
200 . DYNAMICAL GEOLOGY. [Book TIT. 
wind blows the dense cloud aside, can a momentary glimpse be had 
of a part of the bottom of the crater; while at the same time the 
rush and roar of the escaping steam remind one of the din of some 
vast factory. Aqueous vapour rises likewise from rents on the 
outside of the volcanic cone. It issues so copiously from some 
flowing lavas that the stream of rock may be almost concealed 
from view by the cloud; and it continues to escape from fissures of 
the lava, far below the point of exit, for a long time after the rock 
has solidified and come to rest. So saturated, as it were, are many 
molten lavas with the vapour of water that Mr. Scrope even 
maintained that their mobility was due to this cause. : 
































































































































































































































































































































































































































































Fic. 33.—Virew or VESsUVIUS AS SEEN FROM NAPLES DURING THE ERUPTION OF 
1872, SHOWING THE DENSE CLOUDS OF CONDENSED AQuEOUS VAPOUR. 
Probably in no case is the steam mere pure vapour of water, 
though when it condenses into copious rain it is fresh and not salt 
water. It is associated with other vapours and gases disengaged 
from the potent chemical laboratory underneath. There seems to 
be always a definite order in the appearance of these vapours, though 
it may vary for different volcanoes. ‘lhe hottest and most active 
fumaroles contain probably all the gases and vapours of a volcano, 
but, as the heat diminishes, the series of gaseous emanations is 
reduced. ‘Thus in the Vesuvian eruption of 1855-56, the lava, as 
it cooled and hardened, gave out successively vapours of hydrochloric 
acid, chlorides, and sulphurous acid; then steam; and, finally, 
carbon dioxide and combustible gases.2 More recent observations tend 
1 Considerations on Volcanoes (1825), p. 110. 
* ©. Sainte-Clane Deville and Leblanc, Ann. Chim. et Phys. 1858, lii. p. 19, et seq. 
