FORMATION OF AGGREGATES 273 



between the same gases when free, may also 

 occur by the condensation of the molecules into 

 complex groups or systems of aggregation in the 

 gas itself, round nuclei, or foreign substances, 

 which by their presence facilitate the formation 

 of these aggregates alone. 



Hence the passage of a discharge of electricity 

 through a gas can be said in certain circumstances 

 to produce not merely ionisation but large groups 

 of molecules possessing a large store of potential 

 energy that may be radiated away, partly in the 

 form of light. The life period, if we might so 

 call it, of these molecules is greatly increased, as 

 will be shown, by removing them from the 

 presence of the ionised gas. 



It is conceivable, and it seems quite probable, 

 that by the passage of a discharge, a mixture of 

 gases can at certain pressures enter into chemical 

 combination of a somewhat unstable kind ; although 

 they may not be known to form such com- 

 binations at ordinary pressures, so that reactions 

 analogous to chemical combination, but unknown 

 at ordinary pressures, are facilitated by the 

 passage of the discharge in high vacua and 

 manifested by the phenomenon of phosphor- 

 escence.^ 



"We have said that, speaking generally, radi- 

 ations which excite phosphorescence also produce 

 ionisation. It is, therefore, a matter of very great 



1 Philosophical Magazine, 1901, p. 352. 



T 



