CARBONIC ACID RENDERS LIME SOLUBLE. 47 



ually dissolves a portion of this earth, equivalent to the quantity of gas 

 it holds in solution, and thus reaches the surface impregnated with cal- 

 careous matter. Or it carries it in its progress below the surface to the 

 roots of plants, where its earthy contents are made available, either di- 

 rectly or indirectly, to the promotion of vegetable growth. To the lime 

 thus held in solution, spring and other waters generally owe their hard- 

 ness, and it is the expulsion of the carbonic acid, by heat, that causes 

 the deposition of the sediment so often observed when such waters are 

 boiled. 



I propose hereafter to devote an entire lecture to the consideration of 

 the action of lime upon land, as it is employed for agricultural pur- 

 poses, but I may here remark, that this solvent action of the carbonic 

 acid in rain water is one of the principal agents in removing the lime 

 from your soils, and in rendering a fresh application necessary after a 

 certain lapse of time. It is the cause also of that deposit of calcareous 

 matter at the mouths of drains which you not unfrequently see in lo- 

 calities where lime is laid abundantly upon the land. The greater the 

 quantity of rain, therefore, which falls in a district, the less permanent 

 will be the effects of liming the land — the sooner will it be robbed of 

 this important element of a fertile soil. Still carbonic acid is only one 

 of several agents which act almost unceasingly in thus removing the 

 lime from the land, a fact I shall hereafter have occasion more fully to 

 explain. 



In nature, carbonic acid is produced under a great variety of circum- 

 stances. It is given off from the lungs of all animals during respira- 

 tion. It is formed during the progress of fermentatiort. Fermented li- 

 quors owe their sparkling qualities to the presence of this gas. Dur- 

 ing the decay of animal and vegetable substances in the air, in com- 

 post heaps, or in the soil, it is evolved in great abundance. In certain 

 volcanic countries it issues in large quantity from springs and from 

 cracks and fissures in the surface of the earth ; while the vast amount 

 of carbon contained in the wood and coal daily consumed by burning, 

 is carried up into the atmosphere, chiefly in the form of carbonic acid. 

 We shall hereafter consider the relation which exists between these 

 several sources of supply and the proportion of carbonic acid per- 

 manently present in the air and so necessary to the support of vegetable 

 life. 



§ 2. Oxalic acidf its properties and relations to vegetable life. 



Oxalic acid is another compound of Carbon and oxygen, which, though 

 not known to minister either to their growth or nourishment, is yet found 

 largely in the interior of many varieties of plants. In an uncombined 

 state it exists in the hairs of the chick pea. Ifi combination with potash 

 it is found in the wood sorrel (oxalis acetosella)j in the common sorrel, 

 and other varieties of rumex, — in which it is the cause of the acidity of 

 the leaves and stems, — in the roots of these plants also, in the leaves and 

 roots of rhubarb, and in the roots of torraentilla, bistort, gentian, saponaria, 

 and many others. It is this combination with potash, formerly extracted 

 from wood sorrel, which is known in commerce by the name of salt of 

 sorrel. In combination with lime it forms the principal solid parts of 



