469 



heat. Without the aid of chemistry, they are inexpUcable ; with it, 

 they become perfectly inteUigible. The vital action creates nothing. 

 It does not produce carbon, oxygen, or hydrogen ; but it puts them in- 

 to activity; and they then arrange themselves according to chemical 

 principles ; each organ of the plant having its own specific influence 

 in the production of the results. 



The author discusses, at large, the nature and action of humus. 

 Humus is merely decayed vegetable substance, whose decay or de- 

 struction is effected by the absorption of oxygen from the air. Exclude 

 it from the external air, and the decay would cease ; but would be 

 renewed again as it should be brought in contact w'nh the oxygen of the 

 air. Woody fibre, in a state ' of decay, consists of carbon and the ele- 

 ments of water. Alkaline substances assist its decay. Humus, how- 

 ever, is not composed exclusively of woody fibre ; other substances 

 are associated with it. We have not the room to follow Liebijj- in his 

 curious and profound remarks on this subject, and can give only a sum- 

 mary of his views. The constant tendency of humus is to form car- 

 bonic acid by the abstraction of oxygen from the air. The stirring of 

 the soil, and opening it to the effects of light and heat and moisture, 

 assist this process, by bringing it in contact with the decaying humus. 

 It forms around itself an atmosphere of carbonic acid, and supplies 

 carbonic acid to the plant in the first period of its growth. The roots 

 of the plants, in the beginning and before the formation perform the 

 functions of the leaves. They extract from the soil the carbonic acid 

 generated by the humus. When a plant is matured, and when the or- 

 gans by which it receives its food from the air, are perfected, the car- 

 bonic acid of the soil is no further required. Humus does not afford 

 nourishment to plants, by being taken up into their vessels in an unal- 

 tered state ; but only by the supply of carbonic acid, which it generates 

 from the presence of atmospheric air. 



Hydrogen is another constituent of plants ; for woody fibre is com- 

 posed of carbon and the elements of water. Water is decomposed 

 under the power possessed by plants of separating its elements, and of 

 assimilating its hydrogen, and dispensing with that portion of its oxy- 

 gen not required by the plant in other processes of its growth. Nitro- 

 gen is another constituent, found in all plants; abounding in some, and 

 supposed to form the principal portion of the nutritive properties of 

 some of the cereal grains. The nitrogen of the air cannot enter into 

 combination with any element excepting oxygen. The combination of 



