364 DOMAIN XII. VOLCANIC. 



tude of ordinary smoke (a certain proof that it 

 was considerably lighter than the ambient air), 

 and, when at a certain height, became extremely 

 rarefied and dispersed. This difference in the 

 appearance of the smoke, as observed by the 

 two authors before mentioned and myself, may 

 arise not only from the gravity of the air on 

 Etna being different at different times, but also 

 from the diversity of the smoke, which may be 

 sometimes lighter and sometimes heavier than 

 the air that surrounds it ; differing in its nature 

 according to the quality of the substances from 

 which it is produced. Such a variation in its 

 specific gravity, must induce us to conclude 

 that the bodies which burn within the crater are 

 specifically different. 



Air. " The effects of the air at the summit of Etna, 



as experienced by myself and some of the tra- 

 vellers I have before cited, were likewise different. 

 Sir William Hamilton tells us, that the thinness 

 of that fluid occasioned a difficulty of respira- 

 tion ; and Count Borch appears to have expe- 

 rienced a still greater inconvenience of that kind, 

 since he says, " The rarity of the air on this 

 mountain is extremely sensible, and almost ren- 

 ders that fluid unfit for respiration." On the 

 contrary, Baron Riedesel felt no such effect, as 



