MOLECULAR AGGREGATION 263 



less be produced by chemical, or what may appear to 

 be semi-chemical, catalytic processes. 



In fact, molecular theory in its relation to that 

 of luminosity must be looked at from a more de- 

 tailed point of view than that in which it is 

 generally regarded. 



Thus, again, the behaviour of a luminous gas or 

 other substance in the formation of large molecules 

 " is analogous " to that of a vapour condensing into 

 a liquid, to the freezing of a liquid, or to the forma- 

 tion of crystals from a saturated solution, and the 

 passage of a discharge through a gas at certain 

 pressures. For in the absence of any foreign 

 substance the temperature and pressure at which 

 these occur are very indefinite ; but when a 

 foreign substance the molecules of which can act 

 as nuclei are present, these efi"ects take place under 

 perfectly definite conditions and at perfectly definite 

 " temperatures and pressures." ^ 



The influence of nuclei in producing these effects 

 will be considered in the sequel. 



The author ^ has found that the after-glow in gases 

 is influenced by the presence of impurities, so that 

 the formation of groups in some cases, perhaps 

 of polymeric forms, is greatly facilitated by the 

 presence of traces of foreign bodies, just as it is in- 

 fluenced by the presence of nuclei and negative ions. 



Sir James Dewar has inferred from his experi- 

 ments at low temperatures that the more complicated 

 the molecular structure of a body, the more likely 



1 J. J. Thomson, Phil. Mag., 1894. 2 Loc. cit. 



