| OCR Text |
Show 392 STRUCTURE, AND MODE OF ACTION times approximating more to basalt and sometimes to trachyte; and (as we learn from the important researches of Mitscherlich, 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 substances 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 ( i. e. the earlier oxidized 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 begun 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 metalloids of those substances. The solid and alreadyoxidized crust of the globe separates the surrounding atmosphere, with the oxygen which it contains, from the inflammable unoxidized substances in the interior of our planet : when those metalloids come in contact with the oxygen of the atmosphere there arises disengagement of heat. The great and celebrated chemist who propounded this explanation of volcanic phenomena soon himself relinquished it. Observations made in mines and caverns in all climates, and which in concert with M. Arago I have collected in a separate memoir, show that, even at what may be considered a very small depth, the temperature of the earth is much above the mean temperature of the atmosphere at the same place. A fact so remarkable, and so generally confirmed, connects itself with that which we learn from volcanic phenomena. The depth at which the globe may be regarded as a molten mass has been calculated. The primitive cause of this subterranean heat is, as in all planets, the process of formation itself, the separation of the spherically condensing mass from a cosmica! gaseous fluid, and the cooling of the terrestrial strata at different depths by the loss of heat parted with by radiation. All |
