424: 



F. A. Perret — Vesuvius. 



ing the vapor. A certain variation does, of course, occur, and 

 it so happens that this is in correspondence with the humidity 

 of the air, for it is when the atmospheric pressure is low that 

 the vapors escape with greater facility, and this condition also 

 brings moisture-laden air in which the fumarolic vapors can- 

 not readily be absorbed. In cold, damp air the vapors con- 

 dense and are rendered fully visible, while in a warm, dry 

 atmosphere they are often absorbed without condensation, 

 although their emission from the volcano may be no less 

 abundant than in the former case. Some fumaroles with aque- 



Fig. 8. 



Fig. 8. Vesuvius — A mud-flow in the Atrio. 



ous exhalations are so hot that the vapors are always expanded 

 and absorbed by the atmosphere without condensation, and I 

 have reproduced this phenomenon at the Solfatara of Pozzuoli 

 by artificially heating the orifice of an aqueous fumarole, after 

 which it was found impossible to effect the condensation of the 

 vapor by the usual means. 



I have recently found an important fumarole on the south- 

 east flank of the cone and about one hundred meters below the 

 crater. This has formed on one of the earlier lava flows of 

 the last eruption, but it is evidently a true, primary fumarole 

 although it is not acid at present. The temperature on July 10 

 was 235° C. Another which is very interesting is situated 

 accessibly inside the crater on this side and, with a tempera- 

 ture of only 160° C, gives hydrochloric acid in considerable 

 quantity. There is not a trace of S0 2 . 



