DOUBLE ELECTIVE AFFINITY. 275 



decompositions occur.* Thus, ammonia, it was shown, 

 would separate sulphuric acid from magnesia, lime 

 remove it from ammonia, potash or soda from lime, and 

 barytes from potash or soda. It was thought the 

 inverse of this order would not take place, but recent 

 researches have shown that the results are modified by 

 quantity and some other conditions. 



It often happens that we have a compound action of 

 this kind in which double election is indicated. Sul- 

 phate of lime and carbonate of ammonia in solution are 

 brought together, and there result a carbonate of lime 

 and a sulphate of ammonia. Now, in such cases 

 nothing more than single elective attraction most 

 probably occurs, and the carbonic acid is seized by the 

 lime, by the great affinity of that earth for carbonic 

 acid, only after it has been set free from the ammonia, 

 and then, by the force of cohesion acting with the com- 

 bining powers, the insoluble salt is precipitated, f There 

 is a curious fact in connection with this decomposition. 

 If carbonate of lime and sulphate of ammonia are mixed 

 together dry, and exposed, in a closed vessel, to a red 

 heat, sulphate of lime and carbonate of ammonia are 



* Of these tables of attraction the following may be taken as a 

 specimen : 



SULPHURIC ACID. 

 Baryta. 

 Strontia. 

 Potassa. 

 Soda. 

 Lime. 

 Magnesia. 

 Ammonia. 



It thus appears that baryta separates sulphuric acid from its 

 compounds with all inferior substances, and that ammonia is 

 separated from the acid by all that are above it. 



f Bertbollet : Essai de Stotique Chimique, 1803. Sir Humphry 

 Davy, in his Elements of Chemical Philosophy, has given an ex- 

 cellent review of the views of Berthollet. 



