356 CHEMISTRY OF HORTICULTURE. — NATIVE OR SPECIAL EARTHS. 
This combining power may be referred, I think, to the electric condition of the two 
elements ; for if silica be in a state of electricity opposite to that of any alkaline or 
earthy base with which it combines, the union will be correctly electro-chemical, and in 
conformity with that attractive energy which induces the two electricities to neutralise 
each other. 
2. Alumina — argil. Argillaceous earth is the base of all clays, constitutes the 
larger portion of them, and confers that plasticity and adhesiveness which denote their 
peculiar character. Alumina is an oxide of a metalloid called aluminum, first discovered 
by Davy, in 1808, but not properly defined till 1827, when Wohler obtained it from its 
chloride, in a perfectly separate state. Alumina, pure, is most readily obtained from 
refined alum, dissolved in distilled water, then decomposed by adding a filtered solution 
of carbonate of potassa (salt of tartar) to excess. The precipitate so obtained, is washed 
with repeated portions of hot distilled water, and re-dissolved in hydrochloric acid (muriatic 
acid or spirit of salt). It is again precipitated by liquor of ammonia, or its carbonate ; 
and the alumina so deposited, is again washed on a paper filter till perfectly free from 
saline matter, after which it is dried with a gentle heat. It is then a hydrate of alumina, 
that is, alumina with about half its weight of water, in a dry form. This water is driven 
off at a red heat, leaving the alumina pure, in the state of a soft, colourless, insipid, 
insoluble powder, the specific gravity of which, compared with distilled water at 60° Fahr. 
is 2. Alumina retains a strong affinity for the water so expelled, absorbing it when 
exposed to air, to the extent of half its own weight. 
Though not found native in the condition above described, it is nevertheless widely 
diffused throughout the earth. “ Native alumina may be said to constitute the sapphire, 
which is either colourless or pale blue, also the oriental ruby and topaz, which are red and 
yellow varieties of sapphire. These gems are mostly found in alluvial deposits in Ceylon 
and Pegu. Corundum, adamantine spar, and emery, are minerals also, consisting chiefly 
of alumina, with less than 2 per cent of oxide of iron, and a little silica. All these 
substances are extremely hard, being in that respect, only second to the diamond.” — 
Brande. 
The reader will find the best representative of alumina in tobacco-pipe, or potter’s 
clay. It is not pure alumina, because it contains a considerable percentage of silica, in 
however, so fine and impalpable a state that none of it subsides when 200 grains are 
intimately mixed by trituration, and boiled with four or five ounces of distilled, or 
rain water. 
From the recent investigation of Professor Johnston, printed in the Journal of 
Agricidture , No. 27, July 1847, we collect much useful information. For example, “ On 
the General Composition of Plastic Clays,” it is stated, that “ pure pipe or porcelain clay 
is the standard with which all other plastic clays are to be compared. In its best and 
purest form, this clay — besides water — consists of silica and alumina only. A specimen 
from Devonshire, gave in the 100 parts — fine silica, 49 ’6 ; alumina, 87‘4 ; combined 
w r ater 11*2. When the proportion of silica is much smaller — say only 19 per cent — 
(provided it exist in a state of chemical union with the alumina) it will produce a clay 
sufficiently plastic for all common mechanical purposes.” But even porcelain clays are 
rarely pure : they contain an appreciable quantity of oxide of iron, of lime, of magnesia, 
of potash, and soda.” 
What then must we say of the ordinary tile-clays, on the one hand, and of the clay 
soils of agriculture, on the other ? Space is not given to cite the authority of Mr. Henry 
Stephens’ “ Book of the Farm ” on the constituents of agricultural clays ; we must be 
content to offer a cursory view of different heads under which they are now classed. 
Thus : — 
(a) Strong clay soil consists of from 5 to 15 per cent, of siliceous sand ; it abounds with 
alumina, coloured with iron, which confers upon it every tint, from pale buff to chocolate 
brown, according to its state of oxidation. Such a clay-soil, comprising perhaps 60, or 
even a higher proportion of alumina, is when fully wetted, greasy to the foot, which slips 
upon it backwards, forwards, and sideways. It has an unctuous feel in the hand, by 
