THE ATMOSPHERE. SOILS. 



II 



inorganic bases, or metallic oxides; for these 

 metalic oxides exist in every plant, and may 

 be detected in its ashes after incineration. 



Nitrogen is an element of vegetable albu- 

 men and gluten; it is a constituent of the 

 acid, and of what are termed the " indiffer- 

 ent substances 1 ' of plants, as well as of 

 those peculiar vegetable compounds which 

 possess all the properties of metallic oxides, 

 and are known as " organic bases." 



Estimated by ; ts proportional weight, ni- 

 trogen forms only a very small part of plants ; 

 but it is never entirely absent from any part 

 of them. Even when it does not absolutely 

 enter into the composition of a particular 

 part or organ, it is always to be found in the 

 fluids which pervade it. 



It follows from the facts thus far detailed, 

 that the development of a plant requires 

 the presence, first, of substances containing 

 carbon and nitrogen, and capable of yield- 

 ing these elements to the growing organism ; 

 secondly, of water and its elements; and 

 lastly, of a soil to furnish the inorganic 

 matters which are likewise essential to ve- 

 getable life. 



OP THE COMPOSITION OF THE ATMOSPHERE. 



In the normal state of growth plants can 

 only derive their nourishment from the 

 atmosphere and the soil. Hence it is of 

 importance to be acquainted with the com- 

 position of these, in order that we may be 

 enabled to judge from which of their con- 

 stituents the nourishment is afforded. 



The composition of the atmosphere has 

 been examined by many chemists with great 

 care, and the result of their researches have 

 shown, that its principal constituents are 

 always present in the same proportion. 

 These are the two gases, oxygen and nitro- 

 gen, the general properties of which have 

 been already described. One hundred parts, 

 by weight, of atmospheric air contain 23'1 

 parts of oxygen, and 76'9 parts of nitrogen; 

 or 100 volumes of air contain nearly 21 

 volumes of oxygen gas. From the exten- 

 sive range of affinity which this gas pos- 

 sesses, it is obvious, that were it alone to 

 constitute our atmosphere, and left un- 

 checked to exert its powerful effects, all na- 

 ture would be one scene of universal destruc- 

 tion. It is on this account that nitrogen is 

 present in the air in so large proportion. It is 

 peculiarly adapted for this purpose, as it does 

 not possess any disposition to unite with oxy- 

 gen, and exerts no action upon the processes 

 proceeding on the earth. These two gases 

 are intimately mixed, by virtue of a pro- 

 perty which all gasses possess in common, 

 of diffusing themselves equally through 

 every part of another gas, with which they 

 are placed in contact. 



Although oxygen and nitrogen form the 

 principal constituents of the atmosphere, 

 yet they are not the only substances found 

 in it. Watery vapour and carbonic acid gas 

 materially modify its properties. The for- 



mer of these falls upon the earth as rain, 

 and brings with it any soluble matter which 

 it meets in its passage through the air. 



Carbonic acid gas is discharged in im- 

 mense quantities from the active volcanoes 

 of America, and from many of the mineral 

 springs which abound in various parts of 

 Europe; it is also generated during the 

 combustion and decay of organic matter. 

 It is not, therefore, surprising that -it should 

 have been detected in every part of the 

 atmosphere in which its presence has been 

 looked for. Saussure found it even in the 

 air on the summit of Mont Blanc, which is 

 covered with perpetual snow, and where it 

 could not be produced by the immediate 

 agency of vegetable matter. Carbonic acid 

 gas performs a most important part in the 

 process of vegetable nutrition, the considera- 

 tion of which belongs to another part of the 

 work. 



Carbonic acid, water, and ammonia (a 

 compound of hydrogen and nitrogen) are 

 the final products of the decay of animal and 

 vegetable matter. In an isolated condition, 

 they usually exist in the gaseous form. 

 Hence, on their formation, they must escape 

 into the atmosphere. But ammonia has not 

 hitherto been enumerated among the con- 

 stituents of the air, although, according to 

 our view, it can never be absent. The rea- 

 son of this is, that it exists in extremely mi- 

 nute quantity in the amount of air usually 

 subjected to experiment in chemical analy- 

 sis ; it has consequently escaped detection. 

 But rain which falls through a large extent 

 of air, carries down in solution all that re- 

 mains in suspension in it. Now ammonia 

 always exists in rainwater, and from this 

 fact we must conclude that it is invariably 

 present in the atmosphere. Nor can we be 

 surprised at its presence when we consider 

 that many volcanoes now in activity emit 

 large quantities of it. This subject will, 

 however, be discussed more fully in anothei 

 part of the work. 



Such are the principal constituents of the 

 atmosphere from which plants derive their 

 nourishment; for although other matters are 

 supposed to exist in it in minute quantity, 

 yet they do not exercise any influence on 

 vegetation, nor has even their presence been 

 satisfactorily demonstrated. 



OP SOILS. 



A soil may be considered a magazine 01 

 inorganic matters, which are prepared by 

 the plant to suit the purposes destined for 

 them in its nutrition. The composition and 

 uses of such substances cannot, however, 

 be studied with advantage, until we have 

 considered the manner in which the organic 

 matter is obtained by plants. 



Some virgin soils, such as those of Ame- 

 rica, contain vegetable matter in large pro- 

 portion ; and as these have been found emi- 

 nently adapted for the cultivation of most 

 plants, the organic matter contained in them 



