440 COSMOS. 



the pressure and of the power of conducting heat exercised 

 by various kinds of rock, render it likely that the geo- 

 thermal degrees of depth increase in value in proportion as 

 the depth itself increases. 



Notwithstanding the ve'ry limited number of points at 

 which the fused interior of our planet now maintains an 

 active communication with the atmosphere, it is still not 

 unimportant to inquire in what manner and to what extent 

 the volcanic exhalations of gas operate on the chemical 

 composition of the atmosphere, and through it, on the or- 

 ganic life developed on the earth's surface. We must, in the 

 first place, bear in mind that it is not so much the summit- 

 craters themselves as the small cones of ejection and the 

 fumaroles, which occupy large spaces and surround so many 

 volcanoes, that exhale gases, and that even whole tracts of 

 country in Iceland, in the Caucasus, in the high land of 

 Armenia, on Java, the Galapagos, the Sandwich Islands and 

 New Zealand, exhibit a constant state of activity through 

 solfataras, naphtha-springs, and salses. Volcanic districts, 

 which are now reckoned among those which are extinct, are 

 likewise to be regarded as sources of gas, and the silent 

 working of the subterranean forces, whether destructive 

 or formative, within them is, with regard to quantity, pro- 

 bably more productive than the great, noisy, and more rare 

 eruptions of volcanoes, although their lava-fields continue to 

 smoke either visibly or invisibly for years at a time. If it 

 be said that the effects of these small chemical processes 

 can be but little regarded, for that the immense volume of 

 the atmosphere, constantly kept in motion by currents of 

 air, could only be affected in its primitive mixture to a very 

 small extent through means of such apparently unimportant 

 additions, 61 it will be necessary to bear in mind the powerful 

 of Cordier, in mean value 56 geographical miles, an amount which, 

 according to Hopkins's mathematical theory of stability, would have 

 to be multiplied fourteen times, and would give between 688 and 

 860 geographical miles. I quite concur on geological grounds in the 

 doubts raised by Naumann in his admirable Lelirluch der Geognosie 

 (vol. i, p. 62 64, 73 76 and 289), against this enormous distance of 

 the fluid interior from the craters of the active volcanoes. 



11 A remarkable example of the way in which perceptible changes 

 of mixture are produced in nature by very minute, but continuous, 

 accumulation is afforded by the presence of silver in sea-water, 

 which was discovered by Malaguti and confirmed by Field. Not- 



