222 



EXPANSIVE POWER OF LIQUID GASES. [Cii. XXXIII. 



power may be acquired sufficient to expel a massive current 

 of lava. After the eruption has ceased, a period of tranquil 

 lity succeeds, during which fresh accessions of heat are com- 

 municated from below, and additional masses of rock fused 

 by degrees, while at the same time atmospheric or sea water 

 is descending from the surface. At length the conditions 

 required for a new outburst are obtained, and another cycle 

 of similar changes is renewed. 



Expansive power of liquid gases. — Although aqueous vapour 

 or steam forms a principal part of the aeriform fluids which 

 rush out for days, months, or even years continuously from 

 volcanic vents, there are other gases, such as the carbonic, 

 sulphurous, and hydrochlorous acids, which are also present, 

 and sometimes in great volume. The experiments of Faraday 

 and others have shown that all these gases may be condensed 

 into liquids by pressure. At temperatures of from 30° to 50° 

 Fahr. the pressure required for this purpose varies from 15 

 to 50 atmospheres; and this amount of pressure we may 

 regard as very insignificant in the operations of nature. A 

 column of Yesuvian lava that would reach from the lip of 

 the crater to the level of the sea, must be equal to about 

 300 atmospheres ; so that, at depths which may be termed 

 moderate in the interior of the crust of the earth, the gases 

 may be condensed into liquids, even at very high tempera- 

 tures. The method employed to reduce some of these gases 

 to a liquid state is, to confine the materials, from the mutual 

 action of which they are evolved, in tubes hermetically sealed, 

 so that the accumulated pressure of the vapour, as it rises 

 and expands, may force some part of it to assume the liquid 

 state. A similar process may, and indeed must, frequently 

 take place in subterranean caverns and fissures, or even in 

 the pores and cells of many rocks ; by which means, a much 

 greater store of expansive power may be pac'ked into a small 

 space than could happen if these vapours had not the pro- 

 perty of becoming liquid. For, although the gas occupies 

 much less room in a liquid state, yet it exerts exactly the 

 same pressure upon the sides of the containing cavity as if it 

 remained in the form of vapour. 



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