56 Mr Townsend, The formation of clouds with ozone. 



5. The disappearance of the cloud when the gas is dried, and 

 its reappearance in the presence of moisture, can be explained by 

 supposing the drop to consist of water with a small percentage of a 

 non-volatile body dissolved in it. Thus when the gas in which 

 the cloud is suspended is passed through sulphuric acid, the water 

 evaporates from the drops and gets absorbed in the acid. There 

 remain behind in the gas very small particles which consist of the 

 non-volatile body which prevented the drops forming the cloud 

 from evaporating under the action of surface tension. These 

 small particles on coming into contact with moisture condense 

 water round themselves again to form drops, the size of which 

 would depend on the relative humidity of the atmosphere. When 

 the atmosphere in which they are suspended is saturated, the 

 drops are largest and of a fixed size. 



For example, let us suppose there is a small percentage of 

 sulphuric acid in a drop, we can calculate what the percentage 

 should be in order that a drop of radius 5 x 10~ 5 centimetres should 

 have a vapour pressure equal to the vapour pressure near a flat 

 surface of water. 



The increase of the vapour pressure outside a drop of radius 

 a due to surface tension is 



dp = x - , 



p — a- a 



where a is the density of water vapour, p the vapour pressure, p 

 the density of water, and T the surface tension. 



The diminution of the vapour pressure outside a solution, in 

 which the soluble body has an osmotic pressure P, is 



Sp=- .P.* 

 P 



= P, 



Thus for equilibrium we get : 



2T 



a 

 approximately, since a is small compared with p. 



When this condition is satisfied there will be no tendency for 

 the drop to grow bigger or smaller when it is surrounded by a gas 

 saturated with water vapour. 



Let a be the weight of acid per c.c. of the drop. The weight 



of an equal number of molecules of hydrogen would be -^r . 



* J. J. Thomson, Applications of Dynamics to Physics and Chemistry, §§ 85, 9i, 

 and 92. 



