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ficient, the land will be " hungry," and the fertilizing salts will 

 be washed away from the roots of the plants. Instance the 

 poms soils of the coast. A very important property also of 

 clay is its power of absorbing ammonia from the atmosphere 

 and conveying it to the roots of plants. The color of clay is 

 caused by the presence of oxide of iron. 



Oxide of Iron. This substance occurs in two forms, as pro- 

 toxide, consisting of a combination of one equivalent of iron 

 and one of oxygen, and peroxide or sesquioxide, which consists 

 of two equivalents of iron to three of oxygen. The former is 

 of a dark color, constituting largely the scales on the anvil of 

 the smith, and exists in the blue clay lands. The latter is 

 familiar as iron rust, and causes the red color of most clays. 

 Lands which contain the protoxide should be frequently culti- 

 vated, so as to expose this oxide to the atmosphere so that it 

 can absorb oxygen and be converted into the harmless red 

 oxide. During this oxidation, hydrogen is set free, which then 

 combines with nitrogen to form ammonia, and as the red oxide 

 has slightly acid properties, it fixes the ammonia for plant food. 

 Soils which contain iron pyrites will have formed in them this 

 protoxide, in the form of sulphate or copperas. The presence 

 of this salt, in more than very small quantity, is poisonous to 

 plant life; if, however, it be well cultivated, or still better, if it 

 be limed, the injurious protoxide will, after a time, be converted 

 into the harmless red oxide. The use of iron in plants seems 

 to be in the formation of the green coloring matter in them. 



Lime. This, one of the most important constituents of soils 

 and also one that is very widely disseminated, is derived from 

 the limestones, which are found in nearly every geological 

 period, and also from gypsum, or land plaster, which in some 

 places occurs in large beds. Limestone is a combination of lime 

 and carbonic acid gas. When this is burnt in kilns, the carbonic 

 acid gas goes off, and a hard, white, caustic substance remains. 

 This is stone lime, or quick lime. If now water is poured on, 

 (one part of water to three of lime,) it grows very hot, swells 

 up and finally falls into an impalpable powder, called slacked 

 lime. If it continue to be exposed to the air, it again absorbs 

 carbonic acid and becomes reconverted into limestone; but it is 

 in very fine powder, and not hard and massive as was the 

 original rock. In this fine state it is readily absorbed and 

 assimilated by plants. 



Gypsum is a combination of lime, sulphuric acid, and water. 

 When this is burnt, the water is driven off, and it is called 

 plaster of Paris. In this state it reabsorbs water with great 

 avidity, and sets or becomes hard, thus making an excellent 

 material for taking casts, &c. If, however, it is burnt at too 

 high a temperature, it loses this property and is incapable of 

 reabsorbing water. 

 Magnesia. This substance resembles lime, and is generally 



