Section A. 



ASTRONOMY, MATHEMATICS, AND 

 PHYSICS. 



ADDRESS BY THE PRESIDENT, 

 J. A. POLLOCK, D.Sc, 



Professor of PI/j/s/cs in the Unirrrsiiij of Siidiici/. 





THE IONS OF THE ATMOSPHERE. 



A.S one of the results of the recent development of electrical 

 science, it is considered that throughout the air in its normal state, 

 and in other gases in a similar condition, there exist a small number 

 of molecules, or groups of molecules, which are distinguished from the 

 vast host of their fellows in being electrified. Each of these electrified 

 entities, whatever its structure, is called an ion, and of ions there are 

 two main classes, the one containing those which are positively, the 

 other those which are negatively electrified. The notion of the ion, 

 in this connection, arises from attempts to reach a simple description 

 of the facts associated with the conduction of electricity through 

 gases, and the hj'pothesis admirably fulfils its purpose. 



The number of ions in the air can be greatly increased by 

 exposing it to the influence of Rontgen rays, or to the radiations 

 from radiimi or other radio-active bodies, and it is from investiga- 

 tions connected with this ai'titicially-produced ionisation that most of 

 our present knowledge of ions is derived. For the most interesting 

 account of these researches I refer you to the Address delivered 

 before this Section at Dunedin in 1904 by the present distinguished 

 President of the Association. For my immediate purpose I have to 

 remind you of one result : in an electric field, in addition to the 

 motion of molecular agitation shared by all the constituents of a gas, 

 the ion.s, in virtue of their charge, acquire a velocity whose average 

 value depends on the electric intensity and on the resistance which is 

 offered to the movement; under the influence of the electrical forces 

 the ions drift, as it were, in a definite direction, the positives 

 travelling to the negative electrode, and vice versa, a motion in which 

 the uncharged molecules have no part. Other things being equal, it 

 is assumed that this drift velocity of the ions is directly pi'oportional 

 to the electric intensity, and, following the suggestion of M. Langevin, 

 the term '^■'mobility" has been adopted for the average velocity 

 acquired by an ion under the influence of imit electric force. At the 

 present time the mobility of a class of ions is its most readily deter- 

 mined property, and it is principally to observations of the mobility 

 of the ions in different gases and under various conditions that we 

 must look for a clue to the nature of the ionic structure. ■ In all caaes 



