366 Analysis of Scientific Books. 



towards each other, while the liberated vapour remains among 

 them in " irregular parcels." Therefore when the increase of 

 pressure or diminution of temperature arrives, the forces that tend 

 to bring together gain an ascendency over the power of the vapour, 

 or else the forces remain "unaugmented," while the elasticity of 

 the vapour diminishes ; and then by the gradual condensation of 

 the fixed vapour, the " disaggregated " laminse are brought to- 

 gether again, and " without any derangement in the positions of 

 their axis, are re-aggregated into whole crystals precisely such as 

 they were before." In the mean time the free vapour " allows 

 the approach of the disintegrated crystals," but as their poles do 

 not now correspond, and their proximate surfaces are not parallel, 

 a confused crystallization, or a granitoidal and more or less porous 

 rock is the result : and in case of extreme slowness on the part 

 of the vapour, other kinds of reversal and mobility take place 

 among the crystals, and a partial re-crystallization occurs, from 

 many " particles uniting into larger crystals." 



This is, as clearly as we can abridge it, our author's theory, both 

 of the fluidity or liquefaction, and the consolidation of lava, with 

 all their phenomena ; while he concludes with remarking how it is 

 confirmed by the experiments of Mr. Watt and Sir James Hall on 

 the fusion and refrigeration of rocke. 



The author proceeds to explain the extrication of air, forming 

 air-bubbles in lavas ; but as we do not perceive any views of the 

 same novelty and interest in this particular portion, we shall 

 refer our readers to the original, and take the liberty of passing 

 it over ; and for the same reason we shall refer them to the book 

 itself for the account of sublimation and amygdaloidal infiltra- 

 tions, as well as of hot springs; all of them being very interesting 

 facts connected with this curious subject. 



The sixth chapter treats of the divisionary structure of lavas, 

 which, in a broad view, is attributed to contraction from the escape 

 or consolidation of the included vapour, and to the establishment 

 of certain centres of attraction. Here, too, we must allow the 

 author, in a great measure, to speak for himself, by a reference ; 

 as the various detailed views under which he has handled the sub- 

 ject, would scarcely receive justice in any abridgment that we 

 could here give. The divisions which he has established, how- 

 ever, are thus termed, first, the prismatic; second, the tubular, la- 

 mellar, and schistose; third, the rhomboidal and cubiform ; fourth, 

 the globiform ; fifth, the angulo-globular. 



The causes which give rise to the different mineral compositions 

 of different lavas are also stated in this chapter; and the author is 

 rather inclined to consider them as arising from circumstances 

 taking place in the erupted lava itself, than from any original 

 difference in the rocks by the fusion of which it was formed. 

 Thus, a mass of granite, under particular circumstances, might 



