PLASTER OF PARIS. ITS ACTION. 103 



temperature, and so husbands the ammonia for the future 

 use of plants. This takes place because ammonia and 

 sulphuric acid have a greater mutual attraction than 

 ammonia and carbonic acid. The ammonia, therefore, 

 leaves the carbonic acid with which it has been united, 

 and unites with sulphuric acid, to form sulphate of 

 ammonia ; and the lime, deprived of the sulphuric acid, 

 unites with carbonic acid, to form carbonate of lime. 

 This is more clearly shown by the following diagram : 



SULPHATE ( Sulphuric Acid, Sulphate of Ammonia. 



OF 



LIME. (Lime, 



CARBONATE ( Ammonia, . . . . 



OF &amp;lt; 

 AMMONIA. ( Carbonic Acid, Carbonate of Lime. 



344. The carbonate of ammonia comes from the air, in 

 which it is formed by the combination of the carbonic 

 acid always floating there, with the ammonia always form 

 ing by the union of hydrogon and nitrogen. Or it may 

 be formed in the earth. 



345. But when and how should plaster be applied? 

 When a soil does not contain naturally any sulphate of 

 lime, or when it has been exhausted by cropping, the 

 addition of that substance may prove of great value in 

 two ways ; 1st, by furnishing food for the plants men 

 tioned, and 2d, by fixing the ammonia of the atmosphere 

 and laying it up in store for the future use of plants by 

 decomposing, as shown above, the carbonate of ammonia 

 contained in rain water, and making soluble sulphate of 

 ammonia and carbonate of lime. 



When applied, plaster should be scattered, in the shape 

 of the finest, impalpable powder, in the spring, just as 



vegetation is beginning, while the dew of the morning or 

 10 



