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If they be considered as necessary to the vegeta- 

 ble, it is as giving hardness and firmness to its organi- 

 zation. Thus, it has been mentioned that wheat, oats, 

 and many of the hollow grasses, have an epidermis 

 principally of siliceous earth; the use of which seems 

 to be to strengthen them, and defend them from the 

 attacks of insects and parasitical plants. 



Many soils are popularly distinguished as cold; 

 and the distinction, though at first view it may appear 

 to be founded on prejudice, is really just. 



Some soils are much more heated by the rays of 

 the sun, all other circumstances being equal, than 

 others; and soils brought to the same degree of heat 

 cool in different times, /. e. some cool much faster 

 than others. 



This property has been very little attended to in 

 a philosophical point of view; yet it is of the highest 

 importance in agriculture. In general, soils that con- 

 sist principally of a stiff white clay are difficultly heated; 

 and being usually very moist, they retain their heat 

 only for a short time. Chalks are similar in one res- 

 pect, that they are difficultly heated; but being drier 

 they retain their heat longer, less being consumed in 

 causing the evaporation of their moisture. 



A black soil, containing much soft vegetable mat- 

 ter, is most heated by the sun and air; and the col- 

 oured soils, and the soils containing much carbonace- 

 ous matter, or ferruginous matter, exposed under 

 equal circumstances to sun, acquire a much higher 

 temperature than pale-coloured soils. 



