AND WITH ALUM. 255 



If a salt of alumina be applied to the soil, therefore, we are 

 justified in anticipating that it would not be directly taken up 

 by the roots, however soluble it may be, and that whatever 

 influence it might exercise upon vegetation should be ascribed 

 mainly to the acid or other substances with which* the 

 alumina is combined. It is desirable, therefore, to make expe- 

 riments with one or other, or with both of two compounds of 

 alumina, now to be met with readily in the market. These are 

 the sulphate of alumina and common alum. 



1 Sulphate of Alumina is a colourless deliquescent salt which 

 has a strongly astringent taste, and consists of 



Sulphuric acid, . . 36.0 

 Alumina, . . . 15.4 



Water, .... 48.6 



100 



The effects of this substance should be tried in comparison 

 with those of gypsum and of sulphuric acid the same quan- 

 tity of acid in each of these forms being applied to the same 

 extent of land. The comparative experiment with sulphuric 

 acid will be especially instructive. 



2 Common alum consists of 



Potash, . . 9.9^ 



).8f 





Alumina, . 10.8 ( . ( Sulphate of alumina (dry,) 36.1 



Sulphuric acid, 33.8 ( *} Sulphate of potash, . 18.4 

 Water, . . 45.5 J (Water, . . . 45.5 



100 100 



I am not aware of any field experiments having been made 

 with alum, but there is little doubt that it would be found to 

 exhibit a specific action upon some soils and crops. 



In the case of alum, however, the presence of sulphate of pot- 

 ash in it will prevent us from ascribing the whole of any effect 

 which may be produced by it to its sulphuric acid alone. The 

 alum must, therefore, be tried in comparison with sulphate of 

 potash, and with gypsum applied at the same time, and in similar 

 circumstances. If 18J lb. of sulphate of potash do not produce 

 an equal effect with 100 lb. of alum, then the acid of the sul- 



