210 ESSAYS AND OBSERVATIONS 



fully or nearly faturated with air. In the firft> 

 cither the charcoal or. the acid, or both to* 

 gether, are almoft wlwlly converted into air j 

 a part of which is probably joined to the al- 

 kali. In the fecond, the acid is not properly 

 feparated, but rdther deftroyed by the fire : a 

 confiderable portion of it is converted into an 

 inflammable fubftance ; and we learn from 

 Dr. HaleSy that the bodies of this clafs contain 

 a large quantity of .fixed air. 



When we confider that the attraftion cf 

 alkalis for fixed air is weaker than that of 

 the calcarious earths, and refled upon the ef- 

 fects of heat in chemift'ry, we are led to ima- 

 gine, that alkalis might be entirely deprived 

 of their air, or rendered perfedly cauftic, by a 

 fire fomewhat weaker than that which is fuf- 

 ficient to produce the fame change upon lime; 

 but this opinion does not feem agreeable to 

 experience. 



The alkalis do, however, acquire fomc 

 degree of caufticity in a ftrong fire, as ap- 

 pears from their being more eafily united with 

 fpirit of wine after having been kept in fu^ 

 fion for fome time. For that fluid, which 

 cannot be tindtured by a mild fait of tartar, 



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