Cast iron occupies a less space in a fluid, than in a solid form. Hence its fitness for castings. This fact depends on crystalline form which the iron takes on cooling.
228. Nickel is perfectly malleable and may be forged when hot, and hammered into plates when cold. Its colour is white, and intermediate between those of silver and tin. It admits of being finely polished, and has a lustre between steel and platina. Its ductility is such, as it may be drawn into very fine wire It cannot easily be soldered on account of the oxide which forms on its surface when heated. It is one of the most infusible of the metals. Like iron, it is magnetic. By long exposure to a red heat it is converted into a dark brown oxide which is still magnetic. Its specific gravity about 9At a sufficiently high heat its oxides are reducible without addition ; nor is it more tarnished by a strong heat than gold, silver, or platina. It ranks therefore among the noble or perfect metals.
229. The principle ore of this metal is a sulphuret. It comes from various parts of Germany, where it is found with cobalt.
OA.?. 1. It is a curious circumstance, that nickel has been found alloyed with iron in all the specimens of stones which have fallen from the atmosphere, where these have been analyzed.
2. Nickel is employed in China in making white copper, which is a very beautiful compound. It has also been proposed to make use of this metal in combination with steel, for the purpose of making surgical instruments, as such an alloy is not liable to oxidize.
The properties of tin must be examined in the state of grain-tin, or block-tin ; what is commonly known by the name of tin being nothing more than iron plates with a thin covering of this metal.
232. Tin has a silvery white colour, which is very little altered by exposure to air. It is ductile and very malleable, being easily rolled, or hammered into thin plates. Next to lead it is the softest and least elastic of all elastic metals. Its specific gravity is a little more than 7- It is easily fuzed, and bends with a peculiar crackling noise.
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224. When iron is exposed to the combined action of air and moisture, it absorbs oxygen at common temperatures.
Illus. The comon rust of iron is an oxide of that metal.
225. Iron combines with the acids, and forms conn pounds of various kinds.
Obs. Among these, the sulphate of iron, called also green vitriol or copperas, is the most important.
226. When sulphate of iron is mixed with an infusion of galls, we obtain a black solution, which is a new compound of oxide of iron with the gallic acid and tan.
Illus. Wriiing ink is a gallale of iron, made with tincture of galls, and sulphate of iron.
Obs. The black colour of ink is owing to its combination with oxygen.
Illus. The addition of iron filings to ink destroys its colour by absorbing its oxygen. Characters written with ink after this treatment, are at first illegible, but become black as the iron acquires oxygen from the air. Writing with ink newly made is well known to grow more intensely black by expoture to the air; this is on the same principle.
227. Ink is decomposed by alkalies, by acids, and by age. In some instances its colour may be restored by adding the ingredient which is lost.
Illus. 1. The alkalies, as potash, or soda, will destroy the colour of writing; but the colour may be restored by moistening the paper with infusion of galls on a camels hair pencil.
2. Ink is decomposed by age, partly in consequence of the farther oxidation of the iron, and partly in consequence of the destruction of the gallic acid. Hence ink stains degenerate into iron-moulds, and these last are immediately produced on an inked spot of linen when washed with soap, because the alkali of the soap abstracts the gallic acid, and leaves only an oxide of iron. Hence also the yellowish colour which is produced by dipping linen into a solution of sulphate of iron and called a copperas colour, is only a precipitation of the oxide of iron on the cloth.
227. Iron combines with carbon in various proportions.
Obs. 1. Steel is a compound of iron and carbon ; the iron absorbs the carbon from the charcoal with which it is heated.
2. Cast, or crude iron, contains oxygen and carbon. When these are got rid of by the process of refining, malleable or bar iron is formed, tron however, always contains more or less oxygen and carbon.
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