QiS ON THE CRYSTALS IN LAVAS. 



and throws hirti into amazement. This is the eiFect of tjie 

 extent of its fires, the noise with which they are accompa- 

 nied, and the sight of the streams of burning lava. But 

 The heat uife- jf ^g examine it with inquisitive attention, he will soon 

 our furnaces; j^iclge from its effects, that this vast furnace has in no part 

 a heat so intense as may be produced in our iron works; 

 and must be and it is easy to perceive, why our furnaces have this greater 

 ent^of atr in^^^^ity of heat: it is produced by the continual current 

 of air blown into them, which by its extreme rapidity is 

 incessantly bringing fresh air, the presence of which im- 

 parts greater activity to the fire ; but this cannot happen to 

 the fire of a volcano, which has no such communication 

 with the air. This is the reason why the pyroxene schoerl, 

 which is xmalterable by the fire of volcanoes, is reduced to 

 the state of glass in a crucible in our iron furnaces, and 

 fragments of lava exposed to a similar trial are more com^- 

 pletely vitrified. 

 Ohsidian a Qf all volcanic substances the obsidian, or compact glass 



jperfect g a&s. ^^ volcanoes, has been exposed to the greatest heat. The 

 vitrification of this is complete. None of the pieces I pos- 

 sess, or have seen, exhibits any thing but glass. All the 

 substances that compose it have been reduced to perfect 

 fusion. These vitrifications therefore come from a part of 

 the fbcus, where the heat has been urged to a higher degree 

 by some particular circumstance. 

 And this in- Now why is it that these obsidians, which must have 

 eludes nocrys- cooled as slowly as other lavas, do not exhibit any crystal- 

 line figure within them ; if it be not for this reason, that, all 

 the substances in them having been fused, there can be no- 

 thing but glass throughout their mass ? 

 We must not I will say in my turn, that it is much more extraordinary 

 reason from f^j. ^jjgjj ^q get out from the operations of our petty manu-. 

 our operations . . ^ , ,. fir. n \ 



to nature's. factories, to determme the torce ot the fires or volcanoes, 



and assign them an unlimited extent; and still more extra- 

 ordinary thence to deduce the origin and formation of roqks 

 and primordial mountains. Let us confine ourselves to the 

 effects, that pur narrow means can produce; and not plunge 

 ourselves into a labyrinth of illusions, by reasoning from 

 small to great; for, as pur means are merely artificial, they 

 are not those that operate in nature, 



" Naturalists," 



