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.'