The most abundant of the substances which are ejected from volcanoes is steam or water-gas, which, as we have seen, issues in prodigious quantities during every eruption. But with the steam a great number of other volatile materials frequently make their appearance. The chief among these are the add gases known as hydrochloric acid, sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid; and with these acid gases there issue hydrogen, nitrogen, ammonia, the volatile metals arsenic, antimony, and mercury, and some other substances. In considering the nature of the products which issue from volcanic fissures, it must be remembered that many substances which under ordinary circumstances do not exhibit marked volatility are nevertheless easily carried away in fine particles when a current of steam is passed over them. As we shall have to point out in the sequel, different volatile substances have a tendency to make their appearance at volcanic vents according as the intensity of the action going on within it varies. The volatile substances issuing from volcanic fissures at high temperatures react upon one another, and many new compounds are thus formed. We have already seen how, by the action of sulphurous acid and sulphuretted hydrogen on each other, the sulphur so common in volcanic districts has been separated and deposited. The hydrochloric acid acts very energetically on the rocks around the vents, uniting with the iron in them to form the yellow ferric-chloride. The rocks all round a volcanic vent are not unfrequently found coated with this yellow substance, which is almost always mistaken by casual observers for sulphur. In many volcanoes the constant passage through the rocks of the various acid gases has caused nearly the whole of the iron, lime, and alkaline materials of the rocks to be converted into soluble compounds known as sulphates, chlorides, carbonates, and borates; and, on the removal of these by the rain, there remains a white, powdery substance, resembling chalk in outward appearance, but composed of almost pure silica. There are certain cases in which travellers have visited volcanic islands where chemical action of this kind has gone on to such an extent, that they have been led to describe the islands as composed entirely of chalk. [Sidenote: GASES EMITTED FROM VOLCANOES.]