1284? M. Gay Lussac's Reflections on Volcanoes . 
tact with water, either in the interior of the volcano, or at the 
surface of the earth, by means of air, it must necessarily be pro- 
duced from the hydrochloric acid. MM. Monticelli and 
Covelli have in fact observed the production of acid vapours in 
crevices nearly incandescent ; but they took them for sulphuric 
acid, although I am convinced that they were essentially formed 
of hydrochloric acid. There is the more reason for calling their 
observations in doubt, that they have often been very uncertain 
regarding the nature of acid vapours, whether they were sul- 
phureous or muriatic. 
It is known that lavas, especially those which are spongy, 
contain mucin specular iron-ore. In 1805, in a gallery formed in 
Vesuvius by the lava of the preceding year, which, after having 
cooled at its surface, had gradually contracted below, I saw, in 
company with MM. de Humboldt and de Buch, so great a 
quantity of it, that it formed as it were a vein. It covered, in 
beautiful micaceous crystals, all the walls of this gallery, the 
temperature of which was still too great to allow one to remain 
long in it. Now, the peroxide of iron being very fixed at tem- 
peratures much higher than that of lava, it is by no means pro- 
bable that it had been volatilized in that state ; it is very pro- 
bable that originally it had been in the state of a chloride. 
If, in fact, we take protochloride of iron, which has been melt- 
ed, expose it to a dull red heat in a glass tube, and then make a 
current of aqueous vapour pass over its surface, we shall obtain a 
great quantity of hydrochloric acid and hydrogen gas, and there 
will remain in the tube a black deutoxide of iron. On employing 
dry oxygen in place of aqueous vapour, we obtain chlorine and 
peroxide of iron. The experiment is easily made by mixing 
the chloride of iron with dry chlorate of potash ; on the slightest 
heat being applied, the chloride is disengaged abundantly. If 
we make humid air pass over the chloride, always at a tempera- 
ture approaching to red, we obtain chlorine, hydrochloric acid, 
and peroxide of iron. 
The perchloride of iron exhibits similar phenomena. If it 
meet with humidity, we presently obtain hydrochloric acid, or 
rather chlorine, if it meet with oxygen ; and it is formed from 
the peroxide of iron. I conceive, therefore, that the iron is in 
the state of a chloride in the fumes exhaled by volcanoes, or by 
