MODIFIED by COMPRESSION. 77 



tions, muft produce, in our times, refults fimilar to thofe afcri- 

 bed, in that Theory, to the former action of fire. 



In comparing the Huttonian operations with thofe of the 

 volcanoes, I fhall avail myfelf of fome fads, brought to light 

 in the courfe of the following inveftigations, by which a pre- 

 cife limit is affigned to the intenfity of the heat, and to the 

 force of compreilion, required to fulfil the conditions of Dr 

 Hutton's hypothefis : For, according to him, the power of 

 thofe agents was very great, but quite indefinite j, it was there- 

 fore impoffible to compare their fuppoied effects in any precife 

 manner with the phenomena of nature. 



My attention was almoft exclufively confined to the Carbo- 

 nate of Lime, about which I reafoned as follows : The carbonic 

 acid, when uncombined with any other fubflance, exifts natural- 

 ly in a gafeous form, at the common temperature of our atmo- 

 fphere ', but when in union with lime, its volatility is reprefied, 

 in that fame temperature, by the chemical force of the earthy 

 fubflance, which retains it in a folid form. When the tem- 

 perature is raifed to a full red-heat, the acid acquires a vola- 

 tility by which that force is overcome, it efcapes from the 

 lime, and afTumes its gafeous form. It is evident, that were the 

 attractive force of the lime increafed, or the volatility of the 

 acid diminifhed by any means, the compound would be enabled 

 to bear a higher heat without decompofition, than it can in the 

 prefent flate of things. Now, prefiure muft produce an effect 

 of this kind 5 for when a mechanical force oppofes the expan- 

 fion of the acid, its volatility muft, to a certain degree, be di- 

 minifhed. Under preffure, then, the carbonate may be expect- 

 ed to remain unchanged in a heat, by which, in the open air, 

 it would have been calcined. But experiment alone can teach 

 us, what compreffing force is requifite to enable it to refift any- 

 given elevation of temperature ; and what is to be the refult of 

 fuch an operation. Some of the compounds of lime with acids 



are 



