4^»- On the diffmnt Sorts of Ume afed In Ajrriculliire. 



much lime, as it would olherwife have been too hard to admit the feeds io penetrate 

 through it. PlaiUs will bear a much larger propoition of magneCa in vegetable foil than 

 'li\ fand ; with twenty grains, however, of calcined niagnefta in as much foil as was equal 

 in biilk to four ounces ol fand, the feeds produced only the feed leaves without any 

 roots J and with about forty grains they were entirely prevented from coming up. 



The countries where the magnefia lime is employed, it was faid that the barrennefs of 

 any fpot on which aheap of it liad been laid would continue for many years. To learn 

 how far it could by time be deprived of its injurious qualities, I procured fomc pieces 

 of mortar made of this fpecies of lime from two houfes, one ot wliich had been built 

 three and the other eight years : they were taken from the outfide of the building where 

 they had been expofcd t0 the air. After they were reduced to powder, feeds were fown 

 in them ; only a few came up, and even thofe produced merely the feed leaves without 

 any roots. As plants would grow in the lime-ftone trom which this fpecies of lime was 

 formed, although not in the mortar made from it, I wiflied to know what proportion of 

 the fixed airorigmaily contained in the lime-llone had beenabforbed by the mortar. For 

 this purpofe a piece of it was finely powdered to render it of an uniform quality; it 

 was then tried how much of this powder and of the lime-Hone would faturate the fame- 

 quantity of acid ; by this meatis I afcertained the proportions of lime-ftone and mortar 

 containing equal quantities of the magnefian lime. The fixed air being obtained 

 from them in thofe proportions and meafured in an inverted veflel with quickfilver, it. 

 was found that the mortar which had been expofed three years had abforbed 43, and that 

 of eight years only 47 hundredths of the quantity originally contained in the lime-ftone. 

 I was not able to obtain any mortar which had been made earlier, thoijgh it deferve to 

 be known how much fixed air it was ultimately capable of abforbing. Common mortar 

 which had been expofed to the air for a year and three quarters, had regained 63 hun- 

 dredths of its full quantity of fixed air. 



As the preceding experiments were tried during the winter, in a room warmed by fire, 

 perhaps under circumftances more favourable to vegetation, the fame quantity of mag- 

 nefia would not be equally pernicious. ' 



Magnefian lime-ftone may be eafily diftinguiflied from that which is purely calcare- 

 6us by the flownefs of its folution in acids, which is fo confiderable, that even the fofteft 

 kind of the former is much longer in diflblving than marble. From this property of 

 the magnefian lime-ftone there appeared to be reafon for fufpe£iing that the kind of 

 marble which had been called Dolomite, from M. Dolomieu, who firft remarked its- 

 peculiarity in diflblving flowly, might alfo be fimilar in. its compofition. An analyfis 

 of this fubftance was lately given in t\\e Journal dt Phyjique, but this is probably er- 

 roneoos ; for, upon examining three fpecimens, they were found to conlift of magnefia 

 snd calcareous earth, like the laagnerian iime<iloae ; fo that it ou^ht, 110 doubt, to be 



eeOf 



