Oo4i ACTION OF HEAT MODIFIEO 



ralljjn agafeousform, atthecomraon temperature orourairno. 

 fphere; but t\'hen in union with lime, its volatility is repreired, 

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

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

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

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

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

 the attradive force of the lime increaled, or the volatility of 

 the acid diminithed by any means, the compound would be en- 

 abled to bear a higher heat without decompofition, than it can 

 Preffbre muft in the prefent flate of things. Now, prefiure muft produce an 

 panfion and effect of this kind; for when a mechanical force oppofes the 

 efcape of the expanfion of the acid, its volatility muft, to a certain degree, 

 ^f^'f^"pj^^^;^^^^^bedimininied. Under prefTure, then, the carbonate maybe 

 Wronger heat, expefted to remain unchanged in a heat, by which, in the open 

 air, it would have been calcined. But experiment alone can 

 teach us whatcompreffing force is requifite to enable it to refift 

 any given elevation of temperature ; and what is to be the relult 

 of fuel) an operation. Some of the compounds of lime with 

 acids are fuiible, others refradorj ; the carbonate, when con- 

 firained by prefTure to endure a proper heal, may be as fufible 

 as the muriate. 

 Probability that One circumftance, derived from the Huttonian Theory, 



the carbonate induced me to hope, that the carbonate was eafily fufible, 

 might not be of . ' , , ■' 



difficult fufion. and indicated a precife point, under which that fufion ought 



to be expeded. Nothing is more common than to meet with 

 nodules of calcareous fpar inclofed in whinftone; and we fup- 

 pofe, according to the Huttonian theory, that the whin and 

 the fpar had been liquid together ; the two fluids keeping fepa- 

 rate, like oil and water. It is natural, at thejunftion of thefe 

 two, to look for indications of their relative fufibililies; and we 

 F & \h' h ^"^' accordingly, that the termination of the fpar is generally 

 jndic.teits globular and fmooth ; which feems to prove, that, when the 

 nielting heat, vvhin became folid, the fpar was ftill in a liquid ftale; for had 

 the fpar congealed firft, the tendency which it fliews, on all oc- 

 cafions of freedom, to flioot out into prominent cryftals, would 

 have made it dart into the liquid whin, according to the pecu- 

 liar forms of its cryftallization; as has happened with the 

 various fubftances contained in whin, much more refractory 

 than iilelf, namely, augite, felfpar, &c.; all of which having 

 congealed in the liquid whin, have alTumed their peculiar 



forms 



