146 Effects »f Heal mcdified'by Compression. 



any means, the compound would be enabled to bear a 

 higher heat without decomposition than it can in the present 

 state of things. Now pressure must produce an elleci ol 

 this kind ; for when a mechanical force opposes the expan- 

 sioo of the acid, ite volatility must, to a certain degree, be 

 diminished. Under pressure, then, the carbonate may be 

 expected to remain unchanged in a heat by which, in the 

 open air, it would have been calcined. But experiment 

 alone can teach us what compressing force is requisite to 

 enable it to resist any given elevation of temperature, and 

 what is to be the result of such an operation. Some of the 

 con)pounds of lime with acids are fusible, others refrac- 

 torv; the carbonate, uhen constrained by pressure to en- 

 dure a proper heat, may be as fusible as the muriate. 



One circumstance, derived from the Huttonian theory, 

 induced me to hope that the carbonate was easily fusible, 

 and indicated a precise point under which that fusion ought 

 to he expected. Nothing is more common than to meet 

 with nodules of calcareous spar inclosed in whinstone ; and 

 we suppose, according to the Huttonian theory, that the 

 whin and the spar had been liquid together, the two fluids 

 keeping separate like oil and water. It is natural, at the 

 junction of these two, to look for indications of their relative 

 fusibilities ; and we find, accordingly, that the termination 

 of the spar is generally globular and smooth; which seems 

 to prove that, when the whin became solid, the spar was 

 still in a liquid state ; for had the spar congealed first, the 

 tendency which it shows, on all occasions of freedoiii, to 

 shoot out into prominent crystals, would have made it dart 

 into the liquid whin, according to the peculiar forms of its 

 crystallization, as has happened with the various substances 

 contained in whin, much more refractory than itself, 

 namely, augite, felspar, &c., all of which having congealed 

 in the liquid whin, have assumed their peculiar forms with 

 perfect regularity. From this I concluded, that when the 

 whin congealed, which must have happened about 28° or 

 30° of Wedgwood, the spar was still liquid. I therefore 

 expected, if I cjuld compel the carbonate to bear a heat of 



