Some of the substances issuing from volcanic vents, such as hydrogen and sulphuretted hydrogen, are inflammable, and when they issue at a high temperature, these gases burst into flame the moment that they come into contact with the air. Hence, when volcanic fissures axe watched at night, faint lambent flames are frequently seen playing over them, and sometimes these flames are brilliantly coloured, through the presence of small quantities of certain metallic oxides. Such volcanic flames, however, are scarcely ever strongly luminous and, as we have already seen, the red, glowing light which is observed over volcanic mountains in eruption is due to quite another cause. The study by the aid of the spectroscope of the flames which issue from volcanic vents promises to throw much new light on the rarer materials ejected by volcanoes. Spectroscopic observations of this kind have already been commenced by Janssen, at Stromboli and Santorin. Some of the volatile substances issuing from volcanic vents, are at once deposited when they come in contact with the cool atmosphere, others form new compounds with one another and the constituents of the atmosphere, while others again attack the materials of the surrounding rocks and form fresh chemical compounds with some of their ingredients. Thus, there are continually accumulating on the sides and lips of volcanic fissures deposits of sulphates, chlorides, and borates of the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the oxides and sulphides of certain metals. The lips of the fissures from which steam and acid gases issue in volcanoes are constantly seen to be coated with yellow and reddish-brown incrustations, consisting of mixtures, in varying proportions, of these different materials, and these sometimes assume the form of stalactites and pendent masses. [Sidenote: DEPOSITS AROUND VOLCANIC VENTS.] Some of these products of volcanic action are of considerable commercial value. At Vulcano regular chemical works have been established in the crater of the volcano, by an enterprising Scotch firm, a great number of workmen being engaged in collecting the materials which are deposited around the fissures, and are renewed by the volcanic action almost as soon as they are removed. In fig. 6, I have given a sketch of this singular spot, taken from the high ground of the neighbouring Island of Lipari. From the village at the foot of the volcano, where the workmen live, a zig-zag road has been constructed leading up the side, and down into the crater of the volcano. On this road, workmen and mules, laden with the various volcanic materials, may be seen constantly passing up and down. [Illustration: Fig. 6.--View of Vulcano, with Vulcanello in the foreground taken from the south end of the Island of Lipari.]