o 



6 ESSAYS AND OBSERVATIONS 



the effervefcence which enfiies upon the 

 application of acids to any calcarious 

 fubftance in its natanal flate, proceeds 

 folcly from the expulfion of air contain^ 

 ed in that fubftance. Calcarious earths 

 are, by calcination, totally deprived of 

 their air ; therefore, tliofe earths, in this 

 ilate, never can excite any effervefcence 

 with acids. Hence the abfence of any 

 liich commotion in the two foregoing 

 experiments is eafily explained. 



As to the other circumftances, calca^ 

 rious earths by calcination lofe about a 

 third of their weight. Thislofs is found 

 to proceed from the expulfion of air 

 and moiilure naturally contained in fuch 

 earths. Both this air and nioifture, acd 

 confequentiy the original weight, are, 

 according to Dr Black's obfervations *, 

 rci-orcd to fuch calcined fubftances by 

 cTiifolving them in acids, and then pre- 

 cipiiating them by an alkahne fait. As 

 the marks employed in the foregoing 

 experiments had undergone the a6lion 



of 



* Treat fornierlj' cired. 



