SOME PHYSICAL PROBLEMS 



since there are no nuclei for the drops to condense 

 round. If there are charged particles in dust-free air, 

 however, the fog will be deposited round these by super- 

 saturation far less than that required to produce any 

 appreciable fog when no charged particles are present. 



"Thus, in sufficiently supersaturated damp air a 

 cloud is deposited on these charged particles and they 

 are thus rendered visible. This is the first step tow- 

 ards counting them. The drops are, however, far too 

 small and too numerous to be counted directly. We 

 can, however, get their number indirectly as follows: 

 suppose we have a number of these particles in dust- 

 free air in a closed vessel, the air being saturated with 

 water- vapor; suppose now that we produce a sudden 

 expansion of the air in the vessel ; this will cool the air, 

 it will be supersaturated with vapor, and drops will 

 be deposited round the charged particles. Now if we 

 know the amount of expansion produced we can calcu- 

 late the cooling of the gas, and, therefore, the amount 

 of water deposited. Thus we know the volume of 

 water in the form of drops, so that if we know the 

 volume of one drop we can deduce the number of drops. 

 To find the size of a drop, we make use of the investiga- 

 tions made by Sir George Stokes on the rate at which 

 small spheres fall through the air. In consequence of 

 the viscosity of the air small bodies fall exceedingly 

 slowly, and the smaller they are the slower they fall." l 



Professor Thompson gives us the formula by which 

 Stokes made his calculation. It is a relatively simple 

 algebraic one, but need not be repeated here. For us 

 it suffices that with the aid of this formula, by merely 

 measuring the actual descent of the top of a vapor 



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