PROPERTIES OF VEGETABLE MOULD. 63 



fertile soil is composed. Thus, for instance, carbonic acid, 

 ammonia, and several vegetable acids are constantly forming, 

 while decomposition proceeds in the mould or manure, and by 

 combining with the lime, iron, magnesia, silica, and other 

 mineral portions of the soil, give birth to a variety of soluble 

 salts, all fit to nourish and enter into the composition of plants. 

 No human eye has as yet been able to pierce all the mysteries 

 of these chemical changes, but so much is certain, that they 

 perfectly agree with the wants of vegetation, and that this 

 beautiful coincidence between the chemical affinities of the soil 

 and the requirements of organic life is another evident proof 

 that the various powers of nature are all instruments of har- 

 mony in the hands of one Almighty power. 



The nutritious salts thus formed would have been in a great 

 measure lost for the purposes of vegetation, if the humus had 

 not attracted them with a still greater power than that which it 

 exhibits towards water. However abundantly the rain may 

 fall, however greedily it may be absorbed by the inferior strata, 

 the soluble substances are retained by the superficial layers of 

 mould as by a sieve, and only the pure water percolates. The 

 fertility of our fields, the luxuriant foliage of our woods, the 

 perennial verdure of our meadows, depend in a great measure 

 upon this wonderful physical property ; for vegetation could 

 never have attained its full development, if mould, like sand, 

 had possessed no binding power over the nutritious particles of 

 the soil. 



The decomposition of the mould and the chemical changes 

 that take place in the mineral substances of which it is partly 

 formed, naturally proceed more rapidly, when the loosened 

 and porous earth, which at the same time allows the spongioles 

 of the roots to ramify more easily in all directions, permits the 

 air to penetrate more freely into its interstices. Hence the 

 evident utility of ploughing and digging ; but these fertilising 

 operations, which the labourer imperfectly performs with so 

 much fatigue and expense, are executed with the utmost per- 

 fection and on the grandest scale by the power of winter. 



As I have already mentioned in a previous chapter, water 

 possesses the remarkable property of expanding as soon as it 

 assumes the solid form of ice; so that when the humid soil 

 becomes hard under the influence of frost, the moisture con- 



