PRACTICAL INFERENCES. 281 



be encouraged to extend its application to other crops 

 besides the Leguminosae, and also to mix it with the dung 

 of our stables, so as to prevent the waste of this valuable 

 material, which is constantly occurring. (See p. 191.) 



*' But the farmer must be reminded, that it will be neces- 

 sary, that the sulphate of ammonia resulting from the 

 action of the gypsum, should be brought into contact with 

 some substance capable of slowly decomposing it, so as to 

 supply ammonia to the plant. 



*'For there is no reason to believe, that the organs of a 

 vegetable can decompose sulphate of ammonia, and if they 

 were able so to do, the disengagement of free sulphuric 

 acid in consequence could hardly fail to be injurious to 

 their structure. 



*'Now a soil consisting of pure sand, or of clay, would 

 be incapable of acting upon this salt, but contradictory as 

 it may seem to the fact, that carbonate of ammonia is 

 decomposed by sulphate of lime, carbonate of lime does 

 appear in a slight degree to disengage ammonia even in 

 the cold, as may be seen by the change of color produced 

 in a piece of turmeric or reddened litmus paper, placed 

 over a vessel containing powdered chalk, as soon as it is 

 moistened with a solution of sulphate of ammonia. 



**And since this interchange of constituents is effected 

 rapidly under the influence of a high temperature, as hap- 

 pens in the common method of obtaining carbonate of 

 ammonia artificially by double decomposition, it is worth 

 inquiry, whether it may not be favored likewise by exposure 

 to solar heat and light. 



** Where calcareous matter, therefore, exists in the soil, 

 ammonia may be slowly supplied in this manner to the 

 growing plant, and it is possible even, that the carbonate 

 of lime, which seems to be generally present in the sap, 

 may act in the same manner. 



'*In this way we may readily explain the use of scatter- 

 ing gypsum over the leaves of clover shortly before a 

 shower of rain. The ammonia present in the latter is thus 

 detained, and converted into sulphate by the action of the 

 gypsum upon it, and when introduced into the system by 

 the absorbing surfaces of the plant, it may be again con- 

 verted into carbonate, by the slow action of the carbonate 

 of lime present in the sap. 



*' When, however, a more rapid disengagement of am- 

 moniacal gas is required for the nutrition of the intended 

 crop, we ought not to trust to the slow action of carbonate 



24* 



