1892.] 



connected ivith Cloudy Condensation. 



423 



We are now in a position to understand our first point, namely, 

 why the degree of supersaturation by which the condensation is pro- 

 duced should have an effect on the appearance of the condensed vapour. 

 For the study of this point the condensation produced by expansion 

 is the most convenient, as it is more under our control than the con- 

 densation in steam jets. Suppose we take a glass flask connected with 

 an air-pump. If we wet the inside of the flask and then fill it with 

 unfiltered air, the slightest expansion of the moist air by the pump 

 will cause condensation to take place. But the density of the con- 

 densation which can be produced by any degree of expansion will 

 depend on the rate at which the expansion is made. If the expansion 

 be made very slowly, the clouding is very thin ; but, if it be made 

 rapidly, it is very thick. If the expansion be done slowly, the amount 

 of supersaturation is only slight, and only the largest dust particles 

 come into action as active centres of condensation ; and after a particle 

 of dust has once become a nucleus, it has then, in virtue of its size, an 

 advantage over the particles which have not begun to have vapour 

 condensed on them. The result of this is that, so long as the degree 

 of supersaturation is very slight, these large particles relieve the 

 tension, and if by any chance other dust particles become active, any 

 reduction in the rate of condensation allows the large particles, after 

 they have relieved the tension, to rob the small ones of their burden 

 of water, so that a slow rate of condensation always produces a small 

 number of drops and a thin form of clouding. 



But now suppose you cause the expansion to be made rapidly ; the 

 supersaturation then becomes much greater, as there is not time for 

 the water molecules to select a resting place, and the small number 

 of large dust particles cannot relieve the tension, and the result is 

 a much greater number of nuclei are forced into action. And all 

 these nuclei continue to grow so long as the supersaturation is kept 

 up, but the larger ones grow most. After the tendency to condense 

 has begun to diminish, those particles which have accumulated least 

 are the first to feel the change, and cease to grow, while the larger 

 ones are still accumulating. But after the tendency to condensation 

 has ceased altogether, the changes in the clouded air are not at an 

 end. The smaller drops begin to lose their accumulated moisture, 

 while the larger ones are still growing — growing at the expense of the 

 gradually diminishing smaller ones. This process goes on till most 

 of the small ones have lost all their burden of water, which has 

 been absorbed by the overgrown larger ones ; and in the end a com- 

 paratively small number of drops have absorbed the moisture which 

 was previously distributed over a vast number of particles. The 

 larger particles have, so to speak, eaten up the smaller ones. How 

 like the above looks to a page in the " struggle for existence " in 

 the animal or vegetable world ! 



