THE planter's GUIDE. 



135 



Water , moreover, is known to be a condenser and solvent 

 of carbonic acid gas, which, when the ground is open, can 

 be carried immediately to the roots of vegetables, and 

 probably contributes to their growth. But if the land be 

 close, and the water lie on or near the surface, then the 

 carbonic acid gas, which always exists in the atmosphere, 

 and is carried down by the rains, will soon be dissipated. 

 Let it be observed also that an open soil, besides being 

 favourable to the transmission of nutriment to the roots 

 of plants, is likewise favourable to their extension, and 

 thereby enlarges the field whence nutriment is derived. 

 Nor are these the only benefits resulting from a friable 

 soil : for, in addition to its being the best adapted to supply 

 vegetables with food, it is always most suitable for effect- 

 ing those changes in the manure itself which are equally 

 necessary to the preparation of such food ; and animal 

 and vegetable substances, exposed to the alternate action 

 of heat, moisture, light, and air, undergo spontaneous 

 decompositions, which independently of it would not take 

 place. 



Soils are surprisingly benefited by aeration, and the free 

 admission of the weather into their interior parts. This 

 is generally considered as the principal use of fallowing ; 

 and its importance in gardening is proved by compost 

 heaps, and both winter and summer ridging up. The 

 precise advantages, however, of exposure to the air, inde- 

 pendently of the concurrent influence of water, heat, and 

 the other eflPects above mentioned as resulting from pul- 

 verisation, do not seem at present to be fully ascertained. 

 It is admitted on all hands that carbonic acid gas is 

 absorbed by calcareous earths ; and Dr Thomson, who is 

 among the ablest of our chemical writers, is of opinion 

 that the earths alone may thus, in all likelihood, administer 

 food to plants. Sir H. Davy seems to consider mere 



