80 ESSAYS BIOGRAPHICAL AND CHEMICAL 



what had been lost in the fire; and as it is found to 

 effervesce with acids, part of the addition must certainly 

 be air.' 



Black here made an enormous stride ; he had weighed 

 a gas in combination. He argues further : ' It seems there- 

 fore evident that the air was forced from the alkali by the 

 acid, and lodged itself in the magnesia/ We may repre- 

 sent the change diagrammatically thus : 



Magnesia \ x Alkali^-Yitriolated alkali. 

 Spirit of vitriol ' ^ Air->Mild magnesia. 



The next step was to try whether mild magnesia lost 

 the same weight on being mixed with acid as it did when 

 heated in the fire. But owing probably to the solubility 

 of the fixed air in the water, a much less loss was found 

 on dissolving the magnesia (35 grains out of 120) than by 

 heating it (78 grains out of 120). The amount of acid 

 required to expel the fixed air was, however, practically 

 the same as that required to dissolve the magnesia usta, 

 or heated magnesia (267 and 262 grains). 



Turning his attention next to chalk, he dissolved some 

 in muriatic acid, and having precipitated with fixed alkali 

 no difference could be detected between the recovered and 

 the original chalk. He had thus first separated the fixed 

 air from the chalk, and then recombined the two. These 

 experiments led Black to conclude that fixed air must be 

 of the nature of an acid, for it converts quick-lime the 

 acrid earth, as he termed it into crude lime, or mild 

 earth, the mildness being due to its union with fixed air. 



The explanation is thus given of the curious fact that 

 mild magnesia, mixed with lime-water, gives pure water ; 

 for the fixed air leaves the magnesia and unites itself to 

 the lime, and both the magnesia usta and the chalk 

 which are formed are insoluble in water. And the action 



