170 HEAT. 



Thus (Nature, March 1, 1888) Aitken found the following numbers 



Again (Nature, Feb. 27, 1890) he found at Kingairloch, on the shore 

 of Loch Linnhe, numbers ranging from 205 to 4000 per c.c. ; at the top 

 of Ben Kevis, 335 to 473 per c.c. In London and Paris, the numbers 

 were counted by the hundred thousand. Fridlander (Quarterly Journal 

 of the Royal Meteorological Society, xxii., July 1896) has tested various 

 specimens of air in a journey round the world, the numbers at sea 

 varying from 200 per c.c. in the Indian Ocean to 4000 in the Atlantic. 



The explanation of the thickness of town fogs as compared with 

 those in the country or at sea is now evident. The number of dust 

 particles is always far greater in town air, and if the smoke keeps near the 

 ground, as it does when the upper air is much warmer than the ground 

 air, the number may be enormously greater than in country air. When 

 condensation occurs the fog is, therefore, exceedingly " dense " through 

 the minute subdivision of the water deposited. 



The action of the nuclei in condensing the vapour probably depends 

 on a principle first pointed out by Lord Kelvin, to which we shall return 

 later (chap. xix.). The principle is that the equilibrium or saturation 

 vapour pressure of a space in contact with a liquid surface depends on 

 the curvature of the surface. If P be the normal saturation pressure at 

 a given temperature, it can be shown that in contact with a spherical 

 surface with radius of curvature r, for moderate values of r the saturation 

 pressure is 



ff = P + ^ (1) 



where T is the surface tension ; p the density of the liquid, and cr that 

 of the vapour. 



If the surface is convex then r is negative. 



When r is exceedingly minute so that jo/P differs largely from 1 



2Tcr 

 it can be shown that log p/~P = (2) 



if we assume that Boyle's law will hold for the vapour. 



It follows that small drops in a space just saturated over a flat 

 surface will find the space under-saturated for their own curved surface, 

 and will evaporate ; while if, by any accident, there is a concave surface 

 formed the space will be over-saturated and condensation will go on. 



Now suppose that in some saturated dust-free air a few vapour mole- 



