32, Operation of Manures. [Book VI I L 



Different theories have been advanced, to account 

 for tne operation of manures in promoting the growth 

 of vegetables, none of which feem altogether fatisr 

 factory. The common opinion is, that the fubftances 

 employed a^ manures are the food of plants, and are 

 abforbed by their roots. This hypothefis may be true 

 to a certain extent, when applied to forne manures, but 

 cannot be true with regard to them all ; for it is well 

 known, that riot only chalk and lime, but even flints, 

 are very beneficial to fome foils. Another opinion is, 

 that manures act by bringing foils to fuch a confiftence 

 as is favourable to the growth of the roots of vegeta- 

 bles, and to the affording of them water in a proper 

 quantity. A third opinion is, that manures act as fti- 

 muli on the roots of vegetables, and thus excite them 

 to more vigorous action. Some authors think that 

 manures act as folvents on matters previoufly contained 

 in the foil, and thus fit them for entering the roots of 

 plants ; and others, that they act chemically, by form- 

 ing combinations which are favourable to vegetation; 

 "Which of thefe hypothefes is beft founded, it is difficult 

 to determine j but it does not feem unlikely that they 

 may be all true to a certain extent.- 



When we attempt to difcover the component prin- 

 ciples of the objects around us, and the fources whence 

 they are fupported, we are loft in the greatnefs and 

 diverfity of the fcenes prefented to us. We fee animals 

 nourifhed by vegetables, vegetables apparently by the 

 remains of animals, and foffils compofed of the relics 



allum, -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 

 fronk a dunghill, corn grew in them profperoufly. Hence the 

 jieceffity of the carboi.ic panciple is apparent:" Kir^an oft 

 Manures, p. 42. 



of 



