316 HEAT. 



and integrating from o>' to w, and h = to h = h 



, w qh 

 log -, = 4 



= Ptr 



(up 



where P is the difference of hydrostatic pressures under the two 

 surfaces. 



We may apply this result to the case of a small spherical drop, 

 radius r surface tension T. The hydrostatic pressure just within the 

 drop will be greater than that under a plane surface at the same level 



2T 

 by P= . Hence the vapour-pressure from the drop will exceed that 



2T cr 

 from the plane surface by if P is not very great or r is not very 



T cop 



small. If we cannot make these limitations, then we must use the 

 logarithmic formula, and put 



co 2T o- 



log ^' = T ' Jo> 



Hence a space over a plane liquid surface and saturated for that 

 surface, is not yet saturated for a small drop, and such a drop if formed 

 in any way will tend to evaporate and disappear. We can see, then, 

 how in a dust-free space vapour can exist without condensation, though 

 supersaturated as far as a plane surface is concerned. 



If, however, dust is present, a particle of it may have such small 

 curvature that if any condensation occurs on it the vapour pressure of 

 the liquid is practically that of a plane surface. The liquid may spread 

 round the particles, and from the beginning form drops of such size that 

 the space is saturated for them and they continue to grow. 



It is to be noted that the alteration of vapour-pressure is very small 

 until the radius of curvature is exceedingly minute. 



Thus, taking water vapour at 0, we have its density - density of air. 



o 



Now, for air at and 760 mm. 



co 101 4,000 

 cr ~ -001296 



whence for water vapour 



5-8?-' 



