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Since plants derive fome proportion of earth from the foil 

 on which they grow we cannot be furprifed that thefe foils 

 Should at length be exhaufted by crops that are carried oiF, fuch 

 as thofe of corn and hay, particularly the former ; even lands 

 paflured muft at laft be exhaufted, as the excrements of animals 

 do not reftore the exad quantity that the animals have confumed ; 

 and hence the utility of mucks, as the reftoration is performed 

 by more animals than have been employed in the confumption. 

 Hence alfo a fucceflion of different crops injures land lefs than 

 a fucceflion of crops of the fame kind, as different proportions 

 of the different earths are taken up by the different vegetables. 

 Finally, we may hence derive the utility of marling land, as the 

 deficient earths are thereby replaced. This fubjed admits of 

 more precifion than has been hitherto imagined, and may even 

 be fubjeded to calculation. The abfolute quantity and rela- 

 tive proportions of the various earths in an acre of land may 

 be determined, fo may that in the crops of different vegetables, 

 and by comparing both, the time alfo may be found in which 

 the land muft be exhaufted, unlefs renovated by various manures ; 

 thus the neceffity of marling, the kind of marie or other ma- 

 nures, and the quantity neceffary to an acre of land may be very 

 nearly afcertained. 



Earths cannot enter into plants but in a ftate of folution, or 

 at Icaft only when fufpendcd in water in a ftate of divifion as 

 minute as if they had been really diffolved. That filiceous earth 

 may be fufpended in fuch a ftate of divifion appears from various 

 experiments, particularly thofe of Mr. Bergman, who found it 

 thus diftufed in the pureft waters of Upfal ; and it is equally cer- 

 tain 



