F. A. Per ret — Vesuvius. 423 



As regards the gas we have atmospheric air which, as is 

 usual in volcauic exhalations, is poor in oxygen. This is read- 

 ily accounted for by its combination with other substances 

 within the mountain — especially with H 2 S in the production 

 of S0 2 . In the deposit the presence of magnesium in fair 

 amount is interesting. Hydrofluoric acid is often found on 

 Vesuvius. 



It was decided to collect and analyze simultaneously with 

 the above the products of a fumarolic area situated on the 

 same side but below the cone itself, and less than two hundred 

 meters from the escarpment of Monte Somma. Fumaroles 

 appeared here through the sand immediately after the erup- 

 tion, and, whether due simply to a continued existence of those 

 on the underlying lavas of 1903-4, or to a revivifying of these 

 by extension of the fissures on the cone to this point, the anal- 

 ysis of their products would form an interesting comparison 

 with the above, because of their location at a greater distance 

 from the central conduit and on a lower level. 



Their temperature averages 98° C, and water vapor is 

 abundant. 



Analysis of gas : 



H 2 S . _ 11-47 per cent. 



C0 2 2-08 " " 



O 11-47 " " 



N 74-98 



100-00 



Analysis of deposit : 



Yellowish-white substance, soluble in water. 



Bases : Al, Fe, Ca, 

 Acids : H 2 S0 4 , S0 2 



Sulphur, from decomposition of H 2 S. 



At a nearby fumarole we found abundant deposits of realgar. 



A veritable battery of fumaroles appeared inside the crater, 

 just below the western rim, some time after the eruption. 

 They are inaccessible, but their emanations consist largely of 

 water vapor which is rendered more or less visible according 

 to the condition of the atmosphere ; and I must here insist 

 upon the importance of taking this into consideration when 

 judging of the activity of fumaroles. Reports are often made 

 of a great increase or decrease in the emanations from day to 

 day, or even during the same day ; but it is evident that, 

 excepting after rain, the output is fairly constant, and the visi- 

 ble changes are due to a varying capacity of the air for absorb- 



