VULCANICITY. 157 



II. 



REACTION OF THE INTERIOR OF THE EARTH UPON ITS SURFACE; 

 MANIFESTING ITSELF:— O. MERELY DYNAMICALLY, BY TREMU- 

 LOUS UNDULATIONS (EARTHQUAKES); b. BY THE HIGH TEM- 

 PERATURE OF MINERAL SPRINGS, AND BY THE DIFFERENCE OF 

 THE INTERMIXED SALTS AND GASES (THERMAL SPRINGS); c. BY 

 THE OUTBREAK OF ELASTIC FLUIDS, SOMETIMES ACCOMPANIED 

 BY PHENOMENA OF SPONTANEOUS IGNITION (GAS AND MUD VOL- 

 CANOES, BURNING NAPHTHA SPRINGS, SALSES); d. BY THE 

 GRAND AND MIGHTY ACTIONS OF TRUE VOLCANOES, WHICH 

 (WHEN THEY HAVE A PERMANENT CONNECTION WITH THE AT- 

 MOSPHERE BY FISSURES AND CRATERS) THROW UP FUSED EARTH 

 FROM THE DEPTHS OF THE INTERIOR, PARTLY ONLY IN THE 

 FORM OF RED-HOT CINDERS, BUT PARTLY SUBMITTED TO VARY- 

 ING PROCESSES t OF CRYSTALLINE ROCK FORMATION, POURED OUT 

 IN LONG, NARROW STREAMS. 



In order to maintain, in accordance with the fundamental 

 plan of this work, the co-ordination of telluric phenomena — 

 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 properties 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 diffusion 

 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 primitive 

 force. They occur in a condition of the greatest independ- 

 ence 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 of its system, be- 

 cause the internal primitive heat, which is probably 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 accord- 

 ance with the most recent hypotheses. 



The second section of this volume is devoted to the entire- 

 ty of those telluric phenomena which are to be ascribed to 

 the constantly active reaction of the interior of the earth upon 



* Cosmos, vol. iii., p. 34. 



