122 T. Hart — Volcanic Explosions. 



Modern steam-engine boilers are now generallj' provided with 

 " Self-acting Injectors " for supplying them with water in preference 

 to the pumps formerlj^ used, because the former have no working 

 parts, and are not liable to derangement. 



In the construction of these Injectors the elastic force of the steam 

 in the boiler is utilized not only to force water into the boiler itself, 

 which it does by imparting to it a considerable velocity, but when 

 required to lift it ten or twenty feet in addition, as from a well.^ 

 The apparatus may often be seen at work upon most locomotive 

 engines. 



I would therefore suggest that this is probably a principle of the 

 conservation of energy which comes into play during volcanic 

 eruptions, the impetuosity of the current from below both carrying 

 the water along with it from fissures in the rocks, and giving rapidity 

 to the slow movement of percolation. 



In addition to these forces, as the water ascends the pipe of the 

 volcano, it will sooner or later (according to the pressure) be con- 

 verted into steam, and produce a hJast which will assist the upward 

 current of molten rock, ashes, and vapour. 



We often have illustrations of this, as when a locomotive discharges 

 glowing ashes through the chimney brought from the furnace by an 

 extra strong blast. 



The water may be supplied either by rain percolation, by that of 

 the sea, or by both combined, as well as through natural fissures ; 

 or in violent eruptions, through fissures pi-oduced by earthquakes 

 ■which precede or accompany explosive volcanic paroxysms. 



It may be well to refer to the account of the great eruption of 

 Vesuvius in a.d. 79-, given by Pliny the Younger. He says : — 

 " There had been noticed many days before a trembling of the earth, 

 which had caused, however, but little fear, because it is not unusual 

 in Campania. But that night it mas so violent that one tkonglit that 

 everything was being not merely moved, but absolutely overtvrned. It 

 was found that the soundings of the Baj' were altered, — an effect 

 attributed to the falling masses, but probably in a great measure 

 owing to an elevation of the sea-bed." 



Therefore it would appear that the rocks may have been so 

 disturbed by the preceding earthquake as to admit water from the 

 Bay of Naples to the heated rock ; and the engine of simjDle 

 emission set to work that continued with such terrible results. 



Further, it appears that water in contact with highly heated 

 rock under enormous pressure may decompose into its elements, 

 and form compounds which produce fuel to greatly intensify that 

 heat. 



We have ocular demonstration now in volcanic districts, as at 

 the Solfatara near Naples, also at Vesuvius, that chemical decom- 

 position of the rock below is active, as witnessed by the more or 

 less pungent and complex emanations there given otf, and by the 

 beautiful crystals there formed from these various vapours. 



1 See " Injector," Encyclopaedia Britannica, last edition, yoL 22, p. 500 ; also 

 "Jet Pump," EucyclopsecUa Britannica, vol. 12, p. 476. 



