FOEJIATION OP NATURAL SOILS. 23 



rubies ; in an uncrystallized state it is a white, tasteless, powdery substance, 

 obtained by adding a solution of carbonate of soda to alum. It constitutes a 

 large proportion of shale and slate rocks, and is a principal ingredient, in 

 combination with silica, in pipe, porcelain, and agricultural clays, to which it 

 gives tenacity and stiffness. It is rarely found in the ashes of plants, and 

 therefore not considered as directly contributing to their nourishment, 

 although useful as a mechanical agent in absorbing ammonia from the atmo- 

 sphere, and in detaining the volatile as well as the alkaline salts of manures, 

 which would otherwise be dissolved by the first heavy shower, and carried 

 into the subsoil beyond the reach of the roots of the plant. 



62. Iron, both in the blaclc or protoxide, and the red or peroxide state, 

 abounds in all soils, the red being most abundant, and easily observable from 

 the red colour it communicates. Even soils in which the protoxide obtains, 

 which are of a bluish-grey colour when brought to the surface, are changed to 

 the red colour by the atmosphere, oxygen uniting with and acting on it. 

 Oxide of iron is found in the ashes of all plants and in the blood of animals. 

 The presence of iron is easily detected in soils by the ochry deposits in the 

 beds of springs and ditches, where the oxide dissolved in carbonic acid pro- 

 duces the metallic-coloured deposit in question. Sulphate of ii'on also occurs 

 in some soils, produced from iron pyrites : such soils are unproductive ; for it 

 is a compound of sulphuric acid with protoxide of iron, better known under 

 the name of green vitriol. Lime added to such soils combines with the sul- 

 phuric acid, forming gypsum : and sweetens them and removes the injurious 

 properties. 



63. Manganese is a metal, which, in combination with oxygen, associated 

 with oxide of iron, occurs naturally in many soils. In the ashes of plants 

 traces of it are also foi;nd ; but iron usually predominates. The ash of the 

 horse chestnut and oak bark is rich in manganese, with no trace of iron. 



64. Silica, or Silex, occurs abundantly in nature, either in a free state or 

 in the form of sand, sandstones, flint, chalcedony, rock-crystal, or quartz, 

 and in combination with lime, magnesia, iron, potash, soda, and other 

 minerals. Silica is insoluble in hot or cold water, and resists the action of 

 some strong acids ; but hydrofluoric acid dissolves it, when mixed with soda 

 or potash, and exposed to the heat of a glass-furnace. Silica is dissolved, or 

 rather enters into combination with the alkali, and forms glass ; or when 

 the alkali is in excess, it dissolves into water. On the addition of muriatic 

 acid, or sulplauric acid, to a solution of this silicate of potash, the silica 

 separates into a gelatinous mass, in which form it is soluble in vrater, and 

 thus becomes the food oi plants. 



65. Suiphiir, in the iorm of sulphuric acid, enters into the composition of 

 all cultivated soils, chiefly in combination with limestone, magnesia, potash, 

 and other bases. With hydrogen it forms sulphuretted hydrogen, a remark- 

 ably disagreeable-smelling gas, the product of the decomposition of organic 

 matter contained in the soil and impregnating many medicinal waters, as at 

 Harrogate, 



66. F/iosjjhoizis is a soft, wax-like, highly-inflammable substance, which 

 combines with atmospheric oxygen, giving rise to phosphoric acid, which 

 enters into the compusition of all our cultivated plants, and is essentially 

 necessai'y to a healthy condition of vegetable life. It exists in trap-rock, 

 granite, basalt, and other igneous rocks, and in lime, ironstone, and most 

 minerals. 



67. Clolorine is a highly-noxious, suffocating, yellowish, gaseous element, 

 particularly disagreeable in smell. In soils it is found in combination with 

 such bases as chloride of sodium, or common salt. 



68. Soils, then, consist of a mechanical mixture of four substances, — 

 1. Silica, silicious sand, or gravel ; 2. Clay ; 3. Lime ; 4. Humus, and many 

 of the chemical substances we have described, in varying proportions. Hazel 



