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abforbcd by foils, or contained in dew. Lands, which contain 

 iron in a femicalcined ftate, are thereby enabled to decompofe 

 fixed air, the iron, by the help of water, gradually attrading the 

 pure air which enters into the compofition of fixed air, as Mr. 

 Gadolin has fhewn *, a difcovery which appears to me among 

 the moft important of thefe later times; but thefe calces of 

 iron may again be reftored to their former ftate by union with 

 oleaginous fubftances, as Mr. Beaume has noticed, and this is 

 one of the benefits refulting from the application of dung before 

 it has fully putrefied f. Hence we may underfland how foils 

 become effete and exhaufted, this effedt arifing in great meafure 

 from the gradual lofs of the carbonic principle dcpofited by 

 vegetable and animal manures, and from them paffing into the 

 growing vegetables, and alfo from the lofs of the fixed air con- 

 tained in the argillaceous part of the foil, which is decompofed 

 by vegetables, and from the calcination of the ferruginous particles 

 contained in the foil. I fay in great meafure^ becaufe other 

 caufes contribute to the diminution of fertility, which fhall pre- 

 fently be mentioned. Hence alfo we fee why lands paftured 

 remain longer fertile than thofe whofe vegetable crop is carried 

 off, as much of the carbonaceous principle is reftored by the ex- 

 crements of the pafturing animals why fome crops exhauft 



more than others, becaufe corn, and particularly wheat, contains 

 more of the carbonic principle than graffes, and very little of 



its exuviae are left behind why fallows are of fome ufe, 



as the putrefadion of the roots of weeds and the abforption of 



fixed air by clays, arc thereby promoted why vegetables 



thrive 



• ift Chym. Ann. 1791. 53. 



t The affinities of coal and iron to pure air vary with the temperature. 



