VULCANISM. 75 



they approach the surface much of the hydrogen and water-vapor escapes 

 and pyroxene minerals crystallize instead of these hydrous micas. 



All of these facts and deductions lead to the general conclusion that 

 our surface-waters have been derived from the interior of the earth, and 

 oppose the idea that to explain the presence of hydrogen, or water, in 

 magmas and rocks, we have merely to appeal to the penetration of surface- 

 waters. The meteoric waters are limited to their superficial place and 

 function, both in the evolution of magmas and in vulcanism; an ultimate 

 source is found for these waters; and a steady supply of water and gases 

 is furnished to the earth to offset the loss of vapor into space, and thus 

 contributes to the globe one of the factors necessary to a long period of 

 habitability for living organisms. 



VOLCANIC GASES. 



The gases which escape from fumarolic vents are in many respects 

 similar to those obtained by heating igneous rocks in vacuo, but with the 

 addition of oxygen and vapors of chlorides, fluorides, boric acid, and other 

 high-temperature volatilizations. Though nitrogen is much more con- 

 spicuous in the analyses of volcanic gases than in those from rocks, this is 

 doubtless due, in the main, to a mixture with atmospheric air. However, 

 the greater heat of the volcano would also favor a higher proportion of 

 nitrogen, as shown by my experiment. Much of the oxygen also is probably 

 from the air. But an analysis of gas escaping from a stream of lava 

 flowing on the sea bottom at Santorin gave Fouque: oxygen, 21.11 per 

 cent; nitrogen, 21.90 per cent; and hydrogen, 56.70 per cent. 1 This would 

 suggest that the dissociation of water also contributes free oxygen. 



Fouque"'s studies at Santorin confirm the law of variation in composi- 

 tion of volcanic gases, first established by Sainte-Claire Deville, 2 namely, 

 that the nature of the gas evolved depends upon the phase of volcanic 

 activity. Hydrochloric acid, with free chlorine and fluorine, is given off 

 only from the hottest fumaroles where the heat is sufficient to liberate 

 these gases from chlorides and fluorides. At less active vents, sulphur 

 dioxide is the most noticeable of the corrosive gases, while the cooler fuma- 

 roles exhale chiefly hydrogen sulphide, carbon dioxide, and nitrogen. 

 Carbon dioxide and nitrogen escape from all the fumaroles. Fouque 

 found that the relative importance of hydrogen increased with rise of 

 temperature, and that his marsh-gas (which, owing to an imperfection in 

 the method of analysis in 1867, may have been carbon monoxide, or a 

 mixture of carbon monoxide and marsh-gas) diminished as the activity 

 increased. These observations are entirely in accord with the results of 

 my differential temperature experiments with rock powders. Hydrogen 

 sulphide and carbon dioxide are the gases expelled from the rocks at the 

 lowest temperatures; carbon monoxide and marsh-gas appear at inter- 

 mediate temperatures, while hydrogen is most prominent when the heat 

 is carried to bright redness. Nitrogen is most abundantly liberated at red 

 heat; hence the presence of that gas at the cooler vents and fissures is 

 chiefly due to atmospheric air. 



1 Fouque", Santorin et ses Eruptions, p. 230. 



2 Sainte-Claire Deville, Ann. de Chim. et Phys., 52 (1858), p. 60. 



