MINERALOGY. 69 
and copper, renders it red short. Iron, of all ductile metals, is the hardest 
and toughest; it cannot be hammered out into very thin plates without 
breaking, but on the other hand it may be drawn out into very thin wire. 
Purified and melted iron has a specific gravity of 7.79, common bar iron of 
7.788. It melts at from 2822 to 2876° F.; and voiatilizes in the flame of 
the oxy-hydrogen blowpipe, and in the heat produced by the galvanic battery. 
Steel, one of the principal products of iron, consists of a similar, but more 
intimate, combination of iron with carbon. The amount of carbon in steel 
varies according to the kind from 0.5 to 1.5 per cent.; and iron with this 
per centage of carbon acquires the property of hardening when rapidly 
cooled. 
2. Silica. 
Silica or silicic acid, since it possesses the distinguishing properties of an 
acid, is a combination of oxygen with a simple substance, silicon, only 
obtainable by the delicate manipulation of the chemist. It is diffused in 
vast quantity over the earth. It occurs, in a very pure state, both 
crystalline and amorphous, and forms combinations with all fixed oxydes. 
The purest crystalline silex or silica is found in rock crystals. These occur 
in perfect single crystals of the hexagonal system, as double six-sided 
pyramids (pl. 34, fig. 36), or as regular hexagonal prisms with six-sided 
pyramidal terminations (fig. 51), or as derivatives from these forms, 
fs. 21, 22,45, 72. Rock crystal frequently occurs in small crystals, more 
rarely several feet in length. Quartz, a common form of crystallized silex, 
and often somewhat impure, is well known to all miners; and in the most 
beautiful specimens presents somewhat the appearance of figs. 2 and 3. 
Rock crystal and quartz pass from perfect transparency into an opake 
milky white, and with foreign admixtures into every color of the rainbow. 
Besides the crystallized varieties of silica, there are others, as chalcedony, 
remarkable for their beauty and hardness, and indispensable in the 
manufacture of various instruments and utensils. The crystallized varieties 
are used as gems, and excellent lenses are obtained from perfectly 
transparent and colorless rock crystal. Quartz, either compact, or as sand, 
is a principal ingredient in glass; chalcedony and agate are converted into 
ornaments, polishing instruments, scale beds, mortars, &c. In sand we find 
one of the most important constituents of the fruitful soils ; and finally all 
building materials, whether stone or brick, owe their solidity to the mortar 
of which this same sand is a most important ingredient. 
3. Tin Ore. 
Tin ore is the oxyde of tin, and at the same time the source from which 
most of the metallic tin of commerce is derived. It occurs in primitive 
rocks, in veins or beds; traversing granite, gneiss, mica slate, and clay 
slate. Tin is not very generally diffused, and occurs in large quantity in 
but few localities, especially in Bohemia, Saxony, Cornwall in England, and 
in the East Indies, in Malacca, and Banca. The ore is sometimes found in 
remarkable crystals of the dimetric system; it is semi-transparent or opake, 
and of various colors, as white, grey, red, brown, or black. Pl. 34, fig. 8, 
represents a group of crystals drawn from nature, and fig. 19 a mass of 
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