CH.VIL] CONDUCTION IN AIR 73 



the wire was raised to about 300 C., even after 

 long-continued previous incandescence of the wire ; 

 whereas in pure hydrogen the wire had to be raised 

 to red heat before a cloud formed. 



Measurements of lonisation Current. 



The whole subject of the dissociation or breaking up 

 of molecules which is effected in gases by Rontgen rays, 

 by radium radiation, and by a great number of other 

 influences a process which is known as ionisation, 

 because the broken constituents of the molecule are 

 oppositely charged is too large to be conveniently 

 entered upon here. It has been worked at by a 

 multitude of experimenters, and for an account of 

 their results the work of J. J. Thomson on The 

 Discharge of Electricity through Gases, must be 

 referred to. It may suffice to say that the products 

 of decomposition, though in the first instance no 

 doubt simple ions, seem to be speedily complicated 

 by the aggregation of other molecules round them ; 

 and accordingly the diffusion or progress of these ions 

 is liable to be retarded, and, when measured, is found 

 slower than might otherwise have been expected. 

 On the whole, the negative ions tend to move faster 

 than the positive ones, but the difference is not 

 necessarily greater than can be observed in liquid 

 electrolysis. Eecent work by H. A. Wilson and 

 E. Gold, on conduction in flames, has however shown 

 that the negative carriers in that case are free 

 electrons. 



One of the easiest tilings to measure is the con- 

 ductivity of air in this ionised condition, that is to 

 say, the total current transmitted .by it between two 

 electrodes immersed in it and connected to some 

 sufficiently sensitive current-measuring device a 



