We have said, at the outset, that there exists no analogy whatever between the action which takes place in volcanoes and the operation of burning or combustion. Occasionally, it is true, certain inflammable substances are formed by the action going on within the volcano, and these inflammable substances, taking fire, produce real flames. Such flames are, however, in almost all cases only feebly luminous, and do not give rise to any conspicuous appearances. What is usually taken for flame during volcanic eruptions is simply, as we have already pointed out, the glowing red-hot surface of a mass of molten rock, reflected from a vapour-cloud hanging over it. The red glow observed over a volcano in eruption is indeed precisely similar in its nature and origin to that which is seen above London during a night of heavy fog, and which is produced by the reflection of the gas-lights of the city from the innumerable particles of water-vapour diffused through the atmosphere. Fires, of course, occur when the molten and incandescent materials poured out from a volcano come in contact with inflammable substances, such as forests and houses, but in these cases the combustion is quite a secondary phenomenon. There is another popular delusion concerning volcanic action, which it may be necessary to refer to and to combat. From the well-known fact that sulphur or brimstone is found abundantly in volcanic regions, the popular belief has arisen that this highly inflammable substance has something to do with the production of the eruptions of volcanoes. In school-books which were, until comparatively recent years, in constant use in this country, the statement may be found that by burying certain quantities of sulphur, iron-pyrites, and charcoal in a hole in the ground, we may form a miniature volcano, and produce all the essential phenomena of a volcanic eruption. No greater mistake could possibly be made. The chemical reactions which take place when sulphur and other substances are made to act upon each other differ entirely from the phenomena of volcanic action. The sulphur which is found in volcanic regions is the result and not the cause of volcanic action. Among the most common substances emitted from volcanic vents along with the steam are the two gases, sulphurous acid and sulphuretted hydrogen. When these two gases come into contact with one another, chemical action takes place, and the elements contained in them--oxygen, hydrogen, and sulphur--are free to group themselves together in an entirely new fashion; the consequence of this is that water and sulphuric acid (oil of vitriol) are formed, and a certain quantity of sulphur is set free. The water escapes into the atmosphere, the sulphuric acid combines with lime, iron, or other substances contained in the surrounding rocks, and the sulphur builds up crystals in any cavities which may happen to exist in these rocks. [Sidenote: VOLCANIC ACTION RESEMBLES BOILING.]