1824.] 
which witnessed the great catastrophes 
of our globe,—epochs at which the 
temperature of the earth must have 
been higher than it now is, the melted 
substances which it contained con- 
sequently more liquid, the resistance 
of its surface less, and the pressure 
exercised by elastic fluids greater,— 
all that could be produced was pro- 
duced; an equilibrium must have 
established itself, the agitated mass 
must have subsided into a state of 
repose which could no longer be 
troubled by intestine causes, and 
which can only now be disturbed by 
fresh contact between bodies acci- 
dentally brought together, and which 
were, perhaps, only added to the mass 
of the globe subsequently to the soli- 
dification of its surface. 
Now the possibility of contact be- 
tween bodies in the interior of the 
earth, the ascent of lava to a consi- 
derable height above its surface, ejec- 
tions by explosion, and earthquakes, 
necessarily imply that those extra- 
neous substances which penetrate 
into volcanic furnaces must be elastic 
fluids, or rather liquids capable of pro- 
ducing elastic fluids, either by means 
of heat which converts them into ya- 
pour, or by aflinity which sets at 
liberty some gaseous elements, Ac- 
cording to analogy, the only two sub- 
stances capable of penetrating into 
the volcanic furnaces in volumes sufli- 
ciently large to feed them, are air, and 
water, or the two together. Many 
geologists have assigned to the air an 
important office in volcanos; its oxy- 
gen, according to them, sustains their 
combustion: but a very simple obser- 
vation will suffice to overthrow this 
opinion entirely. 
How, indeed, is it possible for the 
air to penetrate into the yolcanic fur- 
naces when there exists a pressure 
acting from within towards the exte- 
rior, capable of raising liquid lava, a 
body three times as heavy as water, 
to the height of more than one thou- 
sand métres, as at Vesuvius, or even 
of more than three thousand, as is the 
case in a great number of volcanos? 
A pressure of one thousand metres of 
lava, equivalent to a pressure of three 
thousand métres of water, or to that 
of about three hundred atmospheres, 
necessarily excludes the introduction 
of any air whatever into volcanos; and 
as this pressure subsists for a long 
series of years, during which the vol- 
canic phenomena continue in the ut- 
Reflections on Volcanos, by M. Gay- Lussac. 
495 
most activity, it follows that tlie air 
can have no share whatever in their 
production. 
It is moreover evident, that, if the 
air had a free communication with the 
volcanic furnaces, the ascent of lava, 
and earthquakes, would be impossible, 
If the air cannot be the cause of 
volcanic phenomena, it is probable, 
on the contrary, that water is a very 
important agent in them. 
It can hardly be doubted that water 
does penetrate into volcanic furnaces. 
A great eruption is invariably followed 
by the escape of an enormous quantity 
of aqueous vapour, which, being con- 
densed by the cold which prevails 
above the summits of volcanos, falls 
again in abundant rains accompanied 
by terrific thunder, as was the case at 
the famous eruption of Vesuvius in 
1794, which destroyed Torre del 
Greco. Aqueous vapours and hy- 
drochloric gas have also frequently 
been observed in the daily ejections of 
volcanos. It is scarcely possible to 
conceive the formation of these in the 
interior of volcanos without the agency 
of water. 
If we admit that water is one of the 
principal agents in volcanos, we must 
proceed to examine the real means by 
which it acts, upon cither ef the hy- 
potheses we have just laid down con- 
cerning the heat of volcanic furnaces, 
If,sve suppose, according to the first 
hypothesis, that the earth continues in 
a state of incandescence, at a certain 
depth below its surface, it is impos- 
sible to conceive the existence of water 
at that depth; for the temperature of 
the earth having formerly been of ne- 
cessity higher, its fluidity greater, and 
the thickness of its solid crust less, 
than at the present time, the water 
must necessarily have disengaged it- 
self from its interior, and have risen to 
the surface. 
If we wish therefore to give any air 
of probability to this hypothesis, and 
to maintain the importance of water 
as a principal agent in volcanos, we 
must assume that it penetrated from 
the surface downwards to the incan- 
descent strata of the earth; but in 
order to come to this conclusion, we 
must suppose that ithad a free com- 
munication with those strata, that it 
gradually acquired heat before it 
reached them, and that the vapour it 
produced, compressed by the weight 
of its whole liquid column, obtained 
a suflicient clastic force to elevate the 
layas, 
