The Advantage* of Chemical Science. 37 



cation of manures, are placed within the power of man as if 

 lor the purpose of awakening his industry and of calling forth 

 his powers. 



The theory of the general operation of the more compound 

 manures may be rendered very obvious by simple chemical • 

 principles, but there is still much to be discovered with regard 

 to the best methods of rendering animal and vegetable sub- 

 stances soluble, and the chemistry of the more simple manures, 

 the manures which act in very small quantities, such as gypsum, 

 alkalies, and various saline substances, has hitherto been ex- 

 ceedingly obscure. It has been generally supposed that these 

 materials act in the vegetable economy in the same manner as 

 condiments or stimulants in the animal economy, and that the y 

 render the common food more nutritive. It seems, however, a 

 much more probable idea that they are actually a part of the 

 true food of plants, aud that they supply that kind of matter to 

 the vegetable (ihre which is analogous to the bony matter in 

 animal structures. 



Hie chemical theory of Fallowing affords no new sourc'" 1 of 

 riches to the soil ; it merely tends to produce an accumulation 

 of decomposing matter, which, in the common course of crops, 

 would be employed, as it is formed, and it is scarcely possible 

 to imagine a single instance of a cultivated soil which can be 

 supposed to remain fallow a year with advantage to the farmer. 

 The only cases where this practice is beneficial, seems to be in 

 the destruction of weeds and for cleaning foul soils. 



Paring and burning is destructive to vegetable matter, and 

 must be principally useful in cases in which there is an excess 

 of this matter in soils. The instances in which it must be ob- 

 viously prejudicial, are those of sandy diy siliceous soils con- 

 taining little animal or vegetable, there it can only be destruc- 

 tive, for it decomposes that on which the soil depends for its 

 productiveness. 



The advantages of Irrigation act not only by supplying use- 

 ful moisture to the grass, but likewise the water carries nourish- 

 ment dissolved in it, and defends the roots from the effects of 

 cold. 



No general principles can be laid d6wn respecting the com- 

 parative merits of the different systems of cultivation, and the 

 different systems of crops adopted in different districts, unless 

 the chemical nature of the soil, and the physical circumstances 

 to which it is exposed, arc fully known. Stiff coherent soils 

 are those most benefited by minute division and aeration, and 

 ir^ the drill system of husbandry these effects are produced to 

 the greatest extent, but still the labour and expense connected 



