134 Stevenson Macadam on a new Theory of the 



principal theories which have been promulgated, in order to 

 give an explanation of the causes of volcanic phenomena. 



The theories referred to are respectively termed the 

 mechanical and the chemical ; the former assuming that 

 volcanic eruptions are but the exudation of the interior fused 

 mass through the crust, — the latter believing that they can 

 be accounted for by the chemical action and reaction of 

 certain elementary substances. 



The adherents of the mechanical theory have put forward 

 many suggestions, in order to account for the propelling 

 power by which the molten mass could be ejected through 

 the crust. Humboldt considers that " all volcanic phenomena 

 are probably the result of a permanent or transient connec- 

 tion between the interior and the exterior of our planet. 

 Elastic vapours press the fused oxidising substances up- 

 wards through deep fissures. Volcanoes therefore are 

 intermittent earth springs."* 



The mechanical theory has thus, it would seem, for its 

 foundation, the supposition of a central nucleus in a state of 

 liquidity in physical contact with the crust. As this theory 

 has been assumed to be untenable, for the reasons alluded to 

 at the beginning of this paper, any explanations of volcanic 

 phenomena based upon it must inevitably fall to the ground. 



Many objections have also been made to the sufficiency of 

 the chemical theory, to account for the phenomena in ques- 

 tion. These objections have received considerable weight, 

 from the circumstance that the talented chemist who ad- 

 vanced the theory afterwards saw reason to relinquish it. If, 

 however, his principal reasons for doing so be carefully con- 

 sidered, and recent scientific discoveries be brought to bear 

 upon them, there seems, I think, every probability that the 

 chemical theory will be found to be a more satisfactory one 

 than its promoter latterly considered it. 



The following sentence, quoted from Sir Humphry Davy's 

 works, will explain his reasons for losing faith in the theory 

 referred to : — " There are, however," says Davy, " distinct 



* Views of Nature. Humboldt, p. 373. 



