Ike Advantages of Chemical Science. 35 



in iU nature to animal matter, and which is the substance that 

 (,'ivcs to wheat its superiority over other grain; — the next in 

 order, as to nourishing power, is oil ; then su^ar, then farina, 

 and, last of all, gelatinous and extractive matters; simple tests 

 of the relative nourishing powers of the different species of 

 food are, the relative cpjantities of these substances that they 

 aflord by analysis, and though taste and appearance must in- 

 fluence the consumption of all articles in years of plenty, yet 

 they arc less attended to in times of scarcity, and on such oc- 

 casions this kind of knowledge may be of the greatest im- 

 portance. 



All the varieties of substances found in plants arc produced 

 from the sap, and the sap of plants is derived from water or 

 from the fluids in the soil, and it is altered by or combined 

 with principles derived from the atmosphere. Soils in all cases 

 consist of a mixture of different finely divided earthy matters 

 with animal or vegetable substances in a state of decomposi- 

 tion, and certain saline ingredients. The earthy matters are 

 the true basis of the soil; the other parts, whether natural or 

 artificially introduced, operate in the same manner as manures. 

 Some earths generally abound in soils, the aluminous, the si' 

 liceous, the calcareous, and the magnesian; these earths con- 

 sist of highly inflammable metals united to pure air or oxygen, 

 and they are not, as far as we know, decomposed or altered in 

 vegetation. The great use of the soil is to afford support to 

 the plant to enable it to fix its roots and to derive nourishment 

 by its tubes slowly and gradually from the soluble and dis- 

 solved substances mixed with the earths. That a particular 

 mixture of the earths is connected with fertility cannot be 

 doubted, and almost all sterile soils are capable of being im- 

 proved' by a modification of their earthy constituent parts. 



Tull advanced the opinion, that minute earthy particles sup- 

 plied the whole nourishment of the vegetable world ; that air 

 and water were chiefly useful in producing these particles from 

 the land, and that manures acted in no other way than in 

 ameliorating the texture of the soil, in short, that their agency 

 > "was mechanical. This ingenious autlror of the new system of 

 agriculture, having observed the excellent effects produced in 

 farming, by a minute division of the soil, and the pulverisation 

 of it by exposure to dew and air, was misled by carrying his 

 principles too far. 



Duhamel adopted the opinion of Tull, and stated, that by 

 finely dividing the soil any number of crops might be raised in 

 succession from the same land. He attempted also to prove, 

 by direct experiments, that vegetables of every kind were ca- 

 pable of being raised without manure. This celebrated uciti- 



