POLAE, LIGHT, OK ATJEORA. Lo9 



on the ground. ( 179 ) In high latitudes a flood of brilliant 

 and often coloured light streams through the atmosphere in 

 polar light or electro-magnetic storm ; and in the torrid zone 

 many thousands of square miles of ocean are also seen to 

 generate at once a light of their own ; in the latter case 

 the magic brightness belongs to organic nature : each 

 breaking wave curls in luminous foam; the whole wide 

 expanse sparkles, and every spark is the vital movement of 

 a minute and otherwise invisible world of animal existence. 

 Manifold, no doubt, are the sources of the terrestrial light ; 

 and we may even imagine it as existing latent and not yet 

 set free from combination with vapours, as a means of 

 explaining Moser's pictures produced at a distance, 

 a discovery in which reality as yet presents itself to us 

 like the unsubstantial images of a dream. 



If, on the one hand, the internal heat of our planet may be 

 connected with the excitement of electro-magnetic currents, 

 and the evolution of terrestrial light accompanying a mag- 

 netic storm, it is also a principal source of geological pheno- 

 mena. We shall trace this connection in passing from the 

 purely dynamic effect manifested in earthquakes, and the 

 elevation of entire continents and mountain masses, to the 

 production and issue of gases and liquids, of hot mud, and 

 of glowing and molten earths which harden into crystalline 

 rocks. It is no small advance made by modern geology (or 

 the mineralogical part of terrestrial physics), to have investi- 

 gated the connection of the phenomena here indicated. The 

 insight which we thus obtain leads away from the unpro- 

 fitable hypotheses by which it was formerly sought to explain 

 each such manifestation of force singly and independently : 



