Chap. II.
COMPOSITION AND USE OP MINERALS.
679
every part of the globe, and are found in small quantities in some animal and vegetablebodies, in several mineral waters, and in every soil; it is also met with in combination withsulphur and with several acids. It is of a bluish grey colour, and a dull fibrous fracture,capable of being brought to a brilliant polish; it is fusible at a white heat, is extremelyductile, but cannot be hammered into very thin plates. It is the most tenacious as wellas the hardest of the malleable metals. It is oxidated by air and by water, and bymany other acids; but air in a dry state, or water free from air and acid, exert but littleinfluence over it; the rust being occasioned by an exposure to a moist atmosphere. It iseasily moulded into any shape, drawn into wires of the greatest fineness and strength,rolled into sheets or plates, hardened or softened, welded by heat and rendered permanentlymagnetic, and converted into tools of every description; indeed it is scarcely possible tolimit the various purposes to which it may be applied, and it may be justly considered asthe most useful of all the metals : its specific gravity is I'll. It is obtained in Englandprincipally from the carbonate of iron of the coal formation, which usually yields about30 per cent, of cast metal, or sometimes as much as 40 per cent.
The carbonate of iron consists of two varieties, the compact and the sparry. Thecompact comprises most of the clay iron-stones, and those found in the coal measures of aflat spheroidal form ; its colour is either a yellowish brown, brick red, or reddish grey,with a fracture close-grained; it yields a yellowish brown powder ; it will not effervescewith either of the acids, and it has a slightly argillaceous smell when breathed upon. Theslaty clay between the seams of coal affords abundant supply of this ore; it is frequentlyfound in continuous beds, sometimes 18 inches in thickness. The sparry carbonate ofiron has a lamellar fracture, a yellowish grey colour or brownish red ; it slightly effervesceswith nitric acid, and changes to a reddish brown: its primitive form, when crystal-lised, is an obtuse rhomboid, and the crystals often contain quantities of carbonate of lime.This ore is found in the mountains of gneiss, in combination or mixed with quartz,copper pyrites, oxide of iron, and carbonate of lime of different varieties. Natural steel isproduced from it, and in England and Scotland it produces from 30 to 33 per cent, of castmetal.
The richest specimen, analysed by Dr. Colquhoun, which had a specific gravity of 3 05,gave, in 100 parts,
Carbonic acid
_
_
- 35-17
Protoxide of iron
_
- 53-03
Lime -
-
-
3-33
Magnesia
_
.
1-77
.
_
1-4
Alumina
_
-
•63
Peroxide of iron -
_
-
■23
Carbonaceous matter
.
.
3-03
Loss -
1*41
100-00
and its contents in metallic iron were 41-25.
Three-fourths of the iron manufactured in Great Britain is obtained from the coal fieldsof Dudley and South Wales; the former is very favourably situated, as the ore, the limestoneflux, and the clay for making fire-bricks, are all obtained with the coal. At Merthyr Tydvil in Wales , the iron-stone is found in abundance in about sixteen beds of slate clay, in nodulesof various size, both below and above the coal seams; in this district there are upwardsof thirty blast furnaces, and the iron is chiefly converted into bars.
Of the Assay of Iron Ores .—To obtain all the iron contained in the ore, it must be firstdeoxidised, and the temperature so raised that the metals and earths will melt, when abutton of iron will be found at the bottom of the crucible ; to effect this, and to overcomeany refractory earth, borax is usually added as a flux, taking care that the ore is finelypowdered before being mixed with it; it is then placed in a crucible which is previouslycoated or lined with hard-rammed damp charcoal dust. The ore .and flux is also coveredwith the same material. The crucible is then closed with a well-luted lid and fire-clay,and placed in a furnace, where the heat should be moderate at first, in order that themoisture of the damp charcoal should be slowly passed off, and the deoxidation completed.This is effected within the hour, when more fire is applied until a white heat is obtained,which is kept up for a quarter of an hour ; the crucible being then allowed to cool, thebutton of cast-iron is weighed, and the result denotes the quality of the ore from which ithas been obtained. This method of assaying is called the dry way, and requires a tempe-rature of 150° of Wedge wood.
To effect the assay in the humid way 100 grains of the ore, finely powdered, are digestedwith nitro-muriatic acid, when, of the numerous compounds mixed with the iron, silica oralumina will alone be thrown down. Any effervescence that takes place in the cold dilute
x x 4