THE GREEN-HOUSE. 



139 



atmospherical changes ; earth, especially if rendered 

 pox'ous and sponge-like by culture, receives and gives 

 out air and heat slovrly ; and Avhile the temperature 

 of the air of a country may vary twenty or thirty 

 degrees in the course of twenty-four hours, the soil 

 at the depth of two inches will hardly be found to 

 have varied one degree. With respect to moisture, 

 every cultivator knows, that in a properly constituted 

 and regularly pulverized soil, whatever quantity of 

 rain may fall on the surface, the soil is never saturated 

 with water, nor in times of great drought burnt up 

 with heat ; the porous texture of the soil and subsoil 

 being at once favourable for the escape of superfluous 

 water, and adverse to its evaporation, by never be- 

 coming so much heated on the surface or conducting 

 the heat so far downwards as a close compact soil. 

 Now these properties of the soil relatively to plants 

 can never be attained by growing them in pots, and 

 least of all when these pots are so placed as to be 

 surrounded by air. In this state, whatever may be 

 the care of the gardener, a continual succession of 

 changes of temperature will take place on the outside 

 of the pot ; and the compact material of which it is 

 composed being a much more rapid conductor of heat 

 than porous earth, those changes of temperature will 

 soon be communicated to the web of roots which line 

 its interior surface. With respect to water, a plant 

 in a pot surrounded by air is equally liable to injury. 

 If the soil be properly constituted, and the pot suffi- 

 ciently drained, the water passes through the mass as 



