IT EVOLVES CARBONIC ACID. 65. 



stances, such as sulphurous acid, the mercurial salts, 

 empyreumatic oils, &c., cause its complete cessation. 



Woody fibre in a state of decay is the substance 

 called humus* 



The property of woody fibre to convert surround- 

 ing oxygen gas into carbonic acid diminishes in 

 proportion as its decay advances, and at last a cer- 

 tain quantity of a brown coaly-looking substance 

 remains, in which this property is entirely wanting. 

 This substance is called mould ; it is the product of 

 the complete decay of woody fibre. Mould consti- 

 tutes the principal part of all the strata of brown 

 coal and peat. 



Humus acts in the same manner in a soil permeable 

 to air as in the air itself; it is a continued source of 

 carbonic acid, which it emits very slowly. An atmo- 

 sphere of carbonic acid, formed at the expense of 

 the oxygen of the air, surrounds every particle of 

 decaying humus. The cultivation of land, by tilling 

 and loosening the soil, causes a free and unob- 

 structed access of air. An atmosphere of carbonic 

 acid is therefore contained in every fertile soil, and 

 is the first and most important food for the young 

 plants which grow in it. 



In spring, when those organs of plants are absent 

 which nature has appointed for the assumption of 

 nourishment from the atmosphere, the component 

 substance of the seeds is exclusively employed in 

 the formation of the roots. Each new radicle fibril 

 which a plant acquires may be regarded as consti- 

 tuting at the same time a mouth, a lung, and a 

 stomach. The roots perform the functions of the 

 leaves from the first moment of their formation : 

 they extract from the soil their proper nutriment, 

 namely, the carbonic acid generated by the humus. 



By loosening the soil which surrounds young 

 plants, we favor the access of air, and the formation 



* The humic acid of chemists is a product of the decomposition of 

 humus by alkalies: it does not exist in the humus of vegetable physi- 

 olocrists. — L. 



6* 



