THE VOLCANIC ROCKS OF ICELAND. 77 



compositions, it is in this instance exclusively the sulphuretted 

 hydrogen and sulphurous acid which, in the presence of heated 

 water, bring about all those alterations of rocks, the most re- 

 markable of which I have already pointed out in a previous paper 

 on the pseudo-volcanic phaenomena of Iceland *. The ana- 

 lysis of the gases which are evolved from the smoking ground 

 in the fumarole district, or from small pools of water and mud, 

 affords the most striking proof of this exclusive action of sulphu- 

 retted hydrogen. Thus, for instance, the considerable per-cent- 

 age of carbonic acid is not diminished, while the quantity of 

 sulphuretted hydrogen, in proportion to that of free hydrogen, 

 decreases more and more. The following analyses of the gases 

 taken from different small pools of boihng water situated in the 

 midst of the Solfatara of Krisuvik, most distinctly show this de- 

 crease of sulphuretted hydrogen : — 



86. 87. 



Nitrogen 1-80 1-44 



Carbonic acid 88*54 86-92 



Sulphuretted hydrogen . . 1*79 3-28 



Hydrogen 7*87 8-36 



Carbonic oxide O'OO 0*00 



Carburetted hydrogen . . Q-QO 0*00 



100-00 100-00 



It may be as well, for the sake of completeness, to add here 

 the analysis of such a gas from Reykjahlidh in the far north 

 of Iceland, which was drawn from the smoking muddy soil of a 

 large fumarole by means of an artificial stream of vapour, and 

 was remarkable for its unusually large quantity of hydrogen : — 



88. 



Nitrogen 0*72 



Carbonic acid .... 30-00 

 Sulphuretted hydrogen . 24*12 



Hydrogen 25-14 



Carbonic oxide .... 0*00 

 Carburetted hydrogen . . O'OO 



100-00 



It is evident from these experiments how little ground there 

 is for denying the presence of combustible gases in the exhala- 



* Liebig's Annalen, vol. l.xii. p. 1. 



