1826.] Mr. Scrope oh Volcanos. 51 



In proceeding to account for these pheenomena, he remarks 

 that they prove the existence of a body of lava in a state of 

 ebullition beneath every volcanic aperture during the period of 

 its activity. He then examines the nature of this ebullition, 

 and finds reason to conclude from the crystalline texture of all 

 lavas, their sudden congelation on exposure, the small quantity 

 of heat radiated by them even when incandescent, and the 

 volumes of aqueous vapour they evolve, that their partial lique- 

 faction is not owing to fusion, but to the vaporization of minute 

 quantities of water interposed between the laminas of their com- 

 ponent crystals. The explosive escape of steam in vast quantity 

 from the orifice of every volcano during an eruption, and from 

 the surface and clefts of every lava current as it Hows in open air, 

 proves the existence of this fluid in the interior of the mass, and 

 the elevation of liquid lava through the chimney of the volcano, 

 the jets of scoriae which accompany its rise, and the numerous 

 vesicular cavities and pores of the rock into which it consoli- 

 dates, all appear the necessary consequences of the generation 

 of steam in this situation. Other aerifjrm fluids, perhaps many 

 permanent gases, are probably evolved at the same time, but the 

 great body of vapour appears to be aqueous. 



The author proceeds to account for this process by supposing 

 the continual accession from below of increments of caloric to a 

 mass of crystalline rock of this nature already at an intense 

 temperature, and confined beneath the solid crust of the o-lobe. 

 The consequence would be the increase of its expansive force 

 without other limit than the yielding of the overlyino- strata. 

 When this occurred, and a fissure was broken through them, and 

 sufficiently enlarged,, the consequent sudden reduction of pres- 

 sure on the heated rock below would vaporize its contained 

 water, liquefy the mass, and force it to rise in an intumescenl^ 

 or ebullient state to discharge itself through this aperture, while 

 exploding volumes of steam escaping from its surface as rapidly 

 as the weight and tenacity of the substance permits them, carry 

 up into the air showers of red-hot fragments and ashes, in the 

 exact manner of a volcanic eruption. 



But the quantity of matter thus protruded by its weight and 

 rapid congelation, tends to choke and seal up the fissure of 

 eruption, and check the progress of the ebullition below ; and 

 in far the greater number of cases these obstructing causes 

 sooner or later prevail, the force of repression obtains the predo- 

 minance over that of expansion, and an interval of outward tran- 

 quillity ensues. During this the subterranean mass of lava 

 which has been cooled down by the sudden partial vaporization 

 of its aqueous particles, has its temperature again raised by the 

 communication of caloric from below, till a second eruption is 

 the consequence of the yielding cf the congealed crust to its 

 increasing force of expansion. 



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