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THE CHEMISTRY OF SOILS AND MANURES. 
acids. Ciystalized pure Alumina constitutes the rare and precious hard stones, sapphire, and ruby ; 
uncrystalized or amorphous Alumina, on the other hand, is a white, tasteless, voluminous, powdery 
substance, which may readily be obtained by adding a solution of carbonate of soda to Alum. The 
white precipitate, produced in this way, is the earth of Alum, or Alumina. It constitutes a large 
proportion of all shaley and slaty rocks, and is the principal ingredient of pipe, porcelain, and agri- 
cultural clays. All clayey soils owe their tenacity, stiffness, and power of retaining moisture to the 
presence of alumina, and exhibit these properties in so much higher a degree, the larger the proportion 
of Alumina which enters into their composition. Alumina, however, is seldom found in soils in a 
free or uncombined state, but exists in them as well as in clays, in combination with Silica. Pure 
pipe or porcelain clay is a finely-divided chemical compound, consisting of variable qnantities of 
Silica, Alumina, and water. Silica always preponderates, amounting to from sixty to seventy per 
cent, of the whole. Agricultural clays are mixtures of pure clays with more or less sand, free alumina, 
lime, magnesia, oxide of iron, and silicates of potash and soda. The colour of these clays is generally 
due to a considerable portion of oxides of iron. The complex nature of such clays will become 
apparent by the subjoined analysis of clays, from Dumbleton, made in my laboratory: — 
No. I. 
No. II. 
No. III. 
Water of combination and 
organic matter 
. 7.69 . 
. 6.62 . 
. 6.68 
Inorganic matter soluble in 
muriatic acid: — 
Oxides of iron, 
. 8.24 . 
. 7.33 . 
. 8.63 
Alumina, 
. 8.04 . 
. 10.62 . 
. 9.25 
Carbonate of lime, . 
. 1.12 . 
.70 . 
.19 
Magnesia 
.62 . 
.12 . 
.56 
Potash and soda, 
.73 . 
1.04 . 
. 1.13 
Silica, 
. 
.09 . 
.00 . 
.OS 
Inorganic matter insoluble 
in acid : — 
Silica 
. 61.71 . 
. 63.36 . 
. 61.42 
Alumina 
" ) " ' ' ' ( 
. 10.04 . 
. 7.11 . 
. 9.66 
Lime 
. J in a state of silicate, \ 
.44 . 
.54 . 
.24 
Magnesia 
. ) . . . . ( 
.34 . 
.39 . 
.34 
Alkalies and loss 

.94 . 
. 2.11 . 
. 1.82 
100.00 
Alumina is not usualty found in the ashes of plants, and is therefore not considered as directly 
contributing to the nourishment of plants. The principal agency of Alumina in soils appeal's to be 
of a mechanical kind ; indirectly clays are regarded as supplying the wants of plants by the remark- 
able power they possess of absorbing ammonia from the atmosphere, and retaining, not only this volatile 
substance, but also the easily soluble alkaline salts of the manures. Without this kind provision of 
nature the highly fertilizing alkaline salts would be dissolved by the first heavy rain falling on the land, 
and deposited in the subsoil ; and the manures would thus, in a great measure, be rendered useless. 
6. Iron. — Except in meteoric stones, Iron is not found in nature in a metallic state ; but combined 
with Oxygen, Sulphur, and other elements, Iron is spread all over the world. When Iron is exposed 
to a moist atmosphere, it soon loses its brightness, and becomes covered with rust — a substance which, 
being a combination of Iron with oxygen, is called by chemists, Oxide of Iron. Oxygen, according 
to circumstances, unites in more than one proportion with Iron, and gives rise to several oxides, of 
which the black-coloured or protoxide of Iron, containing the smaller amount of oxygen, and the rcd- 
eoloured or peroxide of Iron, containing the larger proportion of oxygen, are the more important. 
Both the black or protoxide, and the red or peroxide of iron appear as constituents of almost all soils. 
The red oxide is the most abundant, and to its presence the red colour of many soils is due. The 
subsoil of red soils often consists of blue or greyish clays, which owe their blue or greyish colour to 
the presence of protoxide of iron. When such a subsoil is brought to the surface by the spade, the 
blue or grey colour gradually turns red. This change in appearance is the result of the atmospheric 
oxygen muting with the protoxide of iron of the clay, and converting it into the red or peroxide. 
Oxide of iron is found in all ashes of plants, in the blood of animals, and appears, therefore, to 
exercise an important influence on vegetable and animal life. 
Carhonate of Iron, which forms the erystalized spathose iron-stone, is found here and there in the 
soil, and more frequently dissolved in the carbonic acid of some natural springs. In such waters an 
ochrey deposit, consisting chiefly of oxide of iron, is formed on exposure to air. A similar oehrey 
y deposit is seen sometimes at the bottom of drains and ditches. On examination, however, it will 
«i generally appear that the ochrey mass is not pure oxide of iron, but that it contains a large quantity 
V\ of organic matters, being principally crenic and apocrenic acid. When spring-waters, containing oxide 
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