Theory of Thunderstorm Electricity, 629 



kind in a cubic metre of air. Now, i£ every negative ion in 

 the cubic metre of air receives some water, 6*5 grains of 

 water will be deposited on 1 electrostatic unit of electricity, 

 L e. the charge per cubic centimetre of water will be 

 1/6'5 = '15 electrostatic unit. 



It is important to notice that this is the maximum charge 

 per c.c. which it is possible for the water to obtain according 

 to the theory, for we have assumed that every negative ion, 

 in what must be granted to be highly ionized air, has been 

 caught, and that only the water which has been deposited at 

 the instant of condensation from four-fold supersaturation 

 has condensed around the ions. We can even go further 

 than this and say that not only is this the maximum charge 

 possible, but that it would be a practical impossibility for the 

 rain which reaches the earth to have so large a charge. The 

 Avater which receives this maximum charge in the condensa- 

 tion layer has to fall through the whole of the lower cloud 

 before it can reach the earth, and in the process it is bound 

 to become mixed with a certain amount of uncharged water. 

 Further, the lower cloud will itself be raining, so that the 

 rain which reaches the earth will be a mixture of the charged 

 precipitation from the condensation layer, and uncharged 

 rain from the lower cloud. Thus, if rain is ever found to 

 have as great a charge as *15 electrostatic unit per c.c. of 

 water we shall be justified in asserting that the charge 

 cannot possibly have been obtained in the manner which the 

 theory supposes. 



Turning now to the results of actual measurements, we 

 find that Gerdien* records charges as large as 2*25 electro- 

 static units per gram of water : Weissf 5*3 units of negative 

 and 13*6 units of positive electricity per gram of rain ; and I 

 myself have measured during rain in Simla charges as high 

 as 19 electrostatic units of negative electricity per gram of 

 rainwater. Thus it appears that in this particular also the 

 theory signally fails to account for the observed facts. 



(iii.) We will now turn to a still more important con- 

 sideration and discuss the probability of any considerable 

 separation of electricity taking place at the condensation 

 layer. 



To do this it will be necessary to go somewhat closely into 

 the processes taking place at the condensation layer. We 

 will imagine a volume of supersaturated air just arriving at 

 the layer. Condensation takes place, and on the 3X10 9 

 negative ions in the cubic metre 6*5 grams of water are 



* Gerdien, Jahrb. der Had. und Elect, vol. i. p. 15 (1904). 

 t Weiss, Wien. Ber. ex v. pp. 128o-1320 (1906). 



