LIFE of Br HUffON. 59 



though, when the latter is permitted to efcape, the former be- 

 comes one of the mod refractory of all bodies. 



In this way, and probably from this very inftance, the 

 effects of compreflion may have fuggefted themfelves to Dr 

 Hutton. He would foon perceive that the fame principle 

 could be very generally applied, and that it afforded the folu- 

 tion of a difficulty concerning limeftone, fimilar to that which 

 has been juft ftated with refpect to coal. Limeftone is not found ; 

 in the bowels of the earth having the caufticity which it ac- 

 quires by the action of fire, and hence one might conclude that 

 it had never been expofed to the action of that element. But 

 the experiments of Dr Black, before his friend was engaged in 

 this geological invefligation *, had proved that the caufticity 

 of lime depends on the expuliion of the aeriform fluid, fince 

 diflinguifhed by the name of carbonic gas, which compofes no 

 lefs than two-fifths of the whole. This great difcovery, which 

 has extended its influence fo widely over the fcience of chemis- 

 try, alfo led to important confequences in geology ; and Dr 

 Hutton inferred from it, that ftrong compreffion might pre- 

 vent the caufticity of lime, by confining the carbonic gas, even 

 when great heat was applied, and that, as has been fuppofed of 

 coal, the whole may have been melted in the interior of the 

 earth, fo as on cooling to acquire that cryftallized or fparry 

 flructure which the carbonate of lime fo frequently pofTeiTes f. 



It 



* Dr Black's paper on magnefia, -which contained this difcovery, was commu- 

 nicated to the Philofophical Society of Edinburgh in June 1755, an< ^ was publifli- 

 ed in the fecond volume of their Effays, in the year following. Dr Hutton had 

 at this time only begun his geological refearches. It was not, I imagine, till after., 

 the year 1760 that they came to take the form of a theory. 



■f- In the view here prefented of the principle of comprtffion, as employed in 

 the Huttonian Theory, it is confidered as a hypothefis, conformable to analogy, 

 aSumed for the purpofe of explaining certain phenomena in the natural hiflory of 



the 



