THE VOLCANIC ROCKS OF ICELAND. 



79 



by means of alkaline or earthy metals, can have had any share 

 in the generation of the volcanic hydrogen ; for both of these 

 processes presuppose the prevalence of a temperature in which 

 carbonic acid cannot exist in contact with hydrogen without 

 suffering a partial reduction to carbonic oxide. But not the 

 minutest trace of this gas is found in the volcanic exhalations. 



Besides sulphuretted hydrogen, it is especially sulphurous acid 

 which determines the character of the action of solfataras. This 

 gas likewise always appears associated with aqueous vapour. 

 On account of the great readiness with which it dissolves in the 

 condensed vapour, it cannot be collected in the gaseous form. 

 But even the smell of the water condensed from such vapour 

 springs, and the reaction which it gives with iodine, testify to 

 the presence of a considerable quantity of this gas. As sul- 

 phuretted hydrogen and sulphurous acid mutually decompose 

 each other with a separation of sulphur, the two gases can never 

 present themselves together. However, they frequently occur 

 in the same fumarole district, close together. 



It has been attempted to connect the formation of volcanic 

 gases, in which the presence of hydrogen has hitherto been en- 

 tirely overlooked, in part with the decomposition of organic 

 substances. But the gases which result from the spontaneous 

 decomposition or dry distillation of organic remains do not pre- 

 sent the remotest similarity to these exhalations. To prove this 

 statement, it will be sufficient to bring forward here a few ana- 

 lyses which I have made of coal-gas, and of some natural exha- 

 lations of combustible gases, with regard to whose organic origin 

 there can be no question. 



Marsh gas from a Pond in the Botanical Garden at Marburg, 



89. 



Nitrogen . . 

 Oxygen . . 

 Carbonic acid 

 Marsh gas . . 

 Hydrogen . . 

 Carbonic oxide 

 Olefiant gas . 



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