654 THEORY AND PRACTICE OF ENGINEERING. Book II.
Berthier has given a table of the compositions of various clays, ami among them, that atStourbridge , which contains —
SilicaAluminaOxide of ironWater
•637
•207
•040
•103
•987
And those met with in the county of Devonshire, used for pottery, —
Silica - -496
Alumina - -374
Water - - - - -112
•982
Clay in general may be considered as a silicate of alumina, or a compound of silica,alumina, and water ; and the best for pottery are those where the proportions are three ofsilica to one of alumina, or by weight 48 to 18.
Sulphate of Alumina and Potassa , or common alum, consists of three equivalents ofsulphate of alumina, one of sulphate of potassa, and twenty-five of water ; its crystals areoctohedrous. Aluminous slate, which is an argillaceous slaty rock, containing sulphuretof iron, when roasted so as to oxidise the iron and acidify the sulphur, produces a sulphateof alumina, and when to this sulphate of potassa is added, alum is obtained.
Alum reddens vegetable blues, and dissolves in five parts of cold water, and in rathermore of its own weight in boiling water: in its crystallised state it consists of—
Sulphate of alumina 3 - 174 - 35-73
Sulphate of potassa I - 88 - 18*07
Water - - - 25 225 - 46*20
1 487 100-
Silicium is a simple non-metallic combustible, and has a strong resemblance to boron.It may be perfectly oxidised, and converted into silica or silicic acid by mixing it with drycarbonate of potassa, and heating it to redness, when a silicate of potassa is obtained.
Silica is the most common, as well as the most abundant, of the earths; it constitutes thechief portion of hard stones and minerals. It is found in solution in many springs, butthe water must be raised to a high temperature before it will hold any quantity in solution ;and retaining a greater heat under the pressure of the sea than in the atmosphere, sub-marine springs are probably more charged with silex than those to which we have access.
The waters of Ireland hold silex in solution in consequence of the presence of thealkali soda. The deposition of silica in an insoluble state is from the water, when cooleddown, not being so able to retain it as at a high temperature,—the evaporation of thewater decomposing the compounds of silica and soda which at first existed. Fragments ofwood and plants are often found completely silicified, or converted into stone, calledpetrifactions.
Oxide of Silica , Silica , or Silicic Acid. — It exists nearly pure in flint, and quite so inrock crystal; it is a white powder, insoluble in water, and infusible except by intense heat;its specific gravity is 2-66.
Oxygen - - 1 8-50
1 16 100
Fluo-silicic acid (Fluoride of Silicium ) _The only acid body which acts energetically on
silica is the hydrofluoric acid ; it is gaseous, transparent, and colourless. Water condenses365 times its volume ; its specific gravity is 3 61 compared with air ; 100 cubic inches ofthe gas weigh 112 grains : —
Fluorine - - -1- 18 - 692
1 26 100-0
Silicate of Potassa . —To obtain this compound silica is generally fused with the carbonateof potassa, the carbonic acid being expelled by the heat; with 3 of carbonate of potassaand 1 of silica, a glass is formed which is soluble in water ; if there is an excess of silica, itis insoluble. Plate glass is made from soda and siliceous matter. Common flint glasscontains 50 of silica, 34 of the oxide of lead, and 14 of potassa.
Vttrium, Glucinum , Zirconium , Thorinium, are all extremely rare metals.