ON ITS EXTERIOR. HOT AND COLD SPRINGS. 207 



and in upper Silesia. At the same early period of generally 

 distributed volcanic activity, there also issued from the 

 earth the enormous quantity of carbonic acid, which, in 

 combination with lime, has formed the limestone rocks, and 

 of which the carbon alone, in a solid form, constitutes 

 about the eighth part of their absolute bulk. ( 199 ) The 

 portion of carbonic acid which was not absorbed by the 

 alkaline earths, but still remained in the atmosphere, was 

 gradually consumed by the luxuriant vegetation ; and the 

 atmosphere being thus purified by the vital action of plants, 

 retained only that extremely minute portion which we 

 now find, and which is not injurious to the present condi- 

 tion of animal life. More abundant exhalations cf the 

 vapours of sulphuric acid, in the inland waters of the ancient 

 world, appear to have occasioned the destruction of the nu- 

 merous species of fish and mollusca which inhabited them, and 

 the formation of the contorted beds of gypsum which have 

 doubtless been subjected to the frequent action of earthquakes. 



Gases, liquids, mud, and melted lavas (the last emitted 

 through volcanic cones, and to be regarded as a kind of 

 " intermittent springs," ( 20 ) all issue from the earth at the 

 present day under similar relations. All these substances 

 owe their temperature and their chemical nature to the place 

 of their origin. The mean temperature of springs is less 

 tbtui that of the air at the points where they issue, when 

 their waters descend from greater elevations , and the tem- 

 perature increases according to the depth of the stratum 

 with which they are in contact at their origin. The nu- 

 merical law of this increase has been already stated ; but 

 I have found, from my own observations and those of my 



VOL. I. Q 



