MODIFIED ly COMPRESSION. "7 
tions, muft produce, in our times, refults fimilar to thofe afcri- 
bed, in that Theory; to the former a¢tion of fire. 
In comparing the Huttonian operations with thofe of the 
volcanoes, I fhall avail myfelf of fome facts, brought to light 
in the courfe of the following inveftigations, by which a pre- 
cife limit is afligned.to the, intenfity of the heat, and to the 
force of compreilion, required. to fulfil|the conditions ,of | Dr 
Hutron’s hypothefis :: For, according to him, the power of 
thofe agents was very great, but quite indefinite ; it was there- 
fore impoflible to compare their fuppofed 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 fubftance, exifts natural- 
ly ima gafeous form, atthe common temperature of our atmo- 
fphere ; but when in union with lime, its yolatility is repreffed,. 
in that fame temperature, by the chemical, force ofthe earthy. 
fubftance, which retains it ina folid, form. When the tem- 
perature is raifed,to a full red-heat, the acid acquires a yola- 
tility by which that force is overcome, it, efcapes from the. 
lime, and affumes 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 ¢an in the 
prefent ftate.of things. Now, preflure muft, produce an effect 
of this kind; for when,a mechanical force oppofes the expan- 
fion of the acid, its volatility muft, to a certain degree, be di- 
minifhed.. Under preflure, 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 comprefling force is requifite to enableit 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 
