AGENCY OP WATER. 96 



mus, oi" vegetable mould of soils. It is also decomposed by 

 the mineral constituents of the soil, and by the putrefaction 

 of animal matters ; hence we may explain the fact, that soils, 

 by being left to rest for a few years, have their fertility re- 

 stored. This is principally due to the oxygen of the water 

 and of the air. This agency of water in the decay of vegeta- 

 ble matter in the soil, is somewhat remarkable, as its hydro- 

 gen combines directly with the oxygen of the air, which, with 

 the hydrogen of the vegetable, return more water to the soil 

 than is abstracted. In the process of germination, water, as 

 we have seen, is decomposed, and yields its oxygen to the car- 

 bon of the germ. 



3. Mechanical agency of icater. (1) The first effect of 

 water, in this respect, is to penetrate the outer covering of the 

 seed, and to divide the soil so as to permit the roots of plants 

 to extend themselves freely in every direction. 



(2) The second is to convey to the roots the matter which 

 it holds in solution. In this latter respect it is equally useful 

 with the atmosphere itself In passing over rocks it wears off 

 their particles, which, with portions of the soil, and other mat- 

 ters, remain mechanically suspended in it. This matter is 

 spread over the valleys by the overflowing of streams. Wa- 

 ter is thus constantly at work in wearing down the mountains 

 and bringing down their valuable contents upon the plains, or 

 forming new land by the sides, or at the mouths of rivers. In 

 this way, it becomes the greatest fertilizer known. 



(3) When however it flows through soils charged with some 

 metallic salt, like the sulphate of iron {copperas), it proves in- 

 jurious to vegetation, and lime, or some alkali, must be added 

 to decompose the salt, and destroy its corrosive or poisonous 

 properties. 



4. Agency of water as nutriment. In addition to the agen- 

 cy of water above described, it is used by plants as food. We 

 know that it constitutes a large portion of the juices of plants, 

 and, although a large portion of that which enters the roots is 



