ON ITS EXTERIOR. VOLCANOES. 419 



sition of the air, and through its medium upon the 

 development of organic life upon the Earth's surface ? 

 We must first bring under consideration, in reference to 

 this question, the circumstance that gases are exhaled 

 less from summit-craters themselves, than from small 

 cones of eruption and from the fumaroles which over 

 wide spaces surround so many volcanoes ; and that even 

 entire districts, in Iceland, in the Caucasus, in the 

 highlands of Armenia, in Java, the Gralapagos, the 

 Sandwich Islands, and New Zealand, manifest uninter- 

 rupted activity through solfataras, naphtha-springs, and 

 salses. Volcanic districts which are now reckoned as 

 extinct are also to be regarded as sources of gas, and it 

 is probable that the silent operation of subterranean 

 decomposing and formative forces taking place in them 

 is, in respect to quantity, more productive than the 

 grander but more rare phenomena of volcanic eruptions ; 

 although, in regard to these also, it is to be remembered 

 that the " fields of lava " continue for years to send 

 forth visible and invisible vapours. If it should be 

 thought that the effects of these small chemical pro- 

 cesses may be disregarded, because the enormous volume 

 of the atmosphere, incessantly impelled to and fro by 

 currents, can be so little altered by the addition to its 

 primitive mixture of such minute fractions appearing 

 severally so unimportant ( 575 ), we should remember the 

 powerful influence exercised, according to the fine inves- 

 tigations of Percival, Saussure, Boussingault, and Liebig, 

 by three or four ten-thousandth parts of carbonic acid 

 gas in our atmosphere upon the existence of vegetable 

 organic life. According to Bunsen's fine investigations 

 on the volcanic gases, some fumaroles, in different stages 



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