INTERNAL HEAT OF THE EABTH. 57 



phenomena which it is the purpose of natural science to 

 demonstrate and explain. We have only to advert to water, 

 to show that this fluid may be reduced to the most varied 

 and dissimilar conditions by the influence of heat, or by 

 chemical decomposition. By lowering the temperature, we 

 freeze it into ice ; by raising it, we thaw it ; by an increase 

 of heat, we convert it into vapour ; while we can again con- 

 dense this vapour into water. By galvanic agency r we reduce 

 it to its elements, the oxygen and hydrogen gases of which 

 it is composed. Metallic substances are capable of like 

 transmutations from the most solid form to that of vapour. 

 These facts, combined with others, the internal heat of our 

 globe, its volcanic eruptions, its vaporous exhalations and 

 thermal springs, serve to demonstrate the extreme proba- 

 bility of our planet having condensed to its present solidity, 

 from a fluid or gaseous state ; which this hypothesis assumes 

 as its original condition. 



INTERNAL HEAT OF THE EAETH. That the earth pos- 

 sesses a source of internal heafc, is a fact which is demon- 

 strated by the phenomena already mentioned. The increased 

 temperature of wells and mines, the warmth of which aug- 

 ments in a known ratio as we descend ; the vaporous exha- 

 lations of the earth, its streams of heated water, and its 

 volcanic eruptions, all prove the existence of such a cause. 

 To an agent thus powerful and universal many of the 

 modifications of the earth's surface are evidently attribut- 

 able, as the fusing of the melted rocks, and the altering of 

 those which are termed metamorphic, while its operation in 

 existing volcanoes is alike evident. The only doubt of im- 

 portance is as to the nature of these subterranean fires: 

 one section of observers, among whom Humboldt, Fourier, 

 Cordier, and Arago, maintain the views of Leibnitz, as to 

 their resulting from the original incandescence of our planet ; 

 the other, attribute them to chemical agencies operating 

 within the depths of the earth. Meantime there are various 

 facts, such as the mean density of our earth, which is too 

 small to allow of its being wholly a solid mass ; together 

 with the undulatory motion observed in earthquakes, with 

 other phenomena of similar nature, which lead to the con- 

 clusion, that a large part ot the interior of our planet is 

 in a state of fusion by heat, and that we are existing on the 



