590 Mr. G. C. Simpson on 



conductor of the positive ions and decreasing the velocity of 

 the negative ones. 



As the air in contact with the conducting surface of the 

 earth is always ionized one would expect the same process to 

 take place in the atmosphere ; i. e. the earth to gradually 

 become charged until such a field is set up in the atmosphere 

 around it that an equal number of positive and negative ions 

 reach the surface in a given time. If this process takes 

 place on the free surface it will do so to a much greater 

 extent in all places where there is a protection against the 

 earth's normal field, so that in all places having a covering 

 of vegetation the absorption of negative ions will go on 

 unhindered. 



Others have found it necessary to enlarge this simple 

 theory of Elster and Geitel. G. T. R. Wilson ('Nature,' 

 vol. xlviii. p. 104, 1903j postulates a wind as necessary to 

 remove the positive ions from the neighbourhood of the 

 surface, and Riecke {Nacliiicliten der k. Ges. der Wissensch. 

 zu Gottingen, Math- iind PJiys.-KlassP^ 1903, Heft 2) has 

 considered that a greater proportion of negative than of 

 positive ions which strike the surface m;iy be absorbed, and 

 so he introduces a special coefficient of absorption. 



On the whole, it may be said that Elster and Geitel's 

 theory has held its own ; but, as recognized by Geitel in his 

 address, it has the weak point of being based upon an ex- 

 periment which has given a different result when repeated 

 by Villari. 



The great interest which belongs to this question of the 

 origin and maintenance of the etirtli's negative charge led 

 me to undertake a series of experiments to find the conditions 

 under which a conductor can become charged, and to what 

 extent, in consequence of ion absorption. The chief work 

 which has so far been done on this subject is that of Zeleny, 

 Townsend, and Villari, the result of which may be summed 

 up as follows. Zeleny found that when air which had been 

 ionized by means of Rontgen rays was passed along a tube, 

 metals over which it streamed became negatively charged. 

 From this and other experiments he drew the conclusion that 

 when positive and negative ions a7'e in an electrical field the 

 neoative ions move throuoh the neutral molecules of air 

 quicker than the positive ones, i, e. they have a greater 

 mobility. Townsend (Proc. Roy. Soc. Ixv. p. 192, 1^99,, & 

 Ixvii. p. 122, 1900) worked from another point of view. 

 He assumed that positive and negative ions may be looked 

 upon as the constituent parts of two separate gases. With 

 this assumption the following is the process to be expected 



