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carbonate of lime. This is the most useful of all the kinds of steel, and employed for razors, surgeons instruments, and similar purposes; its grain is the most compact, and it takes the highest polish.
It is the particular property of steel to become extremely hard, if it be heated red hot, and then suddenly plunged into cold water; but different instruments made of steel require to be of different degrees of hardness; and they are, therefore, subjected to a process called tempering, which is heating them again to a certain point after having been hardened. The tempering of steel, for some purposes, is a delicate process.
A polished bit of steel, when heated with access of air, acquires very beautiful colours. It first becomes of a pale yellow, then of a deeper yellow, next reddish, then deep blue, and at last bright blue. At this period it becomes red hot, and the colours disappear: at the same time the metallic scales, or the black imperfect oxide of iron which is formed, incrusts its surface. All these different shades of colour indicate the different tempers the steel has acquired by the increase of heat. Artists have availed themselves of this property, to give to surgical and other instruments those degrees of temper which their various uses require. Iron may be alloyed with most of the metals, but these alloys are not much used.
Wootz is the name given to a kind of steel made in the East Indies, which is of a very superior quality for cutting-instruments.
Copper is sometimes found native, but in very small quantities. It is generally met with in the state of oxide, or united to sulphur, or to acids. In Cornwall there are very rich mines of copper.
Pure copper is of a red colour, very tenacious, ductile, and malleable. It melts at 27 of Wedgewood's pyrometer, and burns with a green flame.
It is not oxided by water. When exposed to a red heat, it becomes covered with a crust of oxide of a blackish colour, this is the peroxide of copper. The first, or protoxide, is of a red colour when found native,but when formed artificially is orange. The oxides of copper are reduced to the metallic state by heating with charcoal or oils.
The nitric acid disolves copper with effervescence, and the solution has a blue colour. The acid first oxidates the metal, a large quantity of nitric oxide (nitrous gas), is disengaged, and the oxide dissolves; this forms the nitrate of copper.
The sulphuric acid does not dissolve copper unless when concentrated, and in a boiling state. Fine blue crystals, which are the sulphate of copper, are the result. This is what is commonly called blue vitriol. This salt is decomposed by iron; for if a piece of iron be immersed in a solution of sulphate of copper, the copper will be precipitated upon the iron. This process is often employed for procuring the copper from the water in copper mines, which has in it a large portion of sulphate of copper.
The muriatic acid does not act upon copper except in a state of ebullition, and then the muriate
of copper is formed, which is of a green colour, and of an astringent taste. A solution of it is used as a sympathetic ink; for letters written with it will become yellow by warming, and will disappear again when cool.
The acetous acid in a sufficient degree of concentration dissolves copper, but when not concentrated, as in vinegar, it acts upon it very slowly, and forms common verdigris, which is an impure acetate of copper. This being dissolved in distilled vinegar, and subjected to evaporation, crystals are produced which constitute what is called distilled verdigris.
Copper is employed in making kitchen utensils; but as these vessels are liable to be corroded by the acids and fatty substances used in culinary preparations, they often become dangerous, as all the salts of copper are poisonous. Culinary utensils of copper should always be well tinned, but those of iron tinned are safer, as iron has no poisonous quality. The alloys of copper with other metals are very useful.
Tombac is formed of copper, arsenic, and tin. Prince's metal, or Pinchbeck, is made of copper and zinc.
Brass is also formed of another proportion of copper and zinc.
Bronze is made of copper and tin.
Bell-metal is also of copper and tin, but with more tin than the latter alloy.
A solder for silver is made of copper and silver.
Tin is a metal of a colour approaching to that of silver, but somewhat duller. It is extremely malleable. When hammered into leaves it constitutes tin-foil. It is not, however, very ductile. It is nearly as soft as lead, and may be easily bent, and then emits a crackling noise, which is peculiar to it. Tin fuses more easily than any other metal : when it has been kept some time in a state of fusion, with access of air, its surface becomes wrinkled and covered with a grey pellicle, which is the first, or grey oxide of tin. This oxide when mixed with melted glass forms white enamel.
The grey oxide, when exposed to a greater degree of heat, takes fire, acquires more oxygen, and becomes of a pure white; the white oxide of tin.
Tin is not oxidized in the air at the common temperature; on account of which property, it is used for covering iron plates, to prevent their rusting.
Tin dissolves in the muriatic acid, forming muriate of tin, much used by dyers.
With nitric acid it forms nitrate of tin.
Tin united with sulphur forms the aurum musium. Alloyed with lead, it forms plumber's solder. The best pewter is composed of tin alloyed with antimony, copper, and bismuth.
Tin is not found native, and its ores are not much distributed. The richest mines are in Cornwall.
This metal is never found in a native state. The ore from which it is chiefly procured is galena, which is lead united to sulphur, or a sulphuret
Pure lead is of a greyish colour. When fresh cut it is bright, but it soon tarnishes in the air. It stains the fingers or paper when rubbed on them. It is easily cut with the knife; has little or no elasticity, and is very malleable, but not very ductile. Water does not act upon lead. It easily fuses; and exposed to the air in a state of fusion, its surface becomes covered with a grey pellicle: if this be removed another succeeds, and in this manner the whole may be converted into a powdery substance. This pellicle is composed of oxide of lead mixed with a portion of metallic lead. If it be subjected to a strong heat, it is changed into a yellow powder, known by the name of massicot; which is the first, or yellow oxide of lead: it is used as a pigment.
If massicot be exposed to the flame of a furnace for some time, and kept stirred, it is converted into a beautiful pigment, called minium, or red lead. This has been called the red oxide of lead; but it is a mixture of the yellow oxide above mentioned, and another, the brown oxide of lead. This brown oxide may be procured by pouring nitric acid on red lead; when the yellow oxide in the red lead will be dissolved by the acid, and the brown oxide will remain, being insoluble.
If the oxides of lead be acted on by a strong heat, they give up their oxygen, and metallic lead remains; but they are more readily reduced by mixing them with combustible matter.
Lead, when procured from its ore, frequently contains so much silver, that the latter is worth extracting. This process is called refining. The lead is played upon by the flame of a furnace, by which the lead is oxidized, and the oxide is partly vitrified, and assumes a scaly form, called litharge. The silver then remains free.