OF VOLCANOS. 237 



mixtures of augite, titaniferous iron, feldspar, and hornblende, 

 may have been the same at different epochs, sometimes 

 approximating more to basalt and sometimes to trachyte ; 

 and, (as we learn from the important researches of Mit- 

 scherlich, and the analogy of artificial igneous products) 

 chemical substances may have united in definite proportions 

 in a crystalline form : in all cases we recognise that sub- 

 stances similar in composition have arrived at the surface of 

 the earth by very different ways ; either simply upheaved, 

 or penetrating through temporary fissures ; and that breaking 

 through the older rocks, (/. e. the earlier oxydized crust of 

 the globe), they have finally issued as lava currents from 

 conical mountains having a permanent crater. To confound 

 together phenomena so different is to throw the geological 

 study of volcanos and volcanic action back into the obscurity 

 from which, by the aid of numerous comparative observations 

 and researches, it has gradually began to emerge. 



The question has often been propounded : What is it 

 that burns in volcanos, What produces the heat which 

 melts and fuses together earths and metals ? Modern 

 chemical science has essayed to answer, that what burns are 

 the earths, the metals, the alkalies themselves ; viz. the me- 

 talloids of those substances. The solid and already-oxydised 

 crust of the globe separates the surrounding atmosphere, with 

 the oxygen which it contains, from the inflammable unoxy- 

 dised substances in the interior of our planet : when those 

 metalloids come in contact with the oxygen of the atmos- 

 phere there arises disengagement of heat. The great and 

 celebrated chemist who propounded this explanation of vol- 



