NOME II. VESICULAR LAVA. 



355 



its heat gradually, would congeal and form a 

 crust or solid plane ; and should the fluid lava 

 beneath, afterwards act forcibly on this crust, it 

 might burst it, or make a passage where it found 

 least resistance ; in which case the melted lava 

 would occupy that aperture. Should then the 

 crust, instead of ascending in a single body, be 

 forced up in small fragments, these cooled in the 

 air, would fall down in immense quantities within 

 the crater; and, from the effect of the laws 

 of gravity, must accumulate in the figure of a 

 cone. These theoretical conjectures, if they do 

 not perfectly explain, may at least enable us to 

 conceive the nature of the causes, which have 

 produced the difference of appearance observed 

 at different times in the crater of Etna. 



" It is much to be regretted that we have no Changes, 

 history of Etna ; which, did we possess it, must 

 greatly contribute to elucidate the theory of 

 volcanoes, and the causes of the various changes 

 which have taken place at different times, in the 

 summit of this mountain. That such changes 

 have happened, is evident from the few but va- 

 luable notices concerning Etna, which we find 

 in ancient authors. Of these I shall briefly state 

 two or three, which appear to be of most im- 

 portance. 



" I shall first produce the authority of Strabo, strabo,&c. 



2 A 2 



