THE CAUSES OF EARTHQUAKES. 517 



place at a temperature of 1,000 C. (1,800 Fahr.), approximately that of 

 lava, and under a volume equivalent to that of the water in the liquid 

 state whence the vapor is derived. Under these conditions, we must 

 suppose the vaporization to be total, for the critical temperature, 

 above which the liquefaction of vapor can not be realized, is, accord- 

 ing to M. Clausius, 332 C. (629 F.). The pressure, of which it is 

 also possible to make an approximate estimate, then becomes compar- 

 able to that of the most powerfully explosive gases, and is, conse- 

 quently, capable of producing very considerable dynamic effects. 

 These effects may also be produced at a much lower temperature than 

 that of lavas at 500 C. (900 F.) ; for example, if we suppose that 

 the volume imposed upon the vapor is so limited as to correspond to a 

 density of 0*8 or 0*9. No doubt such conditions are realized in the 

 lower regions of the globe, where water is confined within limited 

 spaces, and as hot as the melted rocks which we see gushing out from 

 the surface at a temperature of 1,000 C. (1,800 F.) or more. We 

 shall see, however, that such depths and such a temperature are not 

 necessary. 



The vapor of water when superheated acquires a power of which 

 the most terrible boiler-explosions could give no idea if we had not 

 the result before our eyes. The tubes of the best quality of iron that 

 I used in observing the action of superheated water in the formation 

 of silicates had an inside diameter of twenty-one millimetres and 

 were eleven millimetres thick. They sometimes exploded, and were 

 projected into the air with a noise like that of the firing of a cannon. 

 Before bursting, the tubes swelled out into bulbous forms, and rents 

 were opened in the middle of the bulbs. If the iron had no flaws and 

 according to the estimate that it would preserve up to 450 C. (810 F.), 

 the temperature to which it was raised, the same tenacity it had when 

 cold, such rents must have indicated a pressure of several thousand 

 atmospheres. A few cubic centimetres of water were sufficient to pro- 

 duce an effect like that ; and, considering the small dimensions of the 

 inside of the tubes as compared with the volume of the water, the 

 vapor must have reached a density of about 0*9. If we apply the data 

 we possess to the depths of the globe, it is not difficult to conceive 

 very simple dispositions in which the vapor of water, under the condi- 

 tions we have just determined, will suddenly provoke shocks or series 

 of shocks that will too often make themselves felt on the surface. 

 Whatever conception we may form of the volcanic reservoirs, we must 

 admit it to be very probable that solutions of continuity exist between 

 the soft or fluid masses in fusion and the solid masses superposed 

 over them. Moreover, cavities may also exist in the solid rocks them- 

 selves that lie over the soft masses. On the other hand, the incessant 

 losses, which these internal reservoirs suffer in consequence of the enor- 

 mous volumes of water in the condition of vapor which they disen- 

 gage every day, are probably repaired by supplies from the surface. 



