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Mr. Ruckert is perfuaded that earth and water in proper 

 proportions form the fole nutriment of plants ; but Mr. Giobert 

 has clearly fiiewn the contrary, for having mixed pure earth of 

 alum, filex, calcareous earth and magnefia in various propor- 

 tions, and moiftened them with water, he found that no grain 

 would grow in them ; but when they were moiftened with water 

 from a dunghill corn grew in them profperoufly *. Hence the- 

 neceflity of the carbonic principle is apparent. 



The abfolute quantity of earth in vegetables is very fmalL 

 Dr. Watfon informs us that io6 avoirdupoife pound = 1696 ozs. 

 of oak, being carefully burned, left but 19 ozs. of afhes, and from 

 thefe we muft dedudt 1,5 for fait, then the earthy part amounts 

 only to 17,5. that is, little more than one per cwt. The com- 

 miflioners appointed to infped the faltpetre manufa£lory found 

 nearly the fame refult, namely 1,2 per cwt. in beech 0,453, and 

 in fir only 0,003. Hence we need not wonder at trees growing, 

 among rocks where fcarce any earth is to be feen ; but in the 

 ftalks of Turkey wheat or maize they found 7 per cwt. of earth, 

 in fun-flower plant 3,7 f ; fb that, upon the whole, weeds and 

 culmiferous plants contain more earth than trees do. Mr. 

 Weftromb found trifolium pratenfe to contain about 4,7 per cwt.- 

 of earth, of which 2 per cwt. was mild calx, nearly 2 more filex, 

 0,7 argill, together with a fmall proportion of phofphorated iron, 

 calx of iron and manganefe J^ 



Since. 



* 



Encyclop. Vegetation^ 274. f See 3 Tranf. Royal Irifli Academy. 



X iftChy. An. 1787. 



