$80 M. Gay Lussac's Reflections on Volcanoes ■, 
of penetrating into the volcanic foci in masses sufficient for feed 
ing them, are air or water, or both together. Many geologists 
have given air a great influence in volcanoes : it is, according to. 
them, its oxygen that produces their combustion ; but a very 
simple observation suffices entirely to overthrow this opinion. 
How, in fact, could air penetrate into volcanic foci, when there 
exists, from within outwards, a pressure which is capable of ele- 
vating the liquid lava, having a specific gravity of about 3, 
to a height of more than 1000 metres, as at Vesuvius, and to 
more than 3000 in a great number of volcanoes P A pressure of 
1000 metres of lava, equivalent to a pressure of 3000 metres of 
water, or to about 300 atmospheres, necessarily excludes all in- 
troduction of air into the interior of volcanoes ; and as this pres- 
sure is kept up for many years, during which the volcanic pheno- 
mena nevertheless preserve a great activity, the action of air in 
producing these phenomena must be absolutely null It is be- 
sides evident, that if the air communicated freely with the foci of 
volcanoes, the ascent of lava and the phenomena of earthquakes 
could not take place. 
If air cannot be the cause of volcanic phenomena, it is how- 
ever probable that water is a very important agent in producing 
them. 
That water penetrates into the foci of volcanoes, is a fact that 
can by no means be called in doubt. There is never a great erup- 
tion that is not followed by an enormous quantity of aqueous 
vapours, which, being afterwards condensed by the cold above 
the summit of the volcanoes, fall in copious rains, accompanied 
with frightful thunders, as was observed in the famous eruption 
of Vesuvius in 1794, which destroyed the Torre del Greco, 
Aqueous vapours and hydrochloric gas have also been often ob- 
served in the daily ejections of volcanoes, the formation of which 
in the interior of volcanoes, it is impossible to conceive without 
the aid of water. 
Admitting that water may be one of the principal agents of 
volcanoes, there remains to examine the true influence that it 
may have, in each of the two hypotheses which we have formed, 
upon the heat of their foci. 
Supposing, according to the first hypothesis, that the earth is 
still in a state of incandescence at a certain depth beneath its 
