ON ITS EXTERIOR. GENERAL REMARKS. 163 



undulations, and electro-magnetic activity) are nearly 

 allied manifestations ( 225 ) of one and the same primary 

 force, among which the first is the most independent of 

 diversity of substance. We have in all the above 

 respects represented our planet in its cosmical relation 

 to the central body of its system : its primitive internal 

 heat, generated possibly by the condensation of a rotating 

 nebulous ring, is modified by the solar action ; and, as 

 has been above stated, according to the latest hypotheses 

 a periodical action is exercised on terrestrial magnetism 

 by a particular condition of the Sun, manifested to us 

 by the periodically varying frequency of the solar spots, 

 or openings in the outer envelopes of the Sun. 



This second division of the present volume will be 

 devoted to the group of telluric phenomena which are 

 attributed to the reaction, still going forward, of the in- 

 terior of the earth on its exterior.( 226 ) I designate the 

 whole of these phenomena by the general name of volca- 

 nism or volcanicity, and I regard it as an advantage not 

 to divide effects having the same causal connection, and 

 differing only by the strength of the manifestation of 

 the acting force, and by varieties in the complication of 

 the physical processes involved. In this generality of 

 view, small and apparently insignificant phsenomena 

 acquire a greater significance. An observer not scienti- 

 fically prepared, who visits for the first time a basin 

 filled by a hot spring, and sees ascend from it gases 

 which extinguish the flame of a candle, or walks 

 between rows of variable cones of mud-volcanoes 

 hardly exceeding in height la is own stature-, would 

 not divine that the place now thus harmlessly occupied 

 has been repeatedly the scene of fiery eruptions as- 



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