Volcanos. 113 



ic In Chap. II. the immediate causes of these phenomena are 

 investigated ; and it is observed, that all their circumsf ances, as 

 well as the direct observations of the author himself, Sipallanza- 

 ni and others, go to prove the existence, beneath every volca- 

 nic vent, of a 'mass of lava, or crystalline rock in a state 

 of actual ebullition ; the generation, or expansion, of electric 

 fluids within its interior producing' intumescence and elevation, 

 and the explosions which take place from its surface. The nature 

 of this elastic fluid has been ascertained by direct experiment, and 

 it appears to consist almost wholly of aqueous vap>or or steam. 

 The uniform dissemination of air-vesicles through many lavas 

 proves the vapor to have been generated throughout every 

 part of their mass. We must then suppose the existence of 

 water in combination with the other elements of the rock. 

 This leads to an examination of the nature of lstvas; and the 

 author finds reason to conclude, that the crystalline particles of 

 which they are composed were not formed as the substance 

 cooled ; that few or no lavas are ever reduced naturally to com- 

 plete fusion (none in fact but the glassy lavas, obsidian, pearlstone, 

 &c.;) but that they consist of crystalline particles of various sizes, 

 which, when the rock is solid, contain very minute portions of 

 water mechanically combined with their substance,* that is, in- 

 tervening between the parallel plane surfaces of *ihe crystals. 

 In this case, any continued accession of caloric to* a mass of 

 such rock confined beneath the crust of the earth, and already 

 at an intense temperature, must sooner or later so increase the 

 expansive force of the confined water, as to reduce; more or less 

 of it to vapor, breaking through or heaving upwards the confi- 

 ning crust, and causing the lava to intumesce and rise outward- 

 ly in a state of imperfect liquefaction through any fractures 

 which the violent expansive effort may create in the overlying 

 beds. The liquidity of lava consists, under this idea, not in its 

 alsolute fusion, but in the mobility afforded to its compotent 



* This at least is the temporary assumption of the author, who in a 

 later part of the work, observes, that he inclines to suppose the water 

 itself may be generated, together with other fluids, by the volatiliza- 

 tion of a superficial pellicle of the proximate crystals and the combi- 

 nation of the oxygen and hydrogen set free by this process, through 

 the intense temperature pervading the mass. Such a supposition, if 

 not supported, is perhaps not opposed by the present state of chemical 

 knowledge ; and would explain all the phenomena of lavas, as well as the 

 idea of a mechanical interposition. In short, the existence and gen- 

 eral dissemination of water, or rather steam, in lavas, is a positive 

 fact susceptible of direct and incontrovertible proof; and it is indiffer- 

 ent to the purpose of the author how, or at what time, we suppose it to 

 be produced there. 



Vol. XIII.— No. 1. 1.5 



