AND WITH LIMES WHICH CONTAIN IT. 235 



finding sulphate of magnesia in the soil and waters of the county 

 of Durham, that the large production of this salt in a soil to which 

 a magnesian lime is added, may be another cause of its alleged 

 burning effect upon crops. Iron pyrites, and other compounds of 

 sulphur, exist abundantly in many soils. These are continually 

 undergoing oxidation, and giving rise to the production of sul- 

 phuric acid, which, with lime, forms gypsum ; and with mag- 

 nesia, sulphate of magnesia. 



It is the prevailing opinion in northern America, where gyp- 

 sum is extensively used in agriculture, that an excess of this 

 .substance in the soil does no harm 5 but such is not likely to be 

 the case with the very soluble sulphate of magnesia. Like 

 other very soluble saline matters, when they abound in the 

 soil, it will produce injurious effects if it be absorbed by growing 

 plants in too large a quantity. Upon those soils, therefore, in 

 which sulphur in any state of combination exists in larger quan- 

 tity than is usual, magnesia may produce more speedy and more 

 marked injurious effects than upon others. This is a point 

 which is capable of being tested by experiment, and is deserving 

 of investigation. I would suggest that experiments should be 

 made. 



1. With caustic, or calcined magnesia, applied alone, in 

 different proportions, to different crops, and at different 

 seasons. 



2. With carbonate of magnesia, applied alone, in proportions 

 and circumstances equally varied. 



3. With caustic magnesia, in comparison with carbonate of 

 magnesia, applied in equivalent proportions. Are their effects 

 different in kind or only in degree, as is the case with carbonate 

 of lime in comparison with quicklime ? 



Carbonate of magnesia is not recommended here for the first 

 time. In consequence of my previous recommendations, it has 

 been tried in numerous experiments by Mr Gardiner, at 

 Barochan in Renfrewshire,* and by Mr Main, at Whitehill in 

 Mid-Lothian.f It was never applied alone by these gentlemen, 



* Transactions of the Highland Society, March 1845, p. 411, et seq. } and July 

 1847, p. 20, et seg. 

 f Ibid. March 1849, p. 532, et seq. 



