THE VOLCANIC ROCKS OF ICELAND. 7l 



and ammonia. I have never been able to detect the minutest 

 trace of carbonic oxide or carburetted hydrogens, although I was 

 enabled to employ methods of investigation which would have 

 detected the presence of only a few thousandths of those gases. 

 The chemical character of the fumarole action depends upon 

 the preponderance of one or other of these gaseous constituents. 

 The hydrochloric acid fumaroles, which not unfrequently occur 

 on a large scale near the Italian volcanoes, and are then gene- 

 rally accompanied by a very considerable sublimation of chloride 

 of sodium, appear to be of less importance in Iceland. I was 

 only able to detect traces of hydrochloric acid in a free state in 

 the crater fumaroles a few months old, which owed their origin 

 to the last eruption of Hecla, as well as in the exhalations of 

 vapour from the lava which was then erupted. As the volcano 

 when I visited it- shortly after the last eruption was already so 

 far in a state of rest that there were no violent exhalations of 

 vapour, the gases could only be drawn up by means of an air- 

 pump from the quietly smoking crater fissures, which were tole- 

 rably open to the access of air. The aqueous vapour condensed 

 while drawing up the gases always contained sensible quantities 

 of free hydrochloric acid, which is put down as an uncertain 

 quantity in the following analyses : — 



75. 76. 77. 



1. Fumaxole 2. Fumarole 3. Fumarole of 



in the great in the great the lava stream 



crater of Hecla. crater of Hecla. of 1845. 



100-00 100-00 100-00 



It follows from these analyses that carbonic and hydrochloric 

 acids constitute one part of these crater gases, and that they are 

 sometimes accompanied by sulphurous acid, which decreases so 



