VULCANIC ITY. 163 



the co-operation of a single system of impelling forces 

 in the descriptive representation, we must here remind the 

 reader, how, starting from the general pr -perties of matter, 

 and the three principal directions of its activity (attraction, 

 vibrations producing light and heat, and electro-magnetic pro- 

 cesses], we have in the first section taken into consideration 

 the size, form, and density of our planet, its internal dif- 

 fusion of heat and of magnetism, in their effects of intensity, 

 dip, and variation, changing in accordance with definite 

 laws. The directions of the activity of matter just mentioned 

 are nearly allied * manifestations of one and the same primi- 

 tive force. They occur in a condition of the greatest inde- 

 pendence of all differences of matter, in gravitation and 

 molecular attraction. We have at the same time represented 

 our planet in its cosmical relation to the central body oi its 

 system ; because the internal primitive heat, which is pro- 

 bably produced by the condensation of a rotating nebular 

 ring, is modified by the action of the sun (Insolation). With 

 the same view, the periodical action of the solar spots (that 

 is to say, the frequency or rarity of the apertures in the 

 solar envelopes) upon terrestrial magnetism, has been referred 

 to, in accordance with the most recent hypotheses. 



The second section of this volume is devoted to the 

 entirety of those telluric phenomena which are to be 

 ascribed to the constantly active reaction of the interior oj 

 the earth upon its surface? To this entirety I give the 

 general name of Vulcanism or Vulcanicity ; and I regard it 

 as advantageous to avoid the separation of that which is 

 causally connected and differs only in the strength of the 

 manifestation of force and the complication of physical pro- 

 cesses. By taking this general view, small and apparently 

 unimportant phenomena acquire a greater significance. 

 The unscientific observer who comes for the first time 

 upon the basin of a thermal spring and sees gases cap- 

 able of extinguishing light rising in it, or who wanders 

 amongst rows of changeable cones of mud volcanoes, scarcely 

 exceeding himself in height, never dreams that in the calm 

 space occupied by the latter, eruptions of fire to the he'ght 

 of many thousand feet have often taken place ; and that one 



1 Cosmos, vol. iii, p. 39. 



2 Cotmos, vol. i, p. 197 199, 



