDRESSES. 



of its own amount than the pressure of vapour in 

 equilibrium at a plane surface of water at. the same 

 temperature. 



For water the value of T at ordinary tempera- 

 tures is about 'oS of a gramme weight per centi- 

 metre ; and p, being the mass of a cubic centi- 

 metre, in grammes, is unity. The value of cr for 

 vapour of water at any atmospheric temperature 

 is so small that we may neglect it altogether in 

 equation (i). In a capillary tube thoroughly wet 

 with water, the free surface is sensibly hemi- 

 spherical, and therefore r and / are each equal to 

 the radius of the inner surface of the liquid film 

 lining the tube above the free liquid surface ; we 

 have, therefore, 



^ = 08 X 2 -. 

 r 



Hence, if h =1300 centimetres, r= '00012 centi- 

 metres. There can be no doubt but that Laplace's 

 theory is applicable without serious modification 

 even to a case in which the curvature is so great 

 (or radius of curvature so small) as this. But in 

 the present state of our knowledge we are not 

 entitled to push it much further. The molecular 

 forces assumed in Laplace's theory to be " in- 

 sensible at sensible distances " are certainly but 

 little, if at all, sensible at distances equal to or 

 exceeding the wave lengths of ordinary light. 

 This is directly proved by the most cursory 

 observation of soap bubbles. But the appearances 



