On the Agency of Water in Volcanic Eruptions. 



151 



the present be more or less conjectural. There can be little doubt 

 that (as before explained) when the volcano is in a state of rest, the 

 beds of volcanic materials surrounding the upper part of the volcanic 

 duct are charged with water to a certain height, and also that, when 

 the volcano stands on sedimentary strata, the duct lower down 

 traverses a certain number of water-logged strata, where the hydro- 

 static pressure is considerable. For the water to rise to the height 

 which it does at the Naples wells requires a pressure of not less 

 than 50 atmospheres ; but the static pressure of the column of lava 

 in the crater of Vesuvius at the depth at which the water-bearing 

 stratum there lies (1520 feet), during a central eruption of the lava, 

 is, of course, far in excess of this. The introduction of the water into 

 the lava must therefore depend on other conditions than hydrostatic 

 pressure, such, for instance, as capillarity, or the elastic force of 

 vapour. Although this increases so rapidly, yet as the law of increase 

 at temperatures exceeding 436° F. has not been experimentally deter- 

 mined, and we have to deal with temperatures approaching to or 

 equalling the melting point of lava, which is not less than 2300° F,, 

 we can only infer what may possibly be the consequences of the 

 passage of a volcanic duct through water-bearing strata; and our 

 remarks on this point must to a certain extent be taken as merely 

 suggestive. 



Under great pressure and friction water may continue to circulate 

 underground until its critical point is reached, or until a point is 

 reached when the elastic force of vapour, or of its disassociated gases, 

 exceeds the hydrostatic pressure. In the former case, taking the 

 critical point of water at 773° F., the depth would be about 35,000 

 feet, but of the limits of the latter we are ignorant. 



If at a depth say of 5000 feet, or at any other depth, a water- 

 bearing stratum should underlie a volcano, the temperature of the 

 water in that stratum will, independently of its temperature of 

 depth, rise rapidly as it approaches the volcanic duct, and pass 

 progressively through an ascending scale until a temperature of 700° 

 to 800° F., or higher, is reached. We will assume that at some 

 point the force of the elastic vapour counterbalances the hydro- 

 static pressure, and stays the further approach of the water. In this 

 case, and supposing the volcano to be at rest, the only underground 

 effect would be that the water to a certain distance, n, fig 6, from the 

 duct, would under that high temperature be at its critical point, or in 

 some state of maximum tension and pressure. This being effected, 

 there will be no further change so long as a state of equilibrium is 

 maintained, and the pressure of the lava at rest in the duct D remains 

 equal to the elastic force of the superheated water or vapour in n. 

 If, however, that state of rest is disturbed, and the lava in the duct 

 begins to move upwards, as represented in fig. 7, then, whatever the 



