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THE POPULAE EDUCATOR. 



It has long been a theory pertinaciously adhered to, that the 

 centre of the earth is now in a state of molten incandescence, 

 and that all volcanoes derive their igneous action from the heated 

 centre. In conjunction with the existence of burning moun- 

 tains, another fact is adduced, which appears to give great 

 weight to the supposition. It is generally found that in 

 descending the crust of the earth the temperature increases. 

 Numerous observations in mines and artesian wells ha ire been 

 made, and it is concluded that in England the thermometer 

 rises 1 Fahr. for every 55 feet descended. 



The reader must not suppose that this increase is at all 

 uniform; indeed, Kupffer, from very extensive comparisons, 

 infers that the increase is 1 Fahr. for every 37 feet, while 

 Cordier gives the same augment of temperature for 45 feet. 

 English geologists, from experiments made in this country, 

 incline to the first-mentioned figure. Admitting that the 

 temperature increases because we descend nearer to the heated 

 nucleus, these differences might be reconciled by taking into 

 consideration local causes, such as the conducting power for heat 

 of the rock in which the mine or well was sunk which would 

 materially affect the observation the proximity of the ocean, 

 and other causes. But cases are recorded where the same mine 

 has altered its temperature very considerably : for example, the 

 Oatfield Mine, in Cornwall, 182 fathoms deep, had a temperature 

 of 77 Fahr. while the mine was working. After the abandon- 

 ment of the working's the temperature was reduced in a few 

 months to 66. In many months after, the equilibrium reached 

 was almost that of the mean temperature of the surface. 

 54 Fahr. 



'It is clear that if the original high temperature were due to 

 the central fire, the mere abandonment of the mine could not 

 have reduced it. Presuming the increase of temperature, as we 

 descend, be due to the approach of the central heat, at a depth 

 of twenty-five miles a temperature sufficient to fuse such rocks 

 as basalt and porphyry must exist. 



The holders of this theory must, therefore, be prepared to 

 admit that our globe has a crust of solid matter, whose thick- 

 ness is represented in Fig. 10, by the breadth of the line c B; 

 that is, if A B be the radius of the earth, 4,000 miles, the thick- 

 ness of the curved line, ^ of this, will represent the solid crust, 

 all beneath being in a state of molten matter at a high tempera- 

 ture. This vast quantity of liquid cannot remain at rest, but 

 must be traversed by currents, which would cause the tempera- 

 ture of the whole to be equalised ; and it cannot be supposed 

 that the mere shell would not be broken up and fused, and still 

 more difficult is it to admit that it is possible that the proximity 

 of such a mass at such a temperature would not affect the heat 

 of the earth's crust, which it certainly does not. The answer 

 given to these objections is, that we do not know the behaviour 

 of solids under the influence of a high heat, and at the same 

 time submitted to a great pressure ; pressure may retard fusion, 

 and that although at a high temperature, yet the rocks may not 

 be molten ; but the point of fusion must be reached a few miles 

 further down, and therefore the objection is not removed. It is 

 also urged that the earth's crust is a good non-conductor of heat, 

 and instances of lava currents are adduced where the surface is 

 solidified and tolerably good, and a few inches below the matter 

 is still molten; but the cases are not parallel, the molten lava 

 is only a dull red, and is at rest ; moreover, in due time it will 

 cool, whereas the temperature of the earth has not altered since 

 the days of Hipparchus, who chronicled certain eclipses. Cal- 

 culation has found that the times of those eclipses are correct, 

 which proves that the length of the day has not altered for 2,000 

 years, that is, the rotation of the earth has not increased, which 

 it must have done had the earth cooled, for then it would have 

 contracted. 



The late Mr. Hopkins attempted to calculate the thickness of 

 the earth's crust from the influence which the moon exerts upon 

 the earth, causing what is called the processional motion of the 

 earth's pole. He found that, assuming the earth to be a fluid 

 nucleus enclosed by a solid shell, it was necessary that this 

 should be at least 800 miles thick to cause the calculation to 

 agree with observation. 



The heat in mines, etc., can be well accounted for, without 

 this "central heat theory," by chemical action. Freshly exposed 

 surfaces, especially metallic lodes, are vigorously acted on by 

 the oxygen of the air; heat is thus developed. This will account 

 for an abandoned mine cooling down to the ordinary temperature 



when the chemical action was exhausted. Moreover, we must 

 remember that air in a condensed state acquires a lower specific 

 heat, that is, its capacity for containing heat is decreased; hence 

 an increment of heat will have a much greater effect in increas- 

 ing the temperature at the bottom of a mine than on the surface. 

 In the case of well-waters, the chemical action is induced by the 

 gases they hold in solution acting upon the rocks through which 

 they percolate, and on the sides of the subterranean reservoirs 

 in which they collect. Sir Humphry Davy, Daubeny, and others, 

 ascribe even volcanic heat to chemical action. The discoverer of 

 the metals of the alkalies, potassium and sodium, supposed that at 

 the base of burning mountains were vast deposits of these metals ; 

 that when water found its way to the metal it became decomposed, 

 the metal retaining the oxygen, and the hydrogen being liberated, 

 flamed out of the crater. The enormous scale of volcanic action, 

 however, precludes the acceptance of this theory, and we must 

 look for another cause of volcanic heat. Probably electricity 

 has much to do with it ; we know that electric currents traverse 

 mineral lodes, and the Aurora Borealis proves that enormous 

 quantities of electricity are in motion in and about the earth. 

 If a powerful current were condensed by any cause, and forced 

 to pass through a limited space, it would fuse the rock and 

 supply the great necessary for the existence of a volcano. We 

 have given a mere outline of these theories, and but very few of 

 the facts by which they are supported. We refer the reader for 

 further information to Lyell's " Principles of Geology," Daubeny 

 on " Volcanoes," and Scrope's " Central France." There are 

 225 active volcanoes, or rather volcanoes which have been 

 known to erupt within the las'u 150 years. The most remark- 

 able European volcanoes are Vesuvius, Etna, Stromboli, one of 

 the Lipari group, Santorin in the Greek Archipelago, and Hecla, 

 in Iceland. 



The products of volcanoes are of two kinds, lava and ashes. 

 They are not always ejected at once. In the great eruption of 

 Vesuvius, A.D. 79, when Herculaneuin and Pompeii were buried, 

 the mountain belched forth nothing but ashes, which so com- 

 pletely covered the doomed cities that their very site was un- 

 known for ages. The first authentic account of a lava current 

 from Vesuvius is in the year 1036. 



Of the quantity of matter ejected by Vesuvius in the eruption 

 of 79, history tells us that a shower of sand, lapilli, and pumice 

 fell for eight days. Very few of the inhabitants perished in 

 the overwhelming of their cities. The skeletons of two soldiers 

 in the barracks of Pompeii were found chained to the stocks, and 

 in the cellars in a villa in the suburbs of the city seventeen 

 skeletons were discovered. The form of a Roman lady, with 

 an infant in her arms, was imprinted in the rock, but nothing 

 but the bones remained, which a chain of gold encircled, and 

 the rings still adorned the fleshless fingers. Herculaneum is 

 buried deeper than Pompeii, but was discovered first by the 

 sinking of a well in 1713, which came down upon the theatre. 

 An illustration of the height to which a volcano can eject ashes 

 was afforded by one of the Mexican mountains in 1835, ashes 

 from which fell in Jamaica, 700 miles distant, and must have 

 been in the air four days. 



In illustration of the rivers of lava which are poured out from 

 the craters, we may quote the instance given by the Eev. T. 

 Coan, of an eruption of Mauna Loa : " From this yawning 

 fissure, from two to thirty yards wide, the molten flood rushed 

 out and spread laterally for four or five miles, filling ravines, 

 flowing over plains, covering all from 10 to 200 feet deep a 

 lava current, including windings, seventy miles long." 



The nature of volcanic rocks will be treated of in another 

 place. 



Before leaving this part of our subject, we must allude to the 

 peculiar phenomenon exhibited in the Geysers, in Iceland. 

 Whatever part steam may play in a volcanic eruption, there 

 is no doubt but that the Geysers owe their action to its 

 agency. The springs are thirty miles from the crater of Hecla: 

 they are springs of hot water, which rise through a bed of lava 

 with such force as to play in the air to a height of 200 feet for 

 five or six minutes, then the column of water subsides into the 

 basin, which is like a volcanic crater, and is in communication 

 with a cavern beneath. The pipe of the Great Geyser descends 

 perpendicularly 78 feet, and is from 8 to 10 feet in diameter. 

 The eruption commences with a distant rumbling noise, which 

 comes nearer, the water in the pool becomes agitated, and at 

 length rises in a jet, with clouds of vapour and a loud explosion. 



