114 Volcanos. 



crystals l>y the intervention of more or less of highly elastic 

 vapor between the opposite facets, and the degree of liquidity 

 will therefore depend on the quantity of vapor generated 

 through the substance, and the size of the component crystals. 

 But at a certain term in the relative proportion of these cir- 

 cumstances, the vapor will become so abundant as to enable a 

 part of it to unite into bubbles, which, by their inferior specific 

 gravity, are urged to rise upwards, and escape from the surface 

 of the liquified mass in which they are formed. u The quanti- 

 ty of vapor discharged in this manner consists therefore at all 

 times of the surplus of that which has been generated in the 

 lava beyond what is necessary to communicate to its component 

 crystals the degree of mobility required for the union of this sur- 

 plus vapor into parcels or bubbles, and the rise of these, when 

 formed, to the surface." The explosions of all volcaDic erup- 

 tions are produced by the rapid ascent and escape of such bub- 

 bles, collecting as they rise through the lava into prodigious 

 volumes of vapor ; and the remainder of these parcels of va- 

 pour, which are prevented from thus escaping by the superfi- 

 cial induration of the exposed masses of lava, occasions the 

 cellular and cavernous structure of such rocks both on the 

 large and small scale. The consolidation of liquified lava, 

 under these circumstances, takes place, not only by the loss of 

 temperature, but also, and chiefly, by the immediate escape of 

 the vapor (which alone occasions the mobility of the crystals) 

 on their superficial exposure to the outward air ; subsequent- 

 ly, by exudation through the pores or interstices of that hard- 

 ened surface, the process is propagated to the interior of the 

 bed of lava. Consolidation may also take place by increase 

 of pressure on the bed of lava, without any change in its tem- 

 perature ; the vapor being condensed till the crystals reunite, 

 more or less conformably, according as they have been more or 

 less broken up, or disintegrated, by mutual friction, when in 

 motion, and by the disaggregative force of vapor generated 

 within them. 



" Having developed these original ideas as to the nature of 

 lava, which are supported by facts and arguments, the author 

 goes on to explain all the phenomena of earthquakes and vol- 

 canos, and the circumstances of disposition, structure, and 

 mineral character in volcanic rocks, by the assumption, which, 

 however, is strongly supported by a large body of evidence, 

 that the interior of the globe, at no great depth from the sur- 

 face, consists of a mass of crystalline rock at an immense tem- 

 perature ; and therefore, that a continual supply of caloric 

 passes off from the centre towards the circumference, wherev- 

 er the nature of the superficial rocks allows of its transmission, 

 or temporary vents are opened for its more free escape. 



