iv CHALK, LIME, AND THE ALKALIS 55 



(A. C. R. I. 32). The caustic alkali therefore resembled 

 slaked lime in its power of absorbing fixed air from the 

 atmosphere, but differed from it in its much greater 

 solubility in water. 



On account of their great solubility the caustic alkalis 

 had not been isolated previously. Black attempted to 

 separate them by evaporating the caustic lye in an earthen- 

 ware bowl, but found that the inside of the bowl became 

 corroded and pitted with holes. By using a silver dish, 

 however, he succeeded in evaporating all the water, and 

 obtained the caustic potash as a fused mass which solidified 

 on cooling, but dissolved again on adding a small quantity 

 of water (A. C. R. I. 33). 



At a later date, strong solutions of the caustic alkalis 

 were used in order to absorb (and so to collect and weigh) 

 fixed air from various sources, the greater concentration of 

 the solutions enabling them to take up much more fixed air 

 than in the case of lime-water; but Black, at that date, did 

 not understand the manipulation of gases, and had no 

 opportunity of making use of this valuable quality. 



The alkalies are not easily decomposed by heat 

 From the above observations it was clear that the relation- 

 ship between the mild and caustic alkalis was much the 

 same as that between chalk and slaked lime. Soda and 

 potash were closely analogous to chalk, from which they 

 differed chiefly in being soluble in water; whilst caustic 

 soda and caustic potash were similar to slaked lime, but 

 differed from it in their extraordinary solubility in water. 



As potash parts with its fixed air so readily when mixed 

 with lime, it seemed probable "that alkalis might be 

 entirely deprived of their air, or rendered perfectly caustic, 

 by a fire somewhat weaker than that which is sufficient to 

 produce the same change " in chalk (A. C. R. I. 37). This 

 was found not to be the case : potash was " exposed, for 

 several hours, in a covered crucible," to the action of a 



