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. [Sidenote: GEYSERS AND HOT-SPRINGS.] When the water issues at a temperature little removed from the boiling point, it is apt to give rise to intermittent springs or geysers, the eruptions of which exhibit a remarkable analogy with those of ordinary volcanoes. Geysers may indeed be described as volcanoes in which heated water, instead of molten rock, is forced out from the vent by the escaping steam. They occur in great abundance in districts in which the subterranean action is becoming dormant or extinct, such as Iceland, the North Island of New Zealand, and the district of the National Park in the Rocky Mountains. Many attempts have been made to explain the exact mechanism by which the intermittent action of geysers is produced, but it is not at all probable that any one such explanation will cover all the varied phenomena exhibited by them. Like volcanic outbursts, geyser eruptions doubtless originate in the escape of bubbles of steam through a liquid mass, and this liberation of steam follows any relief of pressure. In districts where vast masses of lava are slowly cooling down from a state of incandescence, and surface waters are finding their way downwards while subterranean waters are finding their way upwards, there can be no lack of the necessary conditions for such outbursts. Sometimes the eruptions of geysers take place at short and regular intervals, at other times they occur at wide and irregular intervals of time. In some cases the outbursts take place spontaneously, and at others the action can be hastened by choking up the vent with stones or earth. Other hot springs, like the Strudel of Carlsbad, rise above the surface in a constant jet, while most of them issue quietly and flow like ordinary springs.