F. A. Ferret — Vesuvius. 



419 



tion and these have been studied by Lacroix, Johnston-La vis, 

 Casoria and others, and I shall not take up this phase of the 

 subject ; but that to which I refer is continued observation at 

 regular intervals of the temperature and chemical composition 

 of the exhalations, both of which are so intimately related to the 

 actual internal condition of the volcano. The little which I 

 have been able to do in this direction is recorded in this section. 

 For measuring temperatures I find an electric pyrometer 

 indispensable, not only on account of its wide range but also 



Fig. 6. 



Fig. 6. Vesuvius -Corded lava of 1881-3. 



because the length of the fire-end permits of its introduction 

 to the depth of a meter or more in the fumarole. 



The chemical investigation naturally divides itself into two 

 branches, viz. field detection and laboratory analysis. For the 

 former I use the following reagents, which are easily carried 

 in a pocket case. Carbon dioxide is detected with limewater 

 and sulphuretted hydrogen with lead acetate paper. For sul- 

 phur dioxide (S0 2 ) I have adopted a reagent proposed by the 

 late Prof. Casoria* consisting of a precipitate formed by mix- 

 ing together solutions of nitroprussiate of soda and chloride 

 of zinc and adding ferrocyanide of potassium. The three 

 salts have the following proportions : 



* " Una nuova carta rivelatrice dell' anidride solforosa," Eugenio Casoria, 

 Portici, 1904. 



