56 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



strong fire, black lead being added to soak up the potash 

 and so prevent it from corroding the vessel ; the potash 

 was found to " lose a part of its air, and acquire a degree of 

 causticity," but this was only slight, and it was not found 

 possible to convert the alkalis into a perfect caustic by the 

 action of heat alone. Black points out, however, that " the 

 alkali newly obtained from the ashes of vegetables is 

 generally of the more acrid kind " (A. C. R. I. 39) owing 

 to its partial conversion into a caustic alkali during 

 calcination. 



The caustic alkalis, prepared in the form of solutions, 

 evidently correspond with slaked lime rather than with 

 quick-lime. But they are even less easily decomposed by 

 heat than the mild alkalis ; the form of the alkali corre- 

 sponding with quick-lime is therefore very difficult to prepare, 

 and was not discovered until Davy (Chapter XII) had 

 succeeded in isolating from the alkalis the very refractory 

 metals which they contain. 



Sal volatile is a mild alkali : spirit of hartshorn is a 

 caustic alkali. It had long been known that sal ammoniac, 

 the volatile salt described in Chapter I, could be converted 

 into a volatile alkali, SAL VOLATILE, by heating it with soda 

 or potash. This volatile alkali resembled the fixed alkalis 

 in that it effervesced when acted upon by acids. In this 

 action the volatile alkali was reconverted into a salt ; for 

 instance, sal ammoniac could be formed from sal volatile 

 and spirit of salt. These facts were used by Mayow in 

 1674 to prove that the alkali as well as the acid still exists 

 in the salt prepared by mixing them. 



"Although [acids] and alkalis pass into a neutral 

 substance when they meet, yet they do not, as is generally 

 supposed, entirely destroy each other. For example, when 

 the acid spirit of salt is coagulated with a volatile 

 [alkali] .... although the mixed salts seem to be des- 

 troyed, yet they may be separated from each other with 



