MANUAL OF AGEICULTURE. 227 



2d. InorganiC; comprising Silicon, Aluminium, Potassium, 

 Sodium, Calcium, Magnesium, Phosphorus, Sulphur, Iron, Man- 

 ganese, Chlorine, Bromine, Iodine, and Fluorine. 



Oxygen. — Of all the elements, oxygen occurs the most abun- 

 dantly throughout nature. It exists free in the atmosphere, of its 

 total bulk contributing l-5th part. In combination with other 

 elements, it constitutes one-half the weight of the solid globe, 

 and 8-9ths of that of water. It is an invisible gas, tasteless, and 

 without smell. Excepting fluorine, it enters into combination 

 with all the elements. In this process, called oxidation, heat is 

 always, light sometimes, evolved. Flame consists of gas in. a high 

 state of ignition, caused by the oxidation of the substance con- 

 sumed. During combustion, new chemical compounds, chiefly 

 gaseous, are being formed, but no element is annihilated. 

 Animals inhale oxygen into their lungs, where it oxidises certain 

 elements in the blood and tissues, and thus keeps up the degree 

 of warmth necessary for life. Oxidation is much more rapid in 

 undiluted oxygen than in the atmosphere. 



Hydrogen is another invisible gas, devoid of taste and 

 smell. Its principal combination is with oxygen, in the form 

 of w^ater — two atoms of hydrogen to one of oxygen, — whence its 

 chemical symbol H^O. Hydrogen is the lightest of all the 

 elements, and it is taken as the unit with which to compare the 

 others. The symbolical letters, it may be remarked, representing 

 the different elements, represent also their combining weights, or 

 the weight of their respective atoms compared with hydrogen's. 

 An atom, or any volume of oxygen, is IG times the weight of an 

 atom or equal volume of hydrogen ; and as water is composed of 

 two parts of hydrogen to one of oxygen, the latter constitutes 

 8-9ths of the w^eight and a third part of the volume of water. 

 Hydrogen has been found free in sundry volcanic gases, and it 

 can be obtained from the decomposition of water, through the 

 agency of certain metals.. Water enters into combination with 

 many substances, and in so doing, in almost every instance, one 

 of the atoms of hydrogen, in the molecule of water, is replaced 

 by some equivalent in the compound into which it enters, and 

 free hydrogen is given off. 



Nitrogen is also an invisible, inodorous, tasteless gas. 

 It exists free in the atmosphere, mixed with oxygen, forming 

 about 4-5ths of the bulk of atmospheric air. It is a most. 

 inert element, incapable of entering into direct combination witli 

 any other except oxygen, and even then with dithculty, and only 

 by means of the electric spark. Piy very indirect processes, bow- 

 ever, it enters into important combinations with hydrogen as well 

 as with oxygen. It forms five several oxides, the principal one 

 of which is a combination of 5 atoms of oxygen with 2 of nitrogen 

 — ^2^5 ' which, combined with a molecule of water, coustinites 



