106 Effects of Heat modified hy Compression. 



and whence it has been propelled upwards ; the liquid lava 

 risino- through this internal channel, so as to fill the crater 

 to the brim, and flow over it. On this occasion, the sides 

 of the mountain must undergo a violent hydrostatical pres- 

 sure outwards, to which they often yield by the formation 

 of a vast rent, through which the lava is discharged in a 

 lateral eruption, and flows in a continued stream sometimes 

 during months. On JElna. most of the eruptions are so per- 

 formed ; few lavas flowing from the summit, but generally 

 breaking out laterally at very elevated stations. At the 

 place of delivery, a quantity of gaseous matter is propelled 

 violently upwards, and, along with it, some liqaid lava ; 

 which last, falling back again in a spongy state, produces 

 one of those conical hills which we see in great number on 

 the vast sides of Mount iEtna, each indicating the discharge 

 of a particular eruption. At the same time, a jet of flame 

 and smoke issues from the main crater, proving the internal 

 communication between it and the lava; this discharge from 

 the summit generally continuing, in a greater or a less de- 

 gree, during the intervals between eruptions. (Fig. 41. re- 

 presents an ideal section of Mount ./Etna j ai is the direct 

 channel, and be is a lateral branch). 



Let us now attend to the state of the lava within the 

 mountain during the course of the eruption ; and let us sup- 

 pose, that a fragment of limestone, torn from some stratum 

 below, has been included in the fluid lava, and carried up 

 with it. By the laws of hydrostatics, as each portion of 

 this fluid sustains pressure in proportion to its perpendicular 

 distance below the point of discharge, that pressure must in- 

 crease with ahe depth. The specific gravity of solid and 

 compact lava is nearly 2-8 ; and its weight, when in a liquid 

 state, is probably little different. The table shows that the 

 carbonic acid of limestone cannot be constrained in heat by 

 a pressure less than that of 1 708 feet of sea, which corre- 

 sponds nearly to GOO feet of liquid lava. As soon, then, as 

 our calcareous mass rose to within 600 feet of the surface, 

 its carbonic acid would quit the lime, anil, assuming a ga- 

 seous form, would add to the eruptive effervescence. And 

 this change would commonly begin in much greater depths, 



in 



