120 THE ART OF CULTURE. 



completely lost the property of giving a colour to 

 water. 



The soluble substance, which gives to water a 

 brown colour, is a product of the putrefaction of 

 all animal and vegetable matters ; its formation is 

 an evidence, that there is not oxygen sufficient 

 to begin or at least to complete the decay. The 

 brown solutions, containing this substance, are 

 decolourized in the air, by absorbing oxygen, and 

 a black coaly matter precipitates the substance 

 named " coal of humus." Now if a soil were im- 

 pregnated with this matter, the effect on the roots 

 of plants would be the same as that of entirely de- 

 priving the soil of oxygen ; plants would as little 

 be able to grow in such ground, as they would if 

 hydrated protoxide of iron were mixed with the 

 soil. All plants die in soils and water which con- 

 tain no oxygen ; absence of air acts exactly in the 

 same manner as an excess of carbonic acid. Stag- 

 nant water 'on a marshy soil excludes air, but a 

 renewal of water has the same effect as a renewal 

 of air, because water contains it in solution. If 

 the water is withdrawn from a marsh, free access 

 is given to the air, and the marsh is changed into 

 a fruitful meadow. 



In a soil to which the air has no access, or at 

 most but very little, the remains of animals and 

 vegetables do not decay, for they can only do so 

 when freely supplied with oxygen ; but they undergo 

 putrefaction, for which air is present in sufficient 



