On the Agency of Water in Volcanic Eruptions. 



153 



as the underground water stands at a sufficiently high level. Mallet 

 was of opinion* that capillary infiltration goes on in all porous rocks 

 at enormous depths, and that the deeply seated walls of the volcanic 

 ducts leading to the crater, if of such materials, may be red hot, and 

 yet continue to pass water from every pore (like the walls of a well 

 in chalk), which is flashed off into steam, and, unable to return by the 

 way the water came down, escapes through the duct and crater. 

 For my own part, I do not think this can happen during a state of 

 undisturbed statical pressure,, but that it follows on any disturbance. 



That capillarity exercises a very important influence on the under- 

 ground percolation of water is undoubted. To a certain point it has 

 been proved experimentally by M. Daubree,f who found that water 

 placed on a disk of fine-grained (Triassic) sandstone, X fastened over 

 a vessel filled with steam under pressure of nearly two atmospheres, 

 infiltrated into the underlying vessel against that/ press ire. He further 

 nouced that in consequence of the heat the action was more rapid than 

 it otherwise would have been ; and — making the experiment inversely 

 — he observed that vapour placed under a pressure of several atmo- 

 spheres in the lower vessel, did not transude through the disk left 

 dry on the upper surface. As before pointed out, however, capillarity 

 is adversely affected by a rise of temperature, and is comparatively 

 inoperative at high temperatures. 



Under these circumstances, it is conceivable that water may readily 

 be carried down through the upper cooler strata to the proximity of 

 the volcanic duct. But no amount of available vapour tension could 

 force it back through the same depth of strata against both friction and 

 capillarity. At the same time when the elastic tension of the vapour 

 of the water reaches the point either of critical temperature, or such 

 higher temperature that it exceeds the hydrostatic pressure^ the further 

 progress or descent of the water will be prevented. The influx of 

 water to the volcanic duct is to a certain point effected under the same 

 conditions as those which effect its general descent to depths through 

 the earth's crust. But at this point, whatever it may be, other 

 causes come into operation, which, while the descent of water to the 

 volcanic foci beneath the solid crust remains, an impossibility, renders 

 its introduction into the volcanic ducts, even, at considerable depths, 

 possible. In any case, when an equilibrium is established between the 

 vapour tension and the hydrostatic pressure, no change will take place 

 unless that condition of equilibrium be disturbed. Such a cause 

 exists in the case of a volcanic duct. 



In fig. 7 the surface-waters pass in the usual manner through the 

 strata b to some point m, where the heat from the lava L in the duct 



* Palmieri's " Eruption of Vesuvius in 1872." Mallet's Introduction, p. 52. 



f " Greologie Experhnentale," 1879, p. 236. 



X The absorbent power of this rock = 69 per cent, by weight. 



