Chap. II.
COMPOSITION AND USE OF MINERALS.
653
parent. White granular limestone, or primitive marble, is another variety ; and among theinferior limestones are several, as the oolite, common limestone, &c.
Magnesium fuses at a red heat, then burns, uniting with the oxygen of the air to formmagnesia.
Oxide of magnesium, or magnesia, consists of—
Magnesium - - - 1 - 12 59*3
Oxygen - - - I 8 407
1 20 100 -
Its specific gravity is 2*3, it is almost infusible, and nearly insoluble in water.
Aluminum is difficult of fusion, requiring a higher temperature to melt it than cast-iron,and it is not oxidised by exposure to the air; water at a common temperature does not actupon it, nor is it affected by nitric or sulphuric acids; but in hot sulphuric acid it rapidlydissolves, and sulphurous acid is evolved. Sir Humphry Davy found that potassa wasformed by passing the vapour of potassium over alumina at a white heat.
When chloride of aluminum mixed with potassium is heated in a platinum crucible bymeans of a spirit lamp, and the substances begin to act, the temperature rises suddenly torednets, and then care must be had that the chloride does not pass off in an undecomposedstate, or that there is an excess of alkali in the residue. After the whole is cooled down,and washed with cold water, pure aluminum is found in the state of finely divided greysubstance, with a small degree of metallic lustre. Aluminous rocks are abundant in everyformation.
Oxide of Aluminum .—Alumina is an insipid and insoluble compound, with a specificgravity of 2. It has a strong attraction for moisture, and will absorb about one-third ofits own weight, which may be again expelled by ignition. Alumina is rendered plasticwhen mixed with water, and has a strong affinity for various organic compounds; andmoist alumina is readily soluble in most of the acids. When ammonia is put to a solutionof alum, in infusion of cochineal or madder, the aluminous earth is precipitated with thered colouring matter, and in this way the colour lake is formed. Alumina is known byits solubility in caustic potassa, and by its octohedral crystals of alum, which are formedon evaporating the sulphuric solution, with the addition of sulphate of potassa, and bythe blue colour which it has when moistened with nitrate of cobalt strongly heated.Alumina does not combine with carbonic acid, hut with other acids and the alkalies, andwith the latter its compounds are soluble in water.
The different hydrates of alumina are found in a native state, though their distinctatomic compounds are not yet ascertained. Alumina as a protoxide may be thus
stated —
Aluminum
- 1
10
55-5
Oxygen
-
- 1
-
8
44-5
1
18
100*0
Many chemists, however, consider its proportions to consist of two equivalents of aluminaand three of oxygen.
Alumina exists in all kinds of clay, and from an analysis made by Professor Faraday ofthe blue alluvial clay of the Medway, in its dark-coloured and moist state, it was found
. that 100 parts whose specific gravity was 1-46, contained —
Water - - - - - 50-9
Sand - - - . 14
particles j Alumina - 10*8
Peroxide of iron - - - 3 4
Carbonate of Lime - - - - 1*5
Fragments of wood - - - -1-5
Organic matter - - - - 3’1
100-0
In the brown pit clay of Upner, he also found, its specific gravity being 2 07,Water - - - . - 19
Sand - 30*5
Finer Silica - 29*8
particlesj Alumina - 16*5
Peroxide of iron - - - - 3 -7
Carbonate of lime - • _ - *5
100*0