108 MAGNESIA. 



decomposition must take place, the carbonic 

 or other gas being liberated or assimilated, 

 and the magnesia united with the phosphoric 

 acid derived from the lime, or from other 

 sources, as has been explained when speaking 

 of phosphorus. 



Magnesia exists also abundantly in lime- 

 stone, and care should be taken by every per- 

 son about to apply a dressing of lime to his land 

 to know whether the stone or chalk from which 

 it may be burnt, contains magnesia or not, as 

 its use combined with lime is very prejudicial 

 to many soils, whilst on others, its use would 

 be equally beneficial. 



Sir Humphrey Davy explains satisfactorily 

 the reasons why the magnesian limestone is to 

 be avoided, or used with great care as a ma- 

 nure. The reasons he gives are, " That the 

 magnesia of the limestone is much sooner de- 

 prived of its carbonic acid than the lime; and, 

 that when applied to land, it rapidly absorbs 

 the gas from the surrounding humus: but if 

 there should not be a sufficient supply of this 

 carbon, the magnesia will then remain in its 

 caustic state, and in this state it acts as a poi- 

 son to many vegetables. '^ 



Mr. Johnson also, in his able work before 

 quoted, has proved that such is the effect of 

 the magnesian limestone, and acting on Sir 

 Humphrey Davy's experiment, he judiciously 

 recommends its use to restore peaty soils, and 

 vice versa^ the use of peat as a remedy to land 

 where the magnesian limestone has been ap- 

 plied. 



