Page:Popular Science Monthly Volume 24.djvu/534

518 I have shown by experiment that these supplies may be transmitted through the pores of some kinds of rocks. Simple capillary action, in conjunction with gravity, may force water to penetrate against very strong counter-pressure, from the superficial and cooler regions of the globe, to deep and hot regions, where, by reason of the temperature and pressure it acquires there, it becomes capable of producing very great mechanical and chemical effects. If we suppose that water penetrates, either directly or after a halt in a reservoir where it has remained liquid, to masses in fusion, so as to acquire there an enormous tension and an explosive force, we shall have the cause of the anterior real explosions and of the instantaneous shocks due to gases at high pressure. If the cavities, instead of forming a single reservoir, are divided into several parts or distinct compartments, there is no reason why the tension of the vapor should be the same in the different receivers, provided they are separated by walls of rock. The pressure may even be very different in two or more of them. This admitted, if a separating wall is broken by excess of pressure or melted by the heat, vapor at high pressure will be set in motion, and in the presence of the solid masses upon which it will strike it will behave just as if there had been an instantaneous formation of vapor, as we supposed in the former case.

It is very hard to establish, as has been attempted, a clear line of demarkation between the character of the earthquakes of volcanic regions proper and of regions without volcanoes, such as Portugal, Asia Minor (Chios, April 3, 1881, five thousand victims), Syria, Algeria, and the rim of the Mediterranean generally. In both classes, the characteristic manifestations which we perceive are the same. If, as some assume, the internal movements of the rocks were a cause of real earthquakes, it could only be because those internal movements mechanically developed heat, and in that way provoked the formation of vapor. But, in the recently disturbed regions we have especially in view, which are the seat of so frequent shocks, another cause is much more probable. There doubtless remain in them interstices and interior cavities that permit the access of water to the hot regions. The depth of the centers of disturbance of earthquakes has been estimated, in different cases, by calculations only grossly approximate, at eleven kilometres, twenty-seven kilometres, and thirty-eight kilometres. In any case, such depth, though very slight in comparison with the length of the radius of the earth, is great enough for the temperature at the normal rate of increase to be very high; and the same will also be the case with the water that may be present there. Now, as we have already seen, a temperature of 500° C. (900° Fahr.) is sufficient to cause water to explode with violence.

It is certainly in the largest number of cases very difficult to admit collisions of solid bodies in the interior as the moving causes of earthquakes. How, for instance, can we conceive that so violent and