630 Dr, Gr. 0. Simpson on the Wilson- Gerdien 



condensed ; now if each ion receives an equal share of 

 this water the volume cf each of the resulting water-drops 

 will be 2'2xl0~ 9 c.c, and each drop will have a radius 

 of 8xl0~~ 3 mm. According to the formula given by Mr. 

 Wilson, in the letter to ' Nature ' which we are discussing, 

 such drops will fall relatively to the air in which they are 

 formed at the rate of 9 mm. a second. Hence, if the air in 

 the supersaturated stratum is rising with a greater velocity 

 than this the drops will be carried upwards, and a cloud will 

 form above the condensation layer. 



It will at once be granted that the air must have a greater 

 upward velocity than 1 cm. a second, for if it had not it would 

 take a given mass of air nearly 70 hours to pass from the 

 lower cloud to the condensation layer above. Hence, over 

 the condensation layer we shall have an ever-increasing cloud 

 containing the greater part of the separated negative ions 

 each bound to a water-drop. Now through this cloud all the 

 positive ions will have to pass, and it is quite obvious that 

 even if the cloud is only a few metres thick very few positive 

 ions will pass out through its upper surface. Thus the cloud 

 will not only contain the negative ions on which the water 

 had been deposited, but it will also contain practically the 

 whole of the corresponding positive charge ; that is, little or 

 no separation of electricity will take place. 



The only escape from this reasoning is to assume that the 

 number of ions in a cubic metre of air has been over-estimated^ 

 and that there are .in reality so few ions present that drops 

 large enough to fall immediately through the ascending 

 current are formed. Such an assumption is perfectly valid, 

 but it would only get over this difficulty by greatly increasing 

 those treated of in fii.) above and (v.) below. 



It may be as well to point out here that the theory we are 

 discussing is only partially based on experiment ; for although 

 experiment has shown that when air is supersaturated to a 

 fourfold degree condensation takes place on negative ions, no 

 experiments have ever been made in which measurable 

 quantities of electricity have been separated by the con- 

 densation process ; until this has been done the theory cannot 

 be said to be entirely satisfactory. 



We will now pass on to discuss the theory from the 

 meteorological point of view. 



(iv.) In all the reasoning up to this point we have assumed 

 the main proposition of the theory, namely, that the forma- 

 tion of a supersaturated stratum in the atmosphere is a pos- 

 sibility. Now it may not be possible to prove that the 

 formation of a supersaturated stratum in the atmosphere would 



