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THE CHEMISTRY OF SOILS AND MANURES. 
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of many others, however, to forward our knowledge of this class of phenomena by experiments "upon 
germination under different physical conditions, various directions of illumination, &c, the repetition 
of Knight's experiments and modifications of them. The structural changes, such as the development 
of the bud and leaves require no especial mention here : it may be remarked, however, that the coty- 
ledons, when they rise above ground, become coloured green like the other parts exposed to light, 
acquiring fully the functions of leaves. 
THE CHEMISTET OF SOILS AND MAMTEES. 
By Dr. A. VOELCKEK, Professor of Chemistry in the Royal Agricultural College, Cirencester. 
INORGANIC MATTERS — MANGANESE, SILICA, SULPHUR, PHOSPHORUS, CHLORINE. 
MANGANESE is a metal which, in combination with Oxygen, as Oxide of Manganese is asso- 
ciated with the oxides of iron, occurring naturally in almost all soils. Manganese is found in all 
iron ores ; and, on the other hand, iron constantly accompanies manganese ores. Generally speaking, 
only traces of oxide of manganese occur in soils. In the ashes of plants likewise traces of manganese 
compounds may be detected; but iron usually preponderates in the same. The ash of the horse- 
chesnut, oak-bark, and a few other barks, however, present us with some exceptions to this rule, 
inasmuch as these ashes are rich in manganese, and contain but traces of iron. 
8. Silica. — Common sand, sandstones, flint, chalcedony, rock-crystal, quartz-rock. These are 
different names given to varieties of one and the same substance, which by the chemist is called 
Silica or Silex. These varieties are dependent on the physical characters of the substance, as 
greater or smaller transparency, the degree of hardness, etc., and the admixtures which are found 
in it naturally. Common red sand and all red sandstones are impure varieties of silica, the im- 
purities being iron and manganese, traces of lime, magnesia, etc. Flints are a purer form of silica ; 
white quartz-sand and quartz-rock are almost pure silica; and the same substance is called rock- 
crystal when it occurs in a state of perfect transparency. Silica occurs in nature abundantly, either 
in a free state in one or the other of the above varieties, or in a state of combination with hrne, 
magnesia, iron, potash, soda, and other mineral matters. Many minerals and rocks are entirely com- 
posed of such natural silicates. In soils, particularly those termed silicious, silica or silex, forms by 
far the largest proportion of its constituent parts, amounting often from 80 to 90 per cent, of the whole 
weight, when dry. Silica is entirely insoluble in cold and hot water ; and resists likewise the action 
of strong acids, with the exception of hydro-fluoric acid, which dissolves it, and which, on account 
of this property, is used for etching glass — an artificial silicate of potash or soda, and silicate of lime 
or lead. Though infusible at the strongest heat, yet, when mixed with potash or soda, and exposed to 
the heat of a glass-furnace, silica dissolves in the potash, or rather enters into combination with the 
alkali, and forms with it a transparent mass, or glass. If an excess of alkali has been employed in 
the process, the resulting fused mass dissolves completely in water. On addition of muriatic or 
sulphuric acid to a solution of silicate of potash or soda ; the silica separates in the form of a vo- 
luminous, white gelatinous mass, in which form it is soluble in a large quantity of water. All fertile 
soils contain more or less silica, in a state in which it is soluble hi water, and thus capable of being 
taken up by the roots of plants. 
9. Sulphur. — This well-known yellow combustible substance does not occur as such in soils, except 
in volcanic districts ; but in the form of sulphuric acid, this element enters into the composition of 
all cultivated land. Sulphur, in small proportions, is a constituent of some vegetable principles, but 
it is chiefly present in plants in the form of sulphuric acid, in combination with lime, magnesia, 
potash, and other bases. The sulphuric acid is probably derived directly from the sulphates in the 
soil ; and there appears good reason to suppose that the sulphates furnish the plants likewise with 
the sulphur requisite for those organic compounds containing this element. 
With hydrogen, sulphur forms a very disagreeable smelling gas — sulphuretted hydrogen, which 
being a product of the decomposition of organic matters, is frequently contained in the soil, and 
dissolved in water. Some springs, for instance, those of Harrogate, are so strongly impregnated 
with this gas as to present the characteristic smell of sulphuretted hydrogen in a marked manner. 
10. Phosphorus. — When phosphorus, a soft, wax-like, highly inflammable substance, which is not 
found in nature in a free state, is lighted, it combines with the oxygen of the atmosphere, and gives 
rise to dense white fumes, which may be condensed by placing a bell-glass over the burning phos- 
phorus. On the sides of the glass they become condensed to white crystalline, very deliquescent, 
flakes, constituting a highly acid compound, namely, phosphoric acid. Phosphoric acid is a most 
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