250 Natural History of Volcanos and Earthquakes. 



small quantities of these only will be raised upwards, for the sur- 

 face of the melted matters will soon sink below the opening of the 

 rent at D, and steam will rise. Thus the elevation of a column 

 of lava to a considerable height by a column of steam will take 

 place. But if the lower opening of the rent CD descends more 

 or less below the surface of the melted matters, considerable quan- 

 tities of these will rise into it before this surface sinks below the 

 opening. The same may take place if between the opening of 

 the rent CD and the other rents, (those down which water flows 

 from the surface,) ridges of solid rock reach downwards from the 

 solid crust into the fluid mass. 



Such ridges may be viewed as occasioned by gradual solidifica- 

 tion of the fluid mass from above downwards, for it is well known 

 that melted matters, if they crystallize by cooling, exhibit on 

 their under surface considerable inequalities ; and the consolida- 

 tion of the melted matters in the interior of the earth is assuredly 

 produced by crystallization. 



There is another circumstance which may cause a continuation 

 of the rent CD into the melted matters. After the rising of the 

 lava and steam in this rent, the walls of it are cooled by the for- 

 mation of steam, and by the atmospheric air having a ready access 

 to the empty channel. Therefore these walls may gradually in- 

 crease by the solidifying of the melted matters ; nay, the rent 

 may be entirely solidified and obstructed, so that it can only now 

 be re-opened by the force of steam previous to a new eruption 

 taking place. If even immense quantities of lava are ejected by 

 the steam, yet the level of the melted matters in the interior may 

 be but slightly changed, for in the same manner, as all seas on 

 the surface of the earth communicate together, so the melted 

 matter in the interior does the same. However, more or less time 

 may elapse, before the melted matter which has sunk at one place- 

 in consequence of ejection, can regain its former level by the 

 aflliix of other melted matters from a distance. Therefore the 

 repose and activity of a volcano, besides depending on the inter- 

 ruption and renewal of the supply of water to the volcanic focus, 

 may also proceed from the alternate obstruction and re-opening of 

 the lava channel by the melted matters. In the latter case, in 

 the state of rest, exhalations of steam will take place, inasmuch 

 as water penetrates continually to the volcanic focus. 



