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Magnesia is never found alone; its diftinguifliing characfler 

 confifls in affording a bitter fait, generally called Epfom fait, 

 when combined with the vitriolic acid. 



Argtll is that part of clay to which this owes its property 

 of feeling foft and undluous, and of hardening in fire ; it is 

 difficultly foluble in acids, and fcarce ever effervefces with them ; 

 when combined with the vitriolic acid it forms alum. 



Siliceous earth is often found in a flony form, fuch as 

 flint or quartz, and flill more frequently in that of a very fine 

 fand, fuch as that wheceof glafs is made ; it does not effervefce, 

 nor is it foluble in any of the common acids.. 



To thefe we may add Iron, in that imperfcdl flate in which 

 it exifts when reduced to ruft, and commonly called calx of 

 iron. • 



The foils moft frequently met with, and which deferve a 

 diftindl confideration, are clay, chalk, fand and gravel, clayey 

 loam, chalkey loam, fandy loam, gravelly loam, ferruginous 

 loam, boggy foil, and heathy foil, or mountain, as it is often 

 calleid. 



Clay is of various colours, for we meet with white, grey, 

 brownifh red, brownifh black, yellowifh or bluifli clays ; it 

 feels fmooth and fomewhat imdluous ; if moid, it adheres to 

 the fingers, and if fufficiently fo it becomes tough and du(5tile. 



If 



