128 PRODUCTION OF CARBONIC ACID FROM VKGFTA15LK MOULD. 



rating carbonic acid, in the progress of its decomposition. Tins 

 is dissolved by the fluid of the soil, and is taken up by the 

 roots. The supply of carbonic acid thus obtained, seems chiefly 

 important to the plant, when its leaves are undeveloped ; as is 

 the case in the early stages of its growth, as well as in every 

 succeeding spring, with all but evergreens. For it will hereafter 

 be shown, that it is almost entirely through their leaves, that 

 plants obtain carbon from the atmosphere ; and when these art? 

 fully expanded, the absorption of carbonic acid by the roots may 

 be dispensed with. 



185. But the decomposition of the Vegetable matter of the 

 soil, requires the free access of air to every part of it. If any 

 substance, however rapid its tendency to decay, be completely 

 secluded from the atmosphere, little or no change in it will take 

 place.* Every particle of the soil needs to be surrounded with 

 oxygen, for the production from it of carbonic acid ; and to pro- 

 duce this condition, is one of the chief objects which is effected, 

 by tilling and loosening the soil. In this respect it is manifest, 

 that a clayey soil is inferior to all other kinds ; and its injurious 

 character can only be remedied, by admixture with other sub- 

 stances, or by laborious cultivation. The necessity of unimpeded 

 access of air to the part of the ground, through which the roots 

 are distributed, is shown in an interesting manner, when trees 

 are planted too deep in the soil, or when their roots have been 

 covered with an additional quantity of earth. If the tree be old 

 or sickly, it generally dies ; but if it be vigorous, it sends out a 

 new set of roots nearer the surface, and the extension of the old 

 ones ceases. 



186. Notwithstanding, however, the gradual conversion of 

 the carbon contained in vegetable mould, into carbonic acid, and 

 the absorption of this by the roots, the quantity of carbon in a 

 soil which supports a flourishing vegetation, is progressively 

 increasing, rather than diminishing. The addition takes place 

 in several ways. The roots themselves throw out (as already 



* It i8 on this principle, that various articles of food arc now preserved for sub- 

 sequent use, in tin cases completely closed ; and possess their perfect flavour, 

 after exposure to all varieties of temperature, for several years. 



