78 



Procredinr/s of the Rni/al Irish A cademi/. 



relation for a range which would saturate ions of mobilities from ^w-o to -j-TnTo 

 cm./ sec. In none of these curves is there any evidence of a bend indicating 

 any heterogeneity in the ions. It jnay be mentioned that in all these cases 

 the small ions present would be removed by a fraction of a volt, and theii- 

 presence is indicated by the fact that the curve produced back does not 

 appear to pass throvigh the origin. 



Average. Ye'Locity ofAir-£last =M-7 Cms pe,r Stc. 



The curve (fig. 7) is plotted with such an air-blast and voltages that 

 complete saturation is reached ; and if we take 450 as the saturation 

 voltage, the value of Q being 832 c.c. per second, we get the value ^^^ for 

 the mobility of the large ions. 



It should be observed that it is often a matter of very considerable 

 difficulty to get a satisfactory long series of observations for graphical repre- 

 sentation. This difficulty arises from the great and sometimes sudden 

 variations that take place in the number of large ions present. The curves 

 referred to above were only obtained after a great number of trials, retaining 

 those only in which the ionisation remained steady during the time required 

 to take a complete series of observations. 



The series of curves just described show that all the large ions present 

 in the atmosphere have approximately the same mobility. Our work gives 

 no indication of the presence of ions of an intermediate size distinguished by 

 a mobility of the order of -j-^ cms. per second as found by Pollock. 



The value of the mobility given above, jj'j-q cm. per second, is in good 

 agreement with Langevin's observations. 



Record of Ohservations on the Nxmiber of large Ions jjer c.c. of the Atmosphere. 



It will be seen later that the number of small ions in the atmosphere is 

 only a very small fraction of the number of large ions. Hence the total 



