358 Analysis of Scientific Books. 



spreads laterally or horizontally, forming, if the air is calm, a 

 shape resembling that of a pine-tree, or if there be wind, a hori- 

 zontal stream. Out of this cloud proceed lightnings of great 

 vividness, while the falling of the dust, added to the density of 

 the cloud, produces darkness over the surrounding country. The 

 melted rock or lava now boils up in the crater, and is often so 

 thrown up into jets by the extricated air, as to resemble flames ; 

 and at length it either boils over the edge of the crater, so as to 

 run down the mountain, or else finds an issue laterally, by some 

 crevice, equally flowing down in a stream, which holds its course 

 as circumstances permit, down to the lower grounds. 



In the night this current is luminous * but in the day, it is 

 generally obscured by vapours, or loses its light by the cooling 

 and blackening of the surface. There are cases, however, in 

 which no torrent of lava occurs, and where no other rocks than 

 scoria are erupted. The greatest period of violence is generally 

 over when the lava has flowed for a little while, or this is the 

 crisis of the volcano. But commonly, the explosions of fragments 

 and dust continue for some time, gradually diminishing, till the 

 whole falls into a state of quiescence, and is, finally, extinguished. 

 Lastly, it must be noticed, that from the action of the volcano on 

 the atmosphere, clouds are generally formed in it, which produce 

 falls of rain, often causing torrents, or even inundations. 



We must refer to the work itself, pp. 13, 14, for a catalogue of 

 eruptions which we could not conveniently introduce. But the 

 intervals of repose are various, reaching in some cases as far as 

 to many centuries ; so that cultivation and population is renewed, 

 to be dispersed again at some future day. In these intervals of 

 repose, however, it is common for vapours to continue to be pro- 

 duced, either from the craters, or in the courses of the currents of 

 lava ; and when these are sulphureous, they deposit sulphur ; and 

 in other cases, from their acid nature, they corrode and decom- 

 pose the rocks through which they find a vent. What are called 

 solfataras and souffrieres are the result. 



The author now proceeds to the Theory of Volcanic Eruptions. 



The force of the eruption consists in aeriform fluids extricated 

 and expanded by heat, and maintaining the lava in a state of 

 ebullition ; as is evinced by the case of Stromboli, described so 

 accurately by Spallanzani, and as is further proved by facts oc- 

 curring at Vesuvius, and in the Isle of Bourbon. But the 

 liquidity of the lava is imperfect, in our author's estimation, 

 resembling that of mud, or arising from a freedom of motion 

 among solid particles, mixed with one or more perfect fluids which 

 form their vehicle. At the moment even of emission, he considers 

 it as radiating very little caloric, though of a brilliant white heat, 

 its fluidity, even when greatest, not exceeding that of honey, and 

 being often much less considerable. In flowing, the crust con- 



