38 president's address — section a. 



2 '4 is twice as great as that for a humidity of lo'-t. The observa- 

 tions are not regular enough to show if there is any difference 

 between the mobihties of the positive and negative ions. Owing 

 to ionisation being caused by phosphorus, it is not advisable to use 

 phorphorus pentoxide as the diying agent in such experiments, and 

 calcium chloride has been employed in all cases. 



The intermediate ion has been under observation for only a com- 

 paratively sliort time; the measures so far made, however, show that 

 the mobility is largely affected by change of the humidity of the air, 

 the magnitude varpng from 1/15 to about one-tenth of that value as 

 the absolute humidity alters from 0'5 to 15 at a temperature of about 

 22° C. To this statement there is a limitation, the extent of which I 

 do not as yet fully know : in air in its natural &tate with the absolute 

 humidity between li and 16 at 22° C, when the ionisation due to this 

 class of ions is relatively weak, the mobility, at least of the positive 

 ions, is of the order of 1/65, while with strong ionisation the value is 

 only about half as great. Unless the limitation just mentioned 

 provides an exception, on further investigation, no definite difference 

 between the mobilities of the positive and negative ions of this class 

 can be deduced from the observations. 



The facts just described prove that there is a detiuite connection 

 between the ions and the water vapour of the air, and open up an 

 interesting field for speculation as to the development and structure 

 of electrified clusters, and as to the nature of the resistance which 

 they experience in drifting through the crowd of molecules. The basis 

 of the structure is, of course, the molecular ion, which, it is well 

 known, originates from effects associated with radio-active transfor- 

 mations occuring in the air, the ionisation being primarily due to the 

 presence of radium and thorium in the material of the earth's surface. 

 The growth to more complex structure apparently occurs by the 

 collection of water molecules round the molecular ion owing to the 

 influence of its charge. 



Seemingly from a consideration of the experimental results, Ave 

 must recognise at least two forms of electrified molecular aggi-egation 

 in the air which are stable under ordinaiy conditions. As the mobili- 

 ties depend on the humidity it might not unreasonably be supposed 

 that the intermediate and large ions represent stages in the develop- 

 ment of the small ions into visible drops of water, which occure if the 

 air becomes sufficiently supersaturated. It seems, therefore, curious 

 that the large ions are not separately apparent as condensation nuclei 

 ' . r.loud experiments. 



Mr. C. T. R. Wilson* has sliown that in such experiments the 

 presence of a moderate electrical field prevents the formation of drops 

 if the expansion ratio does not exceed the value 1'27. This proves 

 that the nuclei for these small expansions are ions which can be 

 removed by the field before the expansion takes place. I have care- 

 fully repeated the observations, with an apparatus similar to that 

 described by Mr. Wilson, in order to determine if the effect of the 

 electric field varies with the time it is on before expansion, and find 

 the full effect whether the interval is one second or twenty minutes. 

 With the fields used it takes several minutes to remove all the large 



* Wilson. Phil. M:iff.. Jiiiie. 1904. 



