On Calcarious and Gypjious Earth's, 53 



difcover a great number of fmall black fpecks, particularly on 

 fandy grounds that have been wet. It is alfo known that beds 

 of oyfter-fiiells, and the thin ftratum of earth that covers lime- 

 ilone rocks, is always black, like the richeft vegetable mould. 

 Now as thefe fubltances are white when reduced to powder, 

 from whence can they derive the power of rendering the earth 

 in contaQ; with them black ? Unlefs in their decompofition they 

 attraft oils from the air, or communicate the phlogiflon they 

 contain to the moift earth, as the chalk appeared to do to the 

 vinegar in the inftance I have mentioned. We may add to 

 this, that ground which lays over a ftratum of limeftone rock 

 is lefs fubje6l to froft, and thaws earlier than any other foil. Both 

 oils and fait have a confiderable power in refifting froft. 



Should the fyftem I have endeavoured to eftablifli be true, 

 it will follow that Calcarious earths are very permanent manures 

 in proportion to the quantity employed. For as I have before 

 obferved, if this is fmall it muft be frequenly renev/cd, becaufe 

 this earth is foluble in v/ater, and will be carried off by it, or 

 imbibed by the plants themfelves. 



As far as experiment has gone, this opinion of its duration is 

 fully juftified. Oyfter fliells, craig, marie, laft for ages in full 

 vigor ; thefe are all different modiiications of this earth. 



Whether my ideas on this fubjefl: are juft or not, I confefs 

 I take a pleafure in thinking them fo. I clafs in my own mind 

 this eifed of Calcarious earths with the provifion which nature 



