158 FUNCTIONS AND EFFECTS OF THE 



b The carbonate of soda of the shops effloresces, or falls to a 

 dry white powder, when exposed to the air at ordinary tem- 

 peratures. This is owing to its giving off water, instead of 

 attracting it from the air as carbonate of potash does. It has 

 a disagreeable alkaline taste, and dissolves in twice its weight of 

 cold, and in less than its own weight of boiling water. 



These two carbonates possess in common the property of 

 dissolving vegetable matter in the soil, and of disposing it to 

 decompose and become soluble in water. They also dissolve 

 silica, decompose mineral substances, and dispose the sulphur of 

 the soil* to form sulphuric acid, and combine with them. They 

 thus form soluble sulphates, silicates, &c., which they convey 

 into the plant. 



3. Functions of these carbonates. 



Thus their functions may be said to be fourfold, 



First) To dispose certain other substances to undergo peculiar 

 chemical changes in the soil. 



Second^ To convey these substances organic acids, sulphuric 

 acid, silica, &c. from the soil into the plant. Under this head 

 it ought particularly to be noticed, that though solutions of 

 these carbonates do not dissolve lime and magnesia directly, 

 yet that, when they have previously dissolved a portion of the 

 ulmic or humic acid of the soil, they have the power of taking 

 up sensible quantities of lime and magnesia also, and of thus 

 fitting them for entering into the roots of the plant. 



Thirdj To supply directly the potash and soda which the 

 plant requires to form or modify its substance. 



Fourth) To perform, promote, or aid in the production of 

 those chemical changes which are continually going on in the 

 interior of the plant, and which are necessary to its growth. 



In regard to the functions which the two carbonates are 

 capable of performing in the soil, there is reason to believe that 

 they may take the place of each other without inconvenience 

 to the plant that it is a matter of comparative indifference, 

 therefore, whether we apply the one or the other to the soil. 

 How far this is the case with the functions they respectively 



* That contained in the sulphuret of iron of the soil, for example, which, 

 slowly combining with the oxygen of the air, forms sulphuric acid. 



