246 
THE MEDITERRANEAN NATURALIST 
result in, the different varieties of eruption. Let 
us suppose that an extensive igneous dyke has, 
from some circumstances which will be discussed 
later on, become plugged at the exit on the 
earth’s surface. The part of the magma that 
retains a sufficiently high temperature will be 
gradually absorbing water; and as assimilation 
proceeds the extension of the magma will propor- 
tionally increase, and the temperature of the mix- 
ture reduced, so that in some cases this loss will 
gradually favour solidification of portion of the 
magma forming the crystals of the felspars, Am- 
phibole, Biotite, and other micas, &c. Such a 
state of things will go on until one of two things 
takes place — either the loss of heat be such that 
the whole igneous mass solidifies, or, on the other 
hand, the tension overcomes the resistance, and an 
eruption take place. 
Other things being equal, we should expect the 
violence of the outburst to be directly proportional 
to the length of contact of the igneous magma 
with water-bearing strata, or in ’other words, the 
longer the quiescence the greater the violence in 
the subsequent eruption. 
'Tile diffusion or solution of water through the 
igneous magma will diminish in a progressive 
manner as saturation increases. The amount that 
may be taken up is demonstrated by the enor- 
mous volumes that escape during an eruption. 
Were it possible to calculate the amount of vapour 
expelled during any great eruption, and to estimate 
the solid ejectamenta — also a difficult matter — 
these added together would give us the composi- 
tion of the paste immediately before its expulsion, 
unless we have the level of the magma surface 
below that of the drainage-line of the country, in 
which case the vapour would be increased by the 
inpour from the porous walls of the chimney, 
and a pulverization of such water similar to the 
action of a spray apparatus, when the amount of 
vapour expelled might be enormously augmented. 
Such calculations have rarely been attempted. 
Cavalleri (1) estimated that from Vesuvius, in 
1856, during a period of eighteen months, pending 
which the strornbolian state of activity had per- 
(1) Consider azioni sul vapore e conseguenie ca- 
lore , <&c. Memoir read, in the Accad. Fisio-medico- 
statistica di Milano , December 27th, 1856. 
\ sisted, that no less than 407 millions of cubic 
i meters of water had been ejected in the form of 
vapour. We may form an idea of the mass by 
imagining a lake 6^ kilometers square, and 10 
meters deep. I cannot form a just conception of 
the amount of vapour issuing from Vesuvius on 
the above occasion, but from a long and intimate 
acquaintance with this volcano during the last six 
years, it seems to me that the above calculation 
is greatly in excess of the truth, such a result being 
quite comprehensible when we take into conside- 
ration the almost insurmountable difficulties of 
finding suitable data to go upon. If we form a 
conception of 516,500 kilogrammes of water escap- 
ing every minute in the form of vapour from an 
aperture of four or five meters in diameter at the 
most, it certainly seems the feeble state of strom- 
bolian action would be out of the question. 
It was also calculated (i) that 22.000 c. metres 
of water were daily dispersed in the form of 
vapour by the lateral openings of Etna in the 
eruption of 1S65, that is, equal to 2,000,000 c. 
metres, for 109 days that that eruption lasted. 
This estimation, which, I believe, is that of 
Fouque, certainly appears more reasonable that 
the former. 
One is apt, however, to attach a greater value as 
regards quantity to volcanic vapour, from the 
peculiar molecular state which it assumes imme- 
diately on its escape, which is probably due to 
conversion of the steam into vapour, by the process 
investigated by Dr. Aiken. Everyone is aware 
that our breath in the hottest weather is converted 
iuto a white cloud when near the vapour of HC1. 
Nevertheless, whatever value we may put upon 
the above calculation, we cannot do otherwise than 
comprehend the very large amount of water that 
may disengage itself from the igneous magma. 
In the case of a fissure whose upper limits are 
very far beneath the surface, and suddenly extends 
thereto, we should expect the eruption to be less 
violent than were the magma in loser proximity; 
since a large part of the energy of tension would 
be lost in the expansion in injecting the extension 
of the fissure. 
(1) Quoted by M. Ch. Velain. Les vo/cans re 
</u’ils sont et ce quits nous apprenent. Paris , 188 4, 
p. Jf5, 
