M. Fouqué, by a careful series of analyses of the gases which he collected at different gaseous vents, or fumaroles as they are called, in the crater of Vulcano, has been able to define the general relations which appear to exist between the temperature at a volcanic orifice and the volatile substances which issue from it. He found that in fumaroles, in which the temperature exceeded 360° centigrade, and in which in consequence strips of zinc were fused by the stream of issuing gas, the analysis of the products showed sulphurous acid and hydrochloric add to be present in large quantities, and sulphuretted hydrogen and carbonic acid in much smaller proportions. Around these excessively heated fumaroles, the lips of which often appear at night to be red-hot, considerable deposits of sulphide of arsenic, chloride of iron, chloride of ammonium, boracic acid, and sulphur were taking place.
It was found, however, that as the temperature of the vent declined, the emission of the sulphurous acid and hydrochloric acid diminished, and the quantity of sulphuretted hydrogen and carbonic acid mingled with them was proportionately increased.
In the same way it appears to be a universal rule that when a volcanic vent sinks into a condition of temporary quiescence or complete extinction the powerfully acid gases, hydrochloric acid and sulphurous acid, make their appearance in the first instance, and at a later stage these are gradually replaced by sulphuretted hydrogen and carbonic acid.
Of these facts we find a very beautiful illustration in the Campi Phlegræi near Naples. With the exception of Monte Nuovo, the volcano which has most recently been in a state of activity in that district is the Solfatara. From certain apertures in the floor of the crater of the Solfatara there issue continually watery vapours, sulphurous acid, sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium. The action of these substances upon one another, and upon the volcanic rocks through which they pass, gives rise to the formation of certain chemical products which, from a very early period, have been collected on account of their commercial value. The action of these add gases upon the surrounding rocks is very marked; efflorescent deposits of various sulphates and chlorides take place in all the crevices and vesicles of the rock; sulphur and sulphide of arsenic are also formed in considerable quantities; and the trachytic tuffs, deprived of their iron-oxide, alkaline earths and alkalies, which are converted into soluble sulphates and chlorides, are reduced to a white, powdery, siliceous mass. Many volcanoes, which have sunk into a state of quiescence or extinction like the Solfatara of Naples, exhibit the same tendency to give off great quantities of the powerfully-acid gases which act upon the surrounding rocks, and deprive them of their colour and consistency. Such volcanoes are said by geologists to have sunk into the 'solfatara stage.'
SOLFATARA-STAGE OF VOLCANOES.
At the Lake of Agnano and some other points in the Campi Phlegræi, however, we find fissures from which the less-powerfully acid gases, sulphuretted hydrogen and carbonic acid, issue. These gases as they, are poured forth from the vents are found to be little, if at all, above the temperature of the atmosphere. Sulphuretted hydrogen is an inflammable gas, and in the so-called salses and mud-volcanoes, at which it is ejected in considerable quantities, it not unfrequently takes fire and bums with a conspicuous flame. Carbonic acid on account of its great density tends to accumulate in volcanic fissures and craters rather than to mingle with the surrounding atmosphere. At the so-called Grotto del Cane, beside the Lago Agnano, it is the custom to show the presence of this heavy and suffocating gas by thrusting a dog into it, the poor animal being revived, before life is quite extinct, by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of Transylvania, carbonic acid and sulphuretted hydrogen are emitted in considerable quantities, and it is possible to take a bath of the heavy gas, the head being kept carefully above the constant level of the exhalations.
Although the stories of the ancient Avernian lake, across which no bird could fly without suffocation, and of the Guevo Upas, or Poison Valley of Java, which it has been said no living being can cross, may not improbably be exaggerations of the actual facts, yet there is a basis of truth in them in the existence of old volcanic fissures and craters which evolve the poisonous sulphuretted hydrogen and carbonic acid gases.
Besides the gases which we have already named, and which are the most common at and characteristic of volcanic vents, there are some others which are not unfrequently emitted. First among these we must mention boracic acid, which, though not a remarkably volatile substance, is easily carried along in a fine state of division in a current of steam. At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of steam jets accompanied by sulphuretted hydrogen and boracic acid issue from the rocks, and these jets being directed into artificial basins of water, the boracic acid is condensed and is recovered by evaporation. We have already noticed that boracic add is evolved with the gases at Vulcano and other craters; and the part which this substance plays in volcanic districts is shown by the fact that many of the rocks, filling old subterranean volcanic reservoirs, are found to be greatly altered and to have new minerals developed in their midst through the action upon them of boracic acid.
Ammonia and various compounds of carbon, nitrogen, and hydrogen are among the gases evolved from volcanic vents. In some cases these gases may be produced by the destructive distillation of organic materials in the sedimentary rocks through which volcanic outbursts take place. But it is far from impossible that under the conditions of temperature and pressure which exist at the volcanic foci, direct chemical union may take place between substances, which at the surface appear to be perfectly inert in each other's presence.
When the temperature at volcanic fissures is no longer sufficiently high to cause water to issue in the condition of vapour or steam, as is the case at the 'stufas' which we have described, it comes forth in the liquid state. Water so issuing from old volcanic fissures may vary in its temperature, from the boiling point downwards.