160 REVIEW OF THE PKINCIPLE8 Or nXTIRlTlON. 



water and from the carbonic acid of the atmosphere ; hydrogen from 

 ■water and ammonia, and nitrogen from ammonia and nitric acid, drawn 

 either from the atmosphere or the soil. 



836. The atmosphere oontains about 2 j oo paft of carbonic acid, diffused through- 

 out its whole extent ; and as this gas contains 27 per cent, of carbon, it is demon- 

 etrable that the whole atmosphere contains more than 600 billions (600,000,000,000) 

 of tons of solid carbon, derived from the sources already mentioned (§ 835), — 

 an amount fuUj adequate to the demands of the vegetable kingdom. 



837. Soil consists of two classes of materials, viz. : mineral, and organic. The 

 former, called earths, consists of disintegrated and pulverized Tocks, including all 

 the various mineral substances which are found to enter into the composition of 

 plants, as potassa, soda, silica, lime, etc., all of which are more or less soluble in 

 water. The organic materials consist of the remains of former tribes of plants and 

 animals mingled with the earths ; and which, having access to tlie air, are decom- 

 posed, evolving carbonic acid and ammonia both to the air and the water. 



838. Water is composed of oxygen and hydrogen (HO) in the proportion of 8 

 to 1 by weiglit, or one atom of each to each. Having pervaded the atmosphere in 

 the state of vapor and rain, and percolated through the soil, it holds in solu- 

 tion carbonic and nitric acids, ammonia, and many of the various minerals above 

 mentioned. , 



839. Ammonia consists of nitrogen and hydrogen combined in the proportion of 

 one atom of the former to three of the latter (N H3). It arises from decaying ani- 

 mal and vegetable matter, as above stated, and is diffused through tho atmosphere 

 in the proportion of about 1 part in 10,000. 



840. Nitric acid is also believed to yield nitrogen to plants. It consists of one 

 atom of nitrogen to five of oxygen (N O3). During thunder-storms it is generated 

 in the air by lightning and brought down by rain. When combined with tho 

 bases, as potassa, soda, etc., it forms nilrates — substances known to be efficient fer- 

 tilizers in soils. 



841. Air plants. Thus it appears that water, carbonic acid and ammonia (or 

 nitric acid) may yield to plants their four essential organic elements. And since aU 

 of them are contained in the, atmosphere, some plants are capable of subsisting on 

 air alone (long moss, lichens) ; but most species are dependent on water, earth, and 

 air, and demand a copious supply. 



842. Tub external circumstances, therefore, first requisite to 

 healthy vegetation are, — 1, free access to an atmosphere which is often 

 agitated by winds ; 2, a proper supply of rain or river water ; 3, a soil 

 possessing the peculiar minerals required by the species to be grown 

 upon it, together with a certain proportion of vegetable mold. 



843. The supply. The first of these is everywhere abundantly 

 supplied by nature, and asks no aid from man. The second and third 

 are often deficient, and are to be supplied by the labors of agriculture. 

 By irrigation streams of water are turned from their natural channels to 

 add to the scanty moisture of fields parched with drought ; by drain- 

 age the inundated bog is converted into a luxuriant lawn. 



844. The object op tillage is to pulverize and lighten the too compact soil; 

 and thus expose every part to the oxygen of the air in order to hasten its decompo- 

 sition. SvisoUing, or deep ploughing, is an operation whereby that stratum of earth 



