282 M. Gay Lussac's Reflections on Volcanoes. 
of volcanic phenomena. But as my object is not to fabricate a 
system, but, on the contrary, to examine the probability of the 
two hypotheses which I have mentioned, and to point out the 
observations which may be made upon volcanoes, I shall now 
state the results of the latter hypothesis. 
One, the most important perhaps, of these consequences, is 
the disengagement, by the crater of volcanoes, of an enormous 
quantity of hydrogen, either free, or combined with some other 
principle, if it be really water which produces, by its oxygen, 
the volcanic fires. However, it does not appear that the dis- 
engagement of hydrogen is of very frequent occurrence in vol- 
canoes. Although, during my stay at Naples in 1805, with my 
friends MM. Alexander de Humboldt and Leopold de Buch, 
I witnessed at Vesuvius frequent explosions, which threw the 
melted lava to a height of more than 200 metres, I never ob- 
served any inflammation of hydrogen. Each explosion was fol- 
lowed by vortices of a dense and black smoke, which would not 
have failed to have been inflamed, had they been formed of hy- 
drogen, being traversed by red matters hotter than would have 
been necessary for their ignition. This smoke, evidently caused 
by the explosions, contained, therefore, other fluids than hydro- 
gen ; but what was its true nature ? 
In admitting that it is water which furnishes oxygen to vol- 
canoes, it follows, since its hydrogen is not disengaged in a 
free state, at least most commonly, that it enters into a state of 
combination. It cannot be in any compound which inflames 
by means of heat on exposure to air ; but it might very well be 
the case that it forms with chlorine the hydrochloric acid. 
We have in fact at the present day many observations on the 
presence of this acid in the vapours of Vesuvius ; and according 
to that excellent observer M. Breislak, it is at least equally abun- 
dant with sulphurous acid. M. Menard de la Groye, whose rash 
ideas regarding volcanoes I am in other respects far from being 
disposed to adopt, and M. Monticelli, to whom we are indebted 
for accurate observations on Vesuvius, also consider the presence 
of hydrochloric acid in the vapours of Vesuvius as incontestible. 
For my own part, I retain no doubt on the subject, although, 
during my stay at Vesuvius, I only distinguished by smell sul- 
phurous acid ; but it might be very possible that the foreign sub- 
