500 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



of their decreasing affinity. But Bergman was obliged to 

 recognise that the order of affinity was not quite constant, 

 and in particular that different results were often obtained 

 when the affinities were compared : (i) in the dry way, by 

 heating the substances together (as Mayow had done) and 

 discovering which combination remained fixed in the product 

 and (2) in the moist way, by mixing the substances in solu- 

 tion (as Baume had done) and finding which base secured 

 possession of the acid. 



Whilst, therefore, there was a fixed order of affinity of 

 acids for bases and vice versa, this order might be altered 

 considerably by changing the conditions of the experi- 

 ment. 



Thus, in the case of sulphuric acid (using modern names) 

 the affinities of the different bases were as follows : 



SULPHURIC ACID 



In the Moist Way In the Dry Way 



Baryta Potash 



Potash Soda 



Soda Baryta 



Lime Lime 



Magnesia Magnesia 



Ammonia Metallic calces 



Alumina Ammonia 



Metallic calces Alumina 



From this table it is seen that ammonia, added " in the 

 moist way " to a solution of a metallic salt, precipitates the 

 calx of the metal and secures possession of the acid with 

 which it was combined. On the other hand, when a 

 metallic calx is heated with an ammonium salt " in the dry 

 way," the volatile alkali is expelled, whilst the acid remains 

 behind in combination with the calx. 



This reversal of the action can again be shown by writing 

 two equations, thus : 



