38 Prof. Bischof on the Natural History of 



It is clear that the elastic force of steam cannot surpass a 

 certain maximum, which it reaches when its density is equal to 

 that of water. This is the case when the elasticity of the va- 

 pour e = 232952 Paris inches of mercury, or nearly 8320 atmo- 

 spheres, which suppoees a temperature of 2786° P.* 



Thus, if aqueous vapour were to reach its greatest possible 

 elasticity, its temperature must exceed that above assumed for 

 the melting point of lava by 504° F. The highest column of 

 lava, which steam at its maximum elasticity is capable of sup- 

 porting, is, therefore, if we suppose the specific gravity of liquid 

 lava three times as great as that of water, 88747 feet. But a teni- 

 perature of 2786° F. will, according to the observations at Ge- 

 neva and in Cornwall^ be met with at a depth of 1 59265 feet, 

 and according to those in the Erzg-ebirge, at a depth of 155613 

 feet (about thirty English miles) below the level of the sea 

 near Vesuvius or Etnarf- 



Supposing, then, the values found for the maximum 

 elasticity of steam for the corresponding temperature, and for 

 the depth at which that temperature must exist, to be correct, 

 it would not be possible, that a column of lava, of the whole 

 height, from the seat of the volcanos to the surface of the 

 earth, should be raised up. On the other hand, in the same 

 manner as a bubble of air let into a barometer, drives the 

 mercury into the Torricellian vacuum far above the barome- 

 tric height, aqueous vapour may raise a column of lava of 

 a height equal to its expansive force into the channels open- 

 ing into the craters. Thus, then, it may happen that aqueous 

 vapour, though far from its maximum elasticity, may yet 

 be able to raise a column of lava equal in height to its elas- 

 ticity from still greater depths to the surface of the earth. 

 A continual alternation of columns of lava and steam in 

 the channels may be very well conceived, the consequence of 

 which would be an alternate ejection of lavas, red hot masses, 



* On steam and steam-engines in the Abliandlungen der Konigl. tech- 

 nischen Deputation fiir Gewerbe, part i. 344. 



t Siijiposing the mean ten-} erature of this locality = Cl° F. 



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