Refining Process.— The tin thus obtained is still very impure; it contains generally iron, copper, arsenic and tungsten. In order to purify it the blocks of tin are placed in a reverberatory furnace and moderately heated to the point where the tin melts and flows into the refining basins, while the greater part of the foreign metals remains in the solid state. The molten tin in the refining basins is then stirred with poles of green wood, whence gases are given off, and the metal is maintained in a state of artificial ebullition. The upper parts of the contents of the basin are oxidized and removed from the surface, while the greater part of the foreign metals collects at the bottom. The metal is allowed to partially cool, during which process it separates into zones, the upper part consisting of nearly pure tin, while the under is so impure that it must be returned to the fur nace and again melted. The upper layer of tin is removed into molds, containing each about three hundredweight, in which it is allowed to solidify; it is then sent into the market as block tin, the purest specimens being called refined tin.
when pure, has a fine white color like silver but with a slightly bluish hue, and when newly melted its brilliancy is great. It has a slightly disagreeable taste, and emits a peculiar smell when rubbed. Its hardness is between that of gold and lead. Specific gravity, 7.28. It is very malleable ; tin leaf, or tin-foil as it is called, is about one one-thousandth part of an inch thick; and it might be beaten out into leaves as thin again, if such were wanted for the purposes of art. Its ductility and tenac ity are much inferior to those of most of the metals known in early times; a bar of tin a quarter of an inch in diameter will not support a greater weight than 294 pounds. Tin is very flexible and produces, while bending, a remark able creaking noise, known as the of tin.' It melts at about 460° F. When cooled slowly it may be obtained crystallized in the form of a rhombodial prism. By washing the surface of a mass of tin with warm dilute aqua regina it becomes covered with a number of crystals, which, from their unequal action upon light, give an appearance to the metal somewhat re sembling that of watered silk. After a short exposure to the air tin loses its lustre and as sumes a grayish-black color, but undergoes no further alteration. Neither is it sensibly altered by being kept under water. When cooled to 54° below zero it undergoes a transformation into what is called tin.' In this form it is very brittle and has lost all its metallic properties. When tin is melted in an open ves sel its surface becomes very soon covered with a gray powder, which is an oxide of the metal. If the heat be continued the color of the powder gradually changes, and at last it becomes yellow.
Tin Ores.— These are but two in number, tin ore and tin pyrites, known respectively by the technical names of cassiterite and stannite. The first of these occurs crystallized, and in a great variety of forms, which may all be de rived from an octahedron with a square base, the angle over the apex being 112° 10'. The majority of the crystals have the general figure of a right square prism, with four-sided pyra mids at each extremity. They occur often in twin form, the twin crystals forming an elbow. The cleavages take place parallel with the sides of this prism and with both its diagonals. The crystals may be cleaved also parallel to the sides of the above-named octahedron, but with difficulty. The prisms are sometimes vertically streaked. Lustre adamantine; color various shades of white, gray, yellow, red, brown and black; pale gray, in some varieties pale brown; semi-transparent, sometimes almost transparent, and in others opaque; brittle; hard ness six to seven, about that of feldspar, specific gravity, 6.96. Tin ore presents itself in a great variety of compound or macled crystals. It also occurs reniform, rarely in botryoidal shapes, and massive, with a granular or columnar composition, the individuals being strongly connected and the fracture tuieven. The wood tin of the Cornish mines is a mere variety of tin ore. It is so-called because of its resem blance to a cross section of the trunk of a tree, with the concentric rings of annual growth. In color it is dark reddish to brown. The follow ing ingredients were found in specimens of crystallized, massive and wood-tin ore:— In its greatest purity cassiterite contains noth ing but oxide of tin. Alone it does not melt before the blowpipe, but is reducible when in contact with charcoal. It occurs disseminated through granite, also in beds and veins, in lode mines. It also occurs in the beds and al luvial deposits of streams in the form of rolled pebbles and is extracted in this shape as °stream-tin)) by placer works and dredges. The variety called wood-tin has hitherto been found only in these repositories, but is occasion ally found in pockets in rock formations. Tin pyrites (Cu3S.FeS.SnS), the other ore of tin, occurs massive, with a granular composition; fracture uneven, imperfectly conchoidal; lustre metallic; color steel-gray, inclining to yellow; streak black; opaque; brittle; hardness about that of fluor-spar; specific gravity, 4.35. Be fore the blowpipe sulphur is driven off and the mineral melts into a blacicish scoria, without yielding a metallic button. It is soluble in nitro-muriatic acid, with precipitation of part of the sulphur. It contains from 14 to 30 per cent of tin. It is found at Saint Agnes in Corn wall, in Saxony and in Bolivia and in Virginia, Idaho and Montana in the United States.