126 Scientific Proceedings, Royal Dublin Society. 



such conditions the order with which differently exposed parts of 

 the fissure's contents reached the surface would be most complex, 

 depending on a large number of collateral circumstances. The 

 tendency will be to shade off sharp irregularities of composition, 

 and render the magma more homogeneous. 



The Main Varieties of Volcanic Outbursts. — Whatever type of 

 activity the volcanic outburst may have taken, we have only so far 

 discussed secondary variations therein, and it now remains to 

 explain what is the acting cause in different varieties of eruptions. 



It is necessary that we diverge from our train of argument to 

 refer to some of the physical phenomena accompanying the relief 

 from pressure of a superheated liquid. Sir Gr. B. Airy and Prof. 

 Rankine 1 showed that in the explosion of a steam boiler the 

 destructiveness was not due to the expansion of the steam already 

 existing enclosed within it, but as soon as the pressure on the 

 superheated water-contents diminishes, that liquid undergoes rapid 

 and violent evaporation, until by such action the remnants are 

 reduced to the normal boiling-point of the locality of the boiler. 

 Mr. Gr. Biddle has demonstrated that, in a boiler containing steam 

 and water at a pressure equal to four atmospheres, when the 

 source of heat was removed, and the pressure suddenly relaxed, 

 one-eighth of the whole liquid contents was immediately converted 

 into the gaseous form. Prof. R. H. Thurston, 2 who has lately 

 worked at the same subject, has shown that although the energy 

 stored in the steam contained in a boiler is far in excess of that 

 of the water at the same temperature, the amount, by weight of 

 the latter, is often proportionally so much greater that it repre- 

 sents an enormous amount of stored energy. He showed, how- 

 ever, that as the temperature rose, the more the energy stored in 

 the water approached that of the steam : at 50 lbs. pressure the 

 ratio is 20 to 1 ; at 100 lbs., 14 to 1 ; at 500 lbs., 5 to 1 ; while 

 at 7500 lbs. the two quantities become practically equal. At 

 60 lbs. pressure, 1 lb. of steam equal ^ lb. of gunpowder ; but at 

 very high temperatures, at which steam and water are equal to 

 each other, they rival gunpowder. 



1 Phil. Mag., November, 1863. 



2 Trans. American Soc. of Median. Engineers, 1872 ; and Joum. Franklin Inst., 

 December, 1872. 



