36 
nEPoar— ia-17- 
must not therefore conclude, that because volcanic forces produce violent 
explosions when acting under the actual conditions which existing volMnw 
present, they would produce similar effects under different condition^ 5>ucli 
would certainly not be the case if the inertia of the masses on which the forces 
acted should be sufficiently great*. , 
The permanent state of ehullitinn and gentle intumeHCcnce observed lo 
Stroinboli, Kirauea, and a few other volcanos, is nmnifcsily the consequenM 
of the constant generation of elastic vapours, and their regular ascent to tb« 
surface through which they escape into the atmosphere; but it is difficult to 
obtain any evidence rrspectiiig the precise cause of that discontinuous Md 
paroxysmal action which constitutes the usual characteristic of vulcanic HCtion. 
Perhaps a careful study of the great intermittent hot spring* of Iceland, the 
Geysers, is as likely as any means we possess, to elucidate the subject. 
One explanation which has be**n given of the intcrinittcnt action of these 
springs is founded on the hypothesis of tlio existence of resorvoii-s of steam 
ill the internal cavities, witji which the external vent, in the form of a looi,’ 
cylindrical tube, communicator. These reservoirs may he conceived to he 
formed, in sonic cases, in those parts of internal eavitii-s where the iw 
rises higher than the outlet. Steam may he formed in them after each 
explosion, of aufficient elastic power (if the heat be great enough) In pti"* 
down the surface of llic vvatcr at first filling the cavity, fill it descend to the 
level of the outlet, througli which a portion of tha steam may then wake 
its escape. The steam thus escaping will rise rapidly through the water, 
to escape l)y the external vent. Under these circumstances, if the quantity 
of steam which escapes at the same iiistunt, and its elastic power, be suffi¬ 
ciently great, and the water rise into the narrow cylindrical tube, the water 
occupying the tube w ill be expelled by the steam hi exactly the same 
as the contents of a gun-barrel arc expelled by tlu* clastic force of the vapour 
insUntancuusly generateil by the ignilioii of the gunpowder employed. 
The conditions assumed respecting the form of tlie internal cavities appear 
lilgldy probable, .and the accumulaliun of steam in them would probably b® 
a necessary coiisequcncc t»f their existence. The part of the explanation 
wdiich appears to me least satisfactory, is that wliich tu^suines the suddm 
of the accumulated vapour in sufficient tjuantity to produce the observed 
effects, ami so far to exhaust these reservoirs ns to render necessary insider- 
able periods of repose bedbre the renewal of a similar action, the above 
explanation w equally applicable to the intermittent action of volcanosiaw 
perhaps the difficulty just mentioned may not be so great as in springs hk® 
the Geysers, because the Umsion of the clastic vapours in the fonmT nio?t 
be enormously greater than in Uie latter ease, and may possibly in the d«p 
regions of a volcano produce disruptions in the containing rocks hr whid* 
liberated in sufficient quantity to produw » 
violent eruption. I ossibly however the explosive action of volcanos may ^ 
unknown to us in the process of gyn®* 
-7 ^77? ' takes place under high pressuref. 
7. J/ieonis oj Voleanog.— Futuiamental ilmiothcses. _It is the ohject o 
phys.cal scoMe i„ goacral ,o explain the pil^a rticl. the -"‘-I 
