40 ESSAYS AND OBSERVATIONS 



the proportion of unfoluble parts mull: 

 be diminiflied by calcination. A proof 

 of the exiftence of iron in the marie un- 

 der Gonfideration, is its acquiring a red- 

 difli caft in the fire *, which all clays 

 that contain iron are obferved to do. 

 That this iron remains with the refidue, 

 when the marie is analyfed in its natural 

 ftate, and pafTes off with the calcarious 

 earth, when the analyfis is performed af- 

 ter the marie is calcined, is evident from, 

 fome circumftances in the foregoing ex- 

 periments. For the calcarious earth, ex- 

 tracted from marie in its natural ftate, 

 was always white fj the refidue by the 

 adlion of fire became red :|:. On the 

 other hand, the calcarious earth of calci- 

 ned marie had a reddifh appearance, the 

 refidue was white 11, 



• Experiment 17. f Experiment 15* 



X Experiment 4. g Experiment 22' 



SECT. 



