pressure of 10 atmospheres, water boils at 180°; under 25 atmospheres at 225°; beyond these limits the law that governs the phenomenon of ebullition is not accurately known, but it is known that the pressure of steam increases in a much more rapid ratio than the temperature, and it may be stated that it approximates to 1,200 atmospheres at about 600°; 5,000 atmospheres at about 1,000°, etc. From this it is clear that there is a point where the pressure of steam will equal the weight, where consequently ebullition will occur. Admitting that the temperature of the soil increases at the rate of 1° to 20 metres, we would reach at twelve kilometres a temperature of 600°; and at this depth the pressure of steam would equal the resistance. It would be necessary to go deeper if we adopt a less rapid rate of increase in temperature. Now, if water commences to boil beneath a certain level, the steam will rise through the mass and condense anew, as in a refrigerant imparting thereto a portion of its heat. The upper portion of the liquid will thus become heated more than the soil at the same level; in fact, the water may boil even at the surface, as in the Iceland geysers.
If, in volcanic regions, the temperature is 1,000° at about twenty kilometres depth, the steam forming at that point may possess a pressure greater than 5,000 atmospheres—which would be sufficient to sustain the weight of a column of lava twenty kilometres high. At a temperature of 1,300° the pressure would doubtless be equal to 10,000 atmospheres. This is very nearly the force of the gas of gunpowder in a cannon of heavy caliber, and it is evident that this force would more than suffice for the mechanical effects of which volcanoes offer the terrifying spectacle.
However we view it, volcanoes are an irrefragable proof of a subterranean fire; they truly seem to be the thousand gates of the burning hell we read of. The number of volcanoes discovered constantly increases with the progress of geographical knowledge; in the least explored countries highly volcanic regions are found. A. von Humboldt enumerated 407, of which 227 were active. We now know of several thousand, and, according to M. Fuchs, the number of active volcanoes at the present time may be set down as 323. It is difficult to draw the line between active and extinct volcanoes, because the greater portion have periods of repose, possibly a century or more in length. We know that the ancients considered Vesuvius a perfectly harmless mountain up to the time of the great eruption of a. d. 79, when Herculaneum and Pompeii were buried, and that it remained quiet for three centuries (1306-1631).
On looking at a map whereon the volcanoes are marked as red points, the attention is at once struck by the fact that almost all are found in proximity to the large bodies of water. The greater number are found to be on islands; and the remainder, with a very few exceptions, near the borders of the sea or of lacustrine basins. Around the
