Natural History of Volcanos and Earthquakes. 237 



crystals of salt. He father notices the spongy lavas which con- 

 tain so much iron-glance, and is of opinion that this may also be 

 a consequence of the sublimation of chloride of iron, and its sub- 

 sequent decomposition, by coming in contact with aqueous vapor 

 and atmospheric air, while at a red heat.* And, lastly, he men- 

 tions that chloride of iron, in contact with water, becomes so ex- 

 ceedingly hot, that it is capable, in large quantities, of raising 

 itself to a white heat, and that the chlorides of silicium and alu- 

 minium must be able to produce a much more extraordinary de- 

 gree of heat. 



It cannot be denied that there is some justness in these conclu- 

 sions. But it. must be remembered, on the other hand, that the 

 premises are only taken from appearances at Vesuvius^\ and that 

 the occurrence of common salt and muriatic acid in the products 

 and exhalations of volcanos, seems by no means to be general. 

 We have already quoted Boussingault's observation, that muriatic 

 acid is not evolved from the volcanos under the equator in the 

 New World. The hot springs in those regions contain but little 



* We may here notice the formation of artificial crystals of oxide of iron in a 

 potter's furnace. Poggendorff's Ann. v. xv, p. 630. Mitscherlich, who gives an 

 account of this, finds an analogy between this formation and similar ones in vol- 

 canos. He explains it by supposing that common salt and steam both act together 

 upon silica or siliceous combinations, and form muriatic acid, and that this comes 

 either alone or with a small quantity of water into contact with oxide of iron, or 

 ferriferous combinations. Thus chloride of iron is formed, which is again decom- 

 posed by the aqueous vapors, and, if the decomposition proceed slowly, the oxide 

 of iron remains behind in large crystals. 



In some volcanic eruptions, the conditions necessary for the formation of iron 

 glance seem, indeed, to have been very frequent, whilst in others they have been 

 entirely wanting. It is not only the lavas of Vesuvius and Aci-reale in Sicily, and 

 the rents in the lava of Stromboli, which contain distinct crystals of iron-mica ; but 

 it is also found in the greatest abundance in Jluvergne, (Volvic, Mont cV Or, Puy 



de Dome, etc ) On the other hand, it has never been found by Nog- 



gerath in the volcanic masses of the Sicbengehirgc, the Laacher See, and the Eifel ; 

 it has only lately been found that some of the slags of the Roderherg, an extiact 

 volcano, about two leagues distant from Bonn, are scantily covered with iron- 

 gl-ince. See Thomas der vulkanische Roderherg, &c. Bonn. 1835. p. 22. It is 

 worthy of notice, and speaks in favor of the probability of the above-mentioned 

 production of iron-glance, that in the places last mentioned, the appearance of 

 combinations of chlorine is very limited. 



t The observations of Von Humboldt, Gay-Lussac, Von Buch, and Monticelli, 

 made at different times, shew also that the exhalations of muriatic acid are very 

 variable. They are sometimes so frequent as to surpass the exhalatiozis of sul- 

 phurous acid, sometimes only a few traces of it are found. 



