48 KEPORT OF MEETINGS FOR 1901 



faction is much lower ; and if alkaline raatters, even in small 

 quantity, are present in the water concerned, the temperature 

 which suffices for softening, or even for liquefaction, is lower 

 still. An increase in the percentage of the dissolved alkali 

 still further lowers the temperature required. On the other 

 hand, liquefaction of most substances is regulated very largely 

 hy pressure ; so that a temperature which would amply suffice 

 for that purpose under moderate pressure, would not suffice 

 in these cases if the pressure is increased. The same remark 

 applies also to water. If water is closely confined within a 

 very strong vessel, it is possible to raise its temperature to 

 nearly red-heat without the superheated water passing into 

 the condition of steam. The chief difficulty attending experi- 

 ments of this kind is connected with the fact, of much 

 importance in the present connection, that the solvent power 

 of water increases with a rise of temperature ; so that the 

 heated fluid dissolves away the walls of its prison, and by 

 an explosive effort sooner or later effects its escape. 



Two other points call for notice. The first is the curious 

 fact that the higher the temperature to which a given mass 

 of rock is raised, the larger the quantity of water it can 

 imbibe or occlude. Heated rock greedily takes in water ; 

 and thus, by degrees, not only is its liquefaction facilitated, 

 but the explosive character of the compound which thus 

 arises constantly tends to increase. 



While the pressure of the portion of the Earth's crust 

 overlying the heated zsne remains constant, it is probably 

 the case that this explosive tendency is kept constantly iu 

 check. But in a region undergoing terrestrial disturbances 

 a state of equilibrium is constantly liable to be disturbed. 

 Somewhere or other in the region where the explosive tendency 

 is greatest a local relief of pressure is sure to arise. Wherever 

 the weight is in the smallest degree eased off, some of the 

 pent-up superheated water instantly flashes into steam, and 

 it does so with a degree of violence to which it is not possible 

 to find a parallel on the surface of the globe. This subter- 

 ranean explosion is the prime cause of an earthquake and 

 all itd attendant phenomena. But such an explosion does 

 more than give rise to disturbances at the surface. It acts 

 i^uderground ia relation to tl^e si^rrouadio^ rooks la exactly 



