360 Analysis of Scientific Books. 



is liberated and " unites into parcels," to that which remains 

 fixed, will vary with the comminution of the grain — and, that the 

 force with which these parcels or bubbles tend to traverse the 

 liquid lava and rise to its surface, varies directly with the specific 

 gravity of the solid lava rock ; and the degree to which they are 

 enabled to obey this impulse must depend on the liquidity of the 

 mass immediately above them. 



■ Under an increase of pressure or a diminution of temperature, 

 part of the vapour which would otherwise escape tends to the re- 

 consolidation of the lava. It becomes water, and the enclosing 

 crystal which was disintegrated by the ebullition of the lava or 

 the action of its vapour is re-aggregated. And the separate 

 crystals are also re-aggregated, in a confused manner, into a solid 

 rock, more or less porous in proportion to the quantity of vapour 

 disengaged. Such is the author's view of the formation of lava 

 rocks, and of the crystals in them, as far as we can condense it 

 into our narrow space. 



A wider general view follows. It is concluded that the interior 

 of the earth, at " no great vertical distance," is of an intense 

 temperature ; that the accumulated caloric is continually tending 

 to an equilibrium, but that this is opposed by the imperfectly con- 

 ducting powers of the rocks or strata, while he also supposes that 

 " the crystalline, and particularly the compact granitoidal rocks," 

 are much better conductors than the " secondary limestones, clays, 

 shales, and sandstones." This caloric thus retained is supposed 

 to act especially on the previous masses of lava, producing all 

 those effects which give rise to the phenomena of volcanoes. 

 Here the author enters into a long and minute detail on the for- 

 mation of fissures, for which we must refer to the work itself, at 

 page 32 and onwards, as we find it unsusceptible of abridgment. 



At page 37, he proceeds to investigate the laws by which the 

 violence and duration of the eruption must be determined. 



What he calls the general force of repression, consists in the 

 external pressure, whether from the atmosphere, the ocean, or 

 whatever else, in the weight of the column of liquefied lava, and 

 in the reaction of the vapour generated from the confining sur- 

 faces. 



And proceeding to apply these rules, he further concludes that 

 the energy of the eruption will vary directly with the tempera- 

 ture of the lava and the width of the aperture, and inversely with 

 the thickness of the solid rock which must be broken through, or 

 with its resistance generally as formed by that thickness and its 

 specific gravity, or with the external pressure. Thus also the 

 duration of the eruption will vary directly with the rate at which 

 the temperature increases downwards, and inversely with the 

 specific gravity of the lava, and the width of the aperture. It 

 equally follows that the expansion and the volume of the dilated 



