454 THE POPULAR SCIENCE MONTHLY. 



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 por- 

 tion 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 gun- 

 powder 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 sub- 

 terranean fire ; they truly seem to be the thousand gates of the burn- 

 ing 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 Hum- 

 boldt 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 Avere 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 excep- 

 tions, near the borders of the sea or of lacustrine basins. Around the 



