364 VEGETABLE MOULD. 



full of air, it extracts the oxygen therefrom with 

 greater rapidity than decayed wood, and replaces it 

 by an equal volume of carbonic acid. When this 

 carbonic acid is removed and fresh air admitted, 

 the same action is repeated. 



Cold water dissolves only To.oogth of its own weight 

 of vegetable mould ; and the residue left on its 

 evaporation consists of common salt with traces of 

 sulphate of potash and lime, and a minute quantity 

 of organic matter, for it is blackened when heated 

 to redness. Boiling water extracts several sub- 

 stances from vegetable mould, and acquires a yellow 

 or yellowish brown color, which is dissipated by 

 absorption of oxygen from the air, a black flocculent 

 deposit being formed. When the colored solution is 

 evaporated, a residue is left which becomes black on 

 being heated to redness, and afterwards yields car- 

 bonate of potash when treated with water. 



A solution of caustic potash becomes black when 

 placed in contact with vegetable mould, and the ad- 

 dition of acetic acid to the colored solution causes no 

 precipitate or turbidness. But dilute sulphuric acid 

 throws down a light flocculent precipitate of a brown 

 or black color, from which the acid can be removed 

 with difficulty by means of water. When this pre- 

 cipitate, after having been washed with water, is 

 brought whilst still moist under a receiver filled with 

 oxygen, the gas is absorbed with great rapidity; and 

 the same thing takes place when the precipitate is 

 dried in the air. In the perfectly dry state it has 

 entirely lost its solubility in water, and even alkalies 

 dissolve only traces of it. 



It is evident, therefore, that boiling water extracts 

 a matter from vegetable mould, which owes its solu- 

 bility to the presence of the alkaline salts contained 

 in the remains of plants. This substance is a pro- 

 duct of the incomplete decay of woody fibre. Its 

 composition is intermediate between woody fibre and 

 humus, into which it is converted, by being exposed 

 in a moist condition to the action of the air. 



