Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 10

At a heat far below ignition, the surface, of copper becomes covered with a range of prismatic colours, the commencement of its calcination; and with more heat a black scale is formed, which easily separates from the metal, and in a strong heat it melts, and burns with a bluish green flame.

Copper rusts by exposure to the air; but the partially-calcined surface adheres to the metal, as in the case of lead, and thus preserves it from farther corrosion.

Copper dissolved in the vitriolic acid forms crystals of a blot colour, called _blue copperas_. From this solution it is precipitated by iron, which by this means becomes coated with copper. The nitrous acid dissolves copper with most rapidity, producing nitrous air. If the solution be distilled, almost all the acid will be retained in the residuum, which is white; but more heat will expel the acid, chiefly in the form of dephlogisticated air, and the remainder will be a black substance, consisting of the pure calx of copper. The vegetable acids dissolve copper as well as the mineral ones, which makes the use of this metal for culinary purposes in some cases dangerous. To prevent this they give it a coat of tin. The solution of copper in the vegetable acid is called _verdigris_.

Alkalies dissolve copper as well as acids. With the volatile alkali a blue liquor is formed, but in some cases it becomes colourless. All the circ.u.mstances of this change of colour have not yet been examined. Both oil and sulphur will dissolve copper, and with the latter it forms a blackish grey compound, used by dyers.

Copper readily unites with melted tin, at a temperature much lower than that which is necessary to melt the copper; by which means copper vessels are easily covered with a coating of tin. A mixture of copper and tin, called _bronze_, the specific gravity of which is greater than that of the medium of the two metals, is used in casting statues, cannon, and bells; and in a certain proportion this mixture is excellent for the purpose of mirrors of reflecting telescopes, receiving a fine polish, and not being apt to tarnish. Copper and a.r.s.enic make a brittle compound called _tombach_; and with zinc it makes the useful compound commonly called _bra.s.s_, in which zinc is about one third of its weight.

Copper is sometimes found native; but commonly mixed with sulphur, in ores of a red, green, or blue colour.

Copper being an earlier discovery than that of iron, was formerly used for weapons and the shoeing of horses; and the ancients had a method, with which we are not well acquainted, of giving it a considerable degree of hardness, so that a sword made of it might have a pretty good edge.

LECTURE XXV.

_Of Iron._

Iron is a metal of a bluish colour, of the greatest hardness, the most variable in its properties, and the most useful of all the metals; so that without it it is hardly possible for any people to make great advances in arts and civilization.

This metal readily parts with its phlogiston, so as to be very subject to calcine, or rust, by exposure to the air. The same is evident by the colours which appear on its surface when exposed to heat, and also when it is struck with flint; the particles that fly from it being iron partially calcined. In consequence of its readily parting with its phlogiston, it is capable of burning, like wood or other fuel, in pure air.

Iron and platina have the property of _welding_ when very hot, so that two pieces may be joined without any solder.

When iron is heated in contact with steam, part of the water takes the place of the phlogiston, while the rest unites with it, and makes inflammable air. By this means the iron acquires one third more weight, and becomes what is called _finery cinder_. This substance, heated in inflammable air, imbibes it, parts with its water, and becomes perfect iron again. If the iron be heated in pure air, it also imbibes the water, of which that air chiefly consists, and also a portion of the peculiar element of the pure air.

The solution of iron in spirit of vitriol produces _green copperas_; which being calcined, becomes a red substance, called _colcothar_.

The precipitate of iron, by an infusion of galls, is the colouring matter in _ink_, which is kept suspended by means of gum. The precipitate from the same solution by phlogisticated alkali, is _Prussian blue_.

Water saturated with fixed air dissolves iron, and makes a pleasant chalybeat.

The calx of iron gives a green colour to gla.s.s.

Iron readily combines with sulphur. When they are found combined by nature, the substance is called _pyrites_.

The union of phosphoric acid with iron makes it brittle when cold, commonly called _cold short_; and the union of a.r.s.enic makes it brittle when hot, thence called _red short_.

Iron unites with gold, silver, and platina, and plunged in a white heat into mercury, it becomes coated with it; and if the process be frequently repeated, it will become brittle, which shews that there is some mutual action between them.

Iron readily unites with tin; and by dipping thin plates of iron into melted tin, they get a complete coating of it, and make the _tinned plates_ in common use.

When crude iron comes from the smelting furnace it is brittle; and when it is white within, it is extremely hard; but when it has a black grain, owing to its having more phlogiston, it is softer, and may be filed and bored.

Cast iron becomes _malleable_ by being exposed to a blast of air when nearly melting; the consequence of which is a discharge of inflammable air, and the separation of a liquid substance, which, when concreted, is called _finery cinder_. The iron generally loses one fourth of its weight in the process. Crude iron contains much _plumbago_, and the access of pure air probably a.s.sists in discharging it, by converting it into air, chiefly inflammable.

Malleable iron, exposed to a red heat in contact with charcoal, called _cementation_, converts it into _steel_, which has the properties of becoming much harder than iron, and very elastic, by being first made very hot, and then suddenly cooled, by plunging it in cold water. By first making it very hard, and then giving different degrees of heat, and cooling it in those different degrees, it is capable of a great variety of _tempers_, proper for different uses. Of the degrees of heat workmen judge by the change of colour on its surface. Steel, like crude iron, is capable of being melted without losing its properties. It is then called _cast steel_, and is of a more uniform texture. Iron acquires some little weight by being converted into steel; and when dissolved in acid, it yields more plumbago. Steel has something less specific gravity than iron. If the cementation be continued too long, the steel acquires a darkish fracture, it is more fusible, and incapable of welding. Steel heated in contact with earthy matters, is reduced to iron.

Iron is the only substance capable of _magnetism_; and hardened steel alone is capable of retaining magnetism. The loadstone is an ore of iron.

LECTURE XXVI.

_Of Tin._

Tin is a metal of a slightly yellowish cast, though harder than lead, very malleable, but of no great tenacity; so that wires cannot be made of it. It easily extends under the hammer, and plates of it, called _tinfoil_, are made only one thousandth part of an inch thick, and might be made as thin again.

Tin has less specific gravity than any other metal. It melts long before ignition, at 410 of Fahrenheit, and by the continuance of heat is slowly converted into a white powder, which is the chief ingredient in _putty_, used in polis.h.i.+ng, &c. Like lead, it is brittle when heated little short of fusion, and may be reduced into grains by agitation as it pa.s.ses from a fluid to a solid state.

The calx of tin resists fusion more than that of any other metal, which makes it useful in making an opaque white enamel.

Tin loses its bright surface when exposed to the air, but is not properly subject to rust; so that it is useful in protecting iron and other metals from the effects of the atmosphere.

Concentrated vitriolic acid, a.s.sisted by heat, dissolves half its weight of tin, and yields vitriolic acid air. With more water it yields inflammable air. During the solution the phlogiston of the tin uniting with the acid, forms sulphur, which makes it turbid. By long standing, or the addition of water, the calx of tin is precipitated from the solution. The nitrous acid dissolves tin very rapidly without heat, and yields but little nitrous air. With marine acid this metal yields inflammable air. With aqua regia it a.s.sumes the form of a gelatinous substance used by dyers to heighten the colour of some red tinctures, so as to produce a bright scarlet in dying wool.

A transparent liquor, which emits very copious fumes, called, from the inventor, _the smoking liquor of Libavius_, is made by distilling equal parts of amalgam of tin and mercury with corrosive sublimate, triturated together. A colourless liquor comes over first, and then a thick white fume, which condenses into the transparent liquor above mentioned. Mr.

Adet has shewn, that this liquor bears the same relation to the common solution of tin, that corrosive sublimate does to calomel, and has given an ingenious solution of many of its properties.

Tin detonates with nitre; and if the crystals made by the solution of copper in the nitrous acid be inclosed in tinfoil, nitrous fumes will be emitted, and the whole will become red hot. Also if five times its weight of sulphur be added to melted tin, a black brittle compound, which readily takes fire, will be formed.

Another combination of tin, sulphur, and mercury, makes a light yellow substance called _aurum musivum_ used in painting.

Tin is the princ.i.p.al ingredient in the composition of _pewter_, the other ingredients being lead, zinc, bis.m.u.th, and copper; each pewterer having his peculiar receipt. It is also used in coating copper and iron plates, and in silvering looking-gla.s.ses, besides being cast into a variety of forms, when it is called _block tin_.

Tin is sometimes found native, but is generally mineralized with sulphur and a.r.s.enic. The latter is thought to be always contained in tin, and to be the cause of the crackling noise made by bending plates of it.

_Of the Semi-metals._

Bis.m.u.th is a semi-metal of a yellowish or reddish cast, but little subject to change in the air; harder than lead, but easily broken, and reducible to powder. When broken it exhibits large s.h.i.+ning facets, in a variety of positions. Thin pieces of it are considerably sonorous.

Bis.m.u.th melts at about 460 of Fahrenheit. With more heat it ignites, and burns with a slight blue flame, while a yellowish calx, called _flowers of bis.m.u.th_, is produced. With more heat it becomes a greenish gla.s.s. In a strong heat, and in close vessels, this metal sublimes.

Vitriolic acid, even concentrated and boiling, has but little effect upon bis.m.u.th; but the nitrous acid acts upon it with the greatest rapidity and violence, producing much nitrous air, mixed with phlogisticated nitrous vapour. From the solution of bis.m.u.th in this acid, a white substance, called _magistery of bis.m.u.th_, is precipitated by the affusion of water. This has been used as a paint for the skin but has been thought to injure it.

The marine acid does not readily act upon bis.m.u.th; but when concentrated, it forms with it a saline combination, which does not easily crystallize, but may be sublimed in the form of a soft fusible salt, called _b.u.t.ter of bis.m.u.th_.

Bis.m.u.th unites with most metallic substances, and in general renders them more fusible. When calcined with the imperfect metals, it unites with them, and has the same effect as lead in cupellation.

Bis.m.u.th is used in the composition of pewter, in printers' types, and other metallic mixtures.

This metal is sometimes found native, but more commonly mineralized with sulphur.