54 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



chalk, and was therefore saturated with air which must 

 have been furnished by the alkali " (A. C. R. I. 28). 



B. THE ALKALIS. 



Mild alkalis are made caustic by the removal of fixed 

 air. The preparation of CAUSTIC ALKALIS by the action of 

 caustic lime on the MILD ALKALIS, soda and potash, was 

 described by Geber. The caustic liquid l thus prepared 

 was used from an early period in the manufacture of soap by 

 the action of the hot alkali on fat ; it was, therefore, generally 

 called a " soap lye." Its caustic properties had been 

 attributed to lime dissolved in it ; but Black was not able 

 to prepare from it any trace either of chalk or of gypsum ; 

 he therefore concluded that " the acrimony of the caustic 

 alkali does not depend on any part of the lime adhering to 

 it" (A. C. R. I. 26). 



On the other hand, Black found that a mild alkali made 

 caustic by lime was acted on by acids " without the least 

 effervescence or diminution of weight" (A. C. R. I. 32) nor 

 did it produce more than a slight cloudiness with lime-water. 

 The caustic alkali, when properly prepared, was thus free 

 both from lime and from fixed air. 



The action of lime in rendering the alkali caustic was 

 therefore due, not to its ability to dissolve in the liquid or 

 to impart to it some fiery principle which it had acquired in 

 the lime-kiln, but on the contrary to its property of remov- 

 ing from the alkali the fixed air which had rendered it mild. 



Caustic alkalis are rendered mild by exposure to air. 

 Black found that after a fortnight's exposure to the air in an 

 open shallow vessel his caustic alkali " became entirely mild, 

 effervesced as violently with acids, and had the same effect 

 upon lime water as a solution of an ordinary alkali " 



1 "A very hellish spirit, in which great mysteries lie hid" (Basil 

 Valentine, Last Will and Testament, p. 302). 



