32 Prof. Bischof o?i the Natural Histori/ of 



celli and Covelli,* which shew that this acid is among the ex- 

 halations of volcanos. He himself, however, observes, that an 

 enormous quantity of muriatic acid must be evolved from the 

 craters, if the hydrogen, which would result from an oxida- 

 tion by means of water, were to enter into combination w^ith 

 chlorine. But it would be strange that such an exhalation 

 should not have been remarked sooner. In order to account 

 for the formation of muriatic acid, he mentions the experi- 

 ments made by him and Thenard, in which they evolved that 

 acid, by introducing aqueous vapour into a mixture of sand and 

 common salt heated to a red heat. In support of his position, 

 he mentions the occurrence of common salt in the lavas, from 

 one of which (that of Vesuvius in 1822), Monticelli and Covelli 

 extracted more than 0.09, and in the slags which cover the 

 white hot lava, and which sometimes contain very beautiful 

 crystals of salt. He farther 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 

 subsequent decomposition, by coming in contact with aqueous 

 vapour and atmospheric air, while at a red heat.-h And, lastly, 



* L. c. p. 172. See also Daiibeny's Description of Active and Extinct 

 Volcanos. Lond. 1826, p. 372, and v. Humboldt's Reise, etc. t. i. p. 195. 



+ We may here notice the formation of artificial crystals of oxide of iron 

 in a potter's furnace. PoggendoriF's Ann. v. xv. p. 630. Mitscherlich, who 

 gives an account of this, finds an analogy between this formation and similar 

 ones in volcanos. 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 Avater 

 into contact with oxide of iron, or ferriferous combinations. Thus chloride 

 of iron is formed, which is again decomposed by the aqueous vapours, 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 vei-y frequent, whilst in others they 

 have been entirely wanting. It is not only the lavas of Vesiimus^ Aci-rcale 

 in Sicily f and the rents in the lava of Stromholij which contain distinct cry- 

 stals of iron-mica ; but it is also found in the greatest abundance in Auvergney 

 {Vohic J Mont d* Or, Puy de Dome, etc. . . . ). On the other hand, it has 

 never been found by Noggerath in the volcanic masses of the Siebengeblrgc, 

 the Laacher See, and the El/el ; it has only lately been found that some of 

 the slags of the Boderhcrg, an extinct volcano, about two leagues distant from 

 Bom J are scantily covered with iron-glance. See Thomso der vulkanische 



