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 silictum and alu- 
minium must be able to produce a much more extraordinary d 
gree of heat. et 
It cannot be denied that there is some justness in these conelu- 
sions. But it must be remembered, on the other hand, that the 
premises are only taken from appearances at Vesuvius,t 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. y. xv, p. 630. Mitscherlich, who gives an 
account of this, finds an analogy between this formation and similar ones in vol- 
ne He explains it by supposing that common salt and steam both act together 
"pon 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 
ofiron remains behind in large crystals. 
the conditions necessary for the formation of iron 
tis also found in the greatest abundance in Auvergne, (Volvic, Mont d'Or, Puy 
Dome, ge On the other hand, it has never been found by Nég- 
Strath in the volcanic masses of the Siebengebirge, the Laacher See, and the Eifel ; 
Rita only lately been found that some of the slags of the Roderberg, an extinct 
s ua” about two. leagues distant from Bonn, are scantily coneeet — sad 
—. = Thome der yulkanische Roderberg &e. Bonn. 1835. p. - Itis 
“Y 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 
sears of chlorine is very limited. kek 
nad ¢ observations of Von Humboldt, Gay-Lussac, Von Buch, and Monticelli, 
: ve at different times, shew also that the exhalations of muriatic acid are very 
~ ph . They are sometimes so frequent as to surpass the exhalations of sul- 
Prous acid, sometimes only a few traces of it are found. 
