CHAPTER XXIX.
THE METALS.

WHAT METALS ARE—CHARACTERISTICS AND GENERAL PROPERTIES OF METALS—CLASSIFICATION—SPECIFIC GRAVITY—DESCRIPTIONS.

We have learnt that the elements are divided into metalloids and metals, but the line of demarcation is very faint. It is very difficult to define what a metal is, though we can say what it is not. It is indeed impossible to give any absolute definition of a metal, except as “an element which does not unite with hydrogen, or with another metal to form a chemical compound.” This definition has been lately given by Mr. Spencer, and we may accept it as the nearest affirmative definition of a metal, though obviously not quite accurate.

A metal is usually supposed to be solid, heavy, opaque, ductile, malleable, and tenacious; to possess good conducting powers for heat and electricity, and to exhibit a certain shiny appearance known as “metallic lustre.” These are all the conditions, but they are by no means necessary, for very few metals possess them all, and many non-metallic elements possess several. The “alkali” metals are lighter than water; mercury is a fluid. The opacity of a mass is only in relation to its thickness, for Faraday beat out metals into plates so thin that they became transparent. All metals are not malleable, nor are they ductile. Tin and lead, for example, have very little ductility or tenacity, while bismuth and antimony have none at all. Carbon is a much better conductor of electricity than many metals in which such power is extremely varied. Lustre, again, though possessed by metals, is a characteristic of some non-metals. So we see that while we can easily say what is not a metal, we can scarcely define an actual metal, nor depend upon unvarying properties to guide us in our determination.

The affinity of metals for oxygen is in an inverse ratio to their specific gravity, as can be ascertained by experiment, when the heaviest metal will be the least ready to oxidise. Metals differ in other respects, and thus classification and division become easier. The fusibility of metals is of a very wide range, rising from a temperature below zero to the highest heat obtainable in the blow-pipe, and even then in the case of osmium there is a difficulty. While there can be no question that certain elements, iron, copper, gold, silver, etc., are metals proper, there are many which border upon the line of demarcation very closely, and as in the case of arsenic even occupy the debatable land.

Specific Gravity is the relation which the weight of substance bears to the weight of an equal volume of water, as already pointed out in Physics. The specific gravities of the metals vary very much, as will be seen from the table following—water being, as usual, taken as 1:—

Some metals are therefore lighter and some heavier than water.

The table underneath gives the approximate fusing points of some of the metals (Centigrade Scale)—

There are some metals which, instead of fusing,—that is, passing from the solid to the liquid state,—go away in vapour. These are volatile metals. Mercury, potassium, and sodium, can be thus distilled. Some do not expand with heat, but contract (like ice), antimony and bismuth, for instance, while air pressure has a considerable effect upon the fusing point. Some vaporise at once without liquefying; others, such as iron, become soft before melting.

Alloys are combinations of metals which are used for many purposes, and become harder in union. Amalgams are alloys in which mercury is one constituent. Some of the most useful alloys are under-stated:—

Metals combine with chlorine, and produce chlorides,
Metals combine with sulphu, and produce sulphides,
Metals combine with oxygen, and produce oxides, and so on.

The metals may be classed as follows in divisions:—

We cannot attempt an elaborate description of all the metals, but we will endeavour to give a few particulars concerning the important ones, leaving many parts for Mineralogy to supplement and enlarge upon. We shall therefore mention only the most useful of the metals in this place. We will commence with Potassium.

Metals of the Alkalies.

Potassium has a bright, almost silvery, appearance, and is so greatly attracted by oxygen that it cannot be kept anywhere if that element be present—not even in water, for combustion will immediately ensue on water; and in air it is rapidly tarnished. It burns with a beautiful violet colour, and a very pretty experiment may easily be performed by throwing a piece upon a basin of water. The fragment combines with the oxygen of the water, the hydrogen is evolved, and burns, and the potassium vapour gives the gas its purple or violet colour. The metal can be procured by pulverizing carbonate of potassium and charcoal, and heating them in an iron retort. The vapour condenses into globules in the receiver, which is surrounded by ice in a wire basket. It must be collected and kept in naphtha, or it would be oxidised. Potassium was first obtained by Sir Humphrey Davy in 1807. Potash is the oxide of potassium, and comes from the “ashes” of wood.

The compounds of potassium are numerous, and exist in nature, and by burning plants we can obtain potash (“pearlash”). Nitrate of potassium, or nitre (saltpetre), (KNO₃), is a very important salt. It is found in the East Indies. It is a constituent of gunpowder, which consists of seventy-five parts of nitre, fifteen of charcoal, and ten of sulphur. The hydrated oxide of potassium, or “caustic potash” (obtained from the carbonate), is much used in soap manufactories. It is called “caustic” from its property of cauterizing the tissues. Iodide, bromide, and cyanide of potassium, are used in medicine and photography.

Soap is made by combining soda (for hard soap), or potash (for soft soap), with oil or tallow. Yellow soap has turpentine, and occasionally palm oil, added. Oils and fats combine with metallic oxides, and oxide of lead with olive oil and resin forms the adhesive plaister with which we are all familiar when the mixture is spread upon linen. Fats boiled with potash or soda make soaps; the glycerine is sometimes set free and purified as we have it. Sometimes it is retained for glycerine soap. Fancy soap is only common soap coloured. White and brown Windsor are the same soap—in the latter case browned to imitate age! Soap is quite soluble in spirits, but in ordinary water it is not so greatly soluble, and produces a lather, owing to the lime in the water being present in more or less quantity, to make the water more or less “hard.”

Sodium is not unlike potassium, not only in appearance, but in its attributes; it can be obtained from the carbonate, as potassium is obtained from its carbonate. Soda is the oxide of sodium, but the most common and useful compound of sodium is the chloride, or common salt, which is found in mines in England, Poland, and elsewhere. Salt may also be obtained by the evaporation of sea water. Rock salt is got at Salzburg, and the German salt mines and works produce a large quantity. The Carbonate of Soda is manufactured from the chloride of sodium, although it can be procured from the salsoda plants by burning. The chloride of sodium is converted into sulphate, and then ignited with carbonate of lime and charcoal. The soluble carbonate is extracted in warm water, and sold in crystals as soda, or (anhydrous) “soda ash.” The large quantity of hydrochloric acid produced in the first part of the process is used in the process of making chloride of lime. A few years back, soda was got from Hungary and various other countries where it exists as a natural efflorescence on the shores of some lakes, also by burning sea-weeds, especially the common bladder wrack (Fucus vesiculosus), the ashes of which were melted into masses, and came to market in various states of purity. The bi-carbonate of soda is obtained by passing carbonic acid gas over the carbonate crystals. Soda does not attract moisture from the air. It is used in washing, in glass manufactories, in dyeing, soap-making, etc.

Sulphate of Soda is “Glauber’s Salt”; it is also employed in glass-making. Mixed with sulphuric acid and water, it forms a freezing mixture. Glass, as we have seen, is made with silicic acid (sand), soda, potassa, oxide of lead, and lime, and is an artificial silicate of soda.

Lithium is the lightest of metals, and forms the link between alkaline and the alkaline earth metals. The salts are found in many places in solution. The chloride when decomposed by electricity yields the metal.

Cæsium and Rubidium require no detailed notice from us. They were first found in the solar spectrum, and resemble potassium.

Ammonium is only a conjectural metal. Ammonia, of which we have already treated, is so like a metallic oxide that chemists have come to the conclusion that its compounds contain a metallic body, which they have named hypothetically Ammonium. It is usually classed amongst the alkaline metals. The salts of ammonia are important, and have already been mentioned. Muriate (chloride) of ammonia, or sal-ammoniac, is analogous to chloride of sodium and chloride of potassium. It is decomposed by heating it with slaked lime, and then gaseous ammonia is given off.

The Metals of the Alkaline Earths.

Barium is the first of the four metals we have to notice in this group, and will not detain us long, for it is little known in a free condition. Its most important compound is heavy spar (sulphate of baryta), which, when powdered, is employed as a white paint. The oxide of barium, BaO, is termed baryta.

Nitrate of Baryta is used for “green fire,” which is made as follows:—Sulphur, twenty parts; chlorate of potassium, thirty-three parts; and nitrate of baryta, eighty parts (by weight).

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Calcium forms a considerable quantity of our earth’s crust. It is the metal of lime, which is the oxide of calcium. In a metallic state it possesses no great interest, but its combinations are very important to us. Lime is, of course, familiar to all. It is obtained by evolving the carbonic acid from carbonate of lime (CaO).

The properties of this lime are its white appearance, and it develops a considerable amount of heat when mixed with water, combining to make hydrate of lime, or “slaked lime.” This soon crumbles into powder, and as a mortar attracts the carbonic acid from the air, by which means it assumes the carbonate and very solid form, which renders it valuable for cement and mortar, which, when mixed with sand, hardens. Caustic lime is used in whitewashing, etc.

Carbonate of Lime (CaCO₃) occurs in nature in various forms, as limestone, chalk, marble, etc. Calc-spar (arragonite) is colourless, and occurs as crystals. Marble is white (sometimes coloured by metallic oxides), hard, and granular. Chalk is soft and pulverizing. It occurs in mountainous masses, and in the tiniest shells, for carbonate of lime is the main component of the shells of the crustacea, of corals, and of the shell of the egg; it enters likewise into the composition of bones, and hence we must regard it as one of the necessary constituents of the food of animals. It is an almost invariable constituent of the waters we meet with in Nature, containing, as they always do, a portion of carbonic acid, which has the power of dissolving carbonate of lime. But when gently warmed, the volatile gas is expelled, and the carbonate of lime deposited in the form of white incrustations upon the bottom of the vessel, which are particularly observed on the bottoms of tea-kettles, and if the water contains a large quantity of calcareous matter, even our water-bottles and drinking-glasses become covered with a thin film of carbonate of lime. These depositions may readily be removed by pouring into the vessels a little dilute hydrochloric acid, or some strong vinegar, which in a short time dissolves the carbonate of lime.

Sulphate of Lime (CaSO₄) is found in considerable masses, and is commonly known under the name of Gypsum. It occurs either crystallized or granulated, and is of dazzling whiteness; in the latter form it is termed Alabaster, which is so soft as to admit of being cut with a chisel, and is admirably adapted for various kinds of works of art. Gypsum contains water of crystallization, which is expelled at a gentle heat. But when ignited, ground, and mixed into a paste with water, it acquires the property of entering into chemical combination with it, and forming the original hydrate, which in a short time becomes perfectly solid. Thus it offers to the artist a highly valuable material for preparing the well-known plaster of Paris figures, and by its use the noblest statues of ancient and modern art have now been placed within the reach of all. Gypsum, moreover, has received a valuable application as manure. In water it is slightly soluble, and imparts to it a disagreeable and somewhat bitterish, earthy taste. It is called “selenite” when transparent.

Phosphate of Lime constitutes the principal mass of the bones of animals, and is extensively employed in the preparation of phosphorus; in the form of ground bones it is likewise used as a manure. It appears to belong to those mineral constituents which are essential to the nutrition of animals. It is found in corn and cereals, and used in making bread; so we derive the phosphorus which is so useful to our system.

Chloride of Lime is a white powder smelling of chlorine, and is produced by passing the gas over the hydrate of lime spread on trays for the purpose. It is the well-known “bleaching powder.” It is also used as a disinfectant. The Fluoride of Calcium is Derbyshire spar, or “Blue John.” Fluor spar is generally of a purple hue. We may add that hard water can be softened by adding a little powdered lime to it.

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Magnesium sometimes finds a place with the other metals, for it bears a resemblance to zinc. Magnesium may be prepared by heating its chloride with sodium. Salt is formed, and the metal is procured. It burns very brightly, and forms an oxide of magnesia (MgO). Magnesium appears in the formation of mountains occasionally. It is ductile and malleable, and may be easily melted.

Carbonate of Magnesia, combining with carbonate of lime, form the Dolomite Hills. When pure, the carbonate is a light powder, and when the carbonic acid is taken from it by burning it is called Calcined Magnesia.

The Sulphate of Magnesia occurs in sea-water, and in saline springs such as Epsom. It is called “Epsom Salts.” Magnesium wire burns brightly, and may be used as an illuminating agent for final scenes in private theatricals. Magnesite will be mentioned among Minerals.

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Strontium is a rare metal, and is particularly useful in the composition of “red-fire.” There are the carbonate and sulphate of strontium; the latter is known as Celestine. The red fire above referred to can be made as follows, in a dry mixture. Ten parts nitrate of strontia, 1½ parts chlorate of potassium, 3½ parts of sulphur, 1 part sulphide of antimony, and ½ part charcoal. Mix well without moisture, enclose in touch paper, and burn. A gorgeous crimson fire will result.

Metals of the Earths.

Aluminium (Aluminum) is like gold in appearance when in alloy with copper, and can be procured from its chloride by decomposition with electricity. It occurs largely in nature in composition with clays and slates. Its oxide, alumina (Al₂O₃), composes a number of minerals, and accordingly forms a great mass of the earth. Alumina is present in various forms (see Minerals) in the earth, all of which will be mentioned under Crystallography and Mineralogy. The other nine metals in this class do not call for special notice.

Heavy Metals

Iron, which is the most valuable of all our metals, may fitly head our list. So many useful articles are made of it, that without consideration any one can name twenty. The arts of peace and the glories of war are all produced with the assistance of iron, and its occurrence with coal has rendered us the greatest service, and placed us at the head of nations. It occurs native in meteoric stones.

Iron is obtained from certain ores in England and Sweden, and these contain oxygen and iron (see Mineralogy). We have thus to drive away the former to obtain the latter. This is done by putting the ores in small pieces into a blast furnace (fig. 402) mixed with limestone and coal. The process of severing the metal from its ores is termed smelting, the air supplied to the furnace being warmed, and termed the “hot blast.” The “cold blast” is sometimes used. The ores when dug from the mine are generally stamped into powder, then “roasted,”—that is, made hot, and kept so for some time to drive off water, sulphur, or arsenic, which would prevent the “fluxes” acting properly. The fluxes are substances which will mix with, melt, and separate the matters to be got rid of, the chief being charcoal, coke, and limestone. The ore is then mixed with the flux, and the whole raised to a great heat; as the metal is separated it melts, runs to the bottom of the “smelting furnace,” and is drawn off into moulds made of sand; it is thus cast into short thick bars called “pigs,” so we hear of pig-iron, and pig-lead. Iron is smelted from “ironstone,” which is mixed with coke and limestone. The heat required to smelt iron is so very great, that a steam-engine is now generally employed to blow the furnace. (Before the invention of the steam-engine, water-mills were used for the same purpose.) The smelting is conducted in what is called a blast furnace. When the metal has all been “reduced,” or melted, and run down to the bottom of the furnace, a hole is made, out of which it runs into the moulds; this is called “tapping the furnace.”

Smelting is often confounded with melting, as the names are somewhat alike, but the processes are entirely different; in melting, the metal is simply liquefied, in smelting, the metal has to be produced from ores which often have no appearance of containing any, as in the case of ironstone, which looks like brown clay.

The cone of the furnace, A, is lined with fire-bricks, i i, which is encased by a lining, l l; outside are more fire-bricks, and then masonry, m n; C is the throat of the furnace; D the chimney. The lower part, B, is called the boshes. As soon as the ore in the furnace has become ignited the carbon and oxygen unite and form carbonic acid, which escapes, and the metal fuses at last and runs away. The coal and ore are continually added year after year. The glassy scum called “slag “ protects the molten iron from oxidation.

The metal drawn from the blast furnace is “pig iron,” or “cast” iron, and contains carbon. This kind of iron is used for casting operations, and runs into sand-moulds. It contracts very little when cooling. It is hard and brittle.

Bar Iron is the almost pure metal. It is remarkably tenacious, and may be drawn into very fine wire or rolled. But it is not hard enough for tools. It is difficult to fuse, and must be welded by hammering at a red heat. Wire-drawing is performed by taking the metal as a bar, and passing it between rollers (fig. 404), which flattens it, and then between a new set, which form cutting edges on the rolled plate (fig. 405), the projections of one set fitting into the hollows of the other closely as in the illustration. The strips of metal come out at the aperture seen at A in the next illustration. These rods are drawn through a series of diminishing holes in a steel plate, occasionally being heated to keep it soft and ductile. When the wire has got to a certain fineness it is attached to a cylinder and drawn away, at the same time being wound round the cylinder over a small fire. Some metals can be drawn much finer than others. Gold wire can be obtained of a “thickness” (or thinness) of only the 5,000th part of an inch, 550 feet weighing one grain! But platinum has exceeded this marvellous thinness, and wire the 30,000th part of an inch has been produced. Ductility and malleability are not always found in the same metal in proportion. The sizes of wires are gauged by the instrument shown in the margin. The farther the wire will go into the groove the smaller its “size.”

Steel contains a certain amount of carbon, generally about 1 to 2 per cent. Cast steel is prepared from cast iron. Steel from bar-iron has carbon added, and is termed bar-steel. The process is called “cementation,” and is carried on by packing the bars of iron in brick-work boxes, with a mixture of salt and soot, or with charcoal, which is termed “cement.” Steel is really a carbide of iron, and Mr. Bessemer founded his process of making steel by blowing out the excess of carbon from the iron, so that the proper amount—1·5 per cent.—should remain.

A brief summary of the Bessemer process may be interesting. If a bar of steel as soft as iron be made red-hot and plunged into cold water, it will become very hard. If it be then gently heated it will become less hard, and is then fitted for surgical instruments. The various shades of steel are carefully watched,—the change of colour being due to the varying thickness of the oxide; for we know that when light falls upon very thin films of a substance,—soap-bubbles, for instance,—the light reflected from the under and upper surfaces interfere, and cause colour, which varies with the thickness of the film. These colours in steel correspond to different temperatures, and the “temper” of the steel depends upon the temperature it has reached. The following table extracted from Haydn’s “Dictionary of Science” gives the “temper,” the colour, and the uses of the various kinds of steel.

The Bessemer process transfers the metal into a vessel in which there are tubes, through which air is forced, which produces a much greater heat than a bellows does. Thus in the process the carbon of the iron acts as fuel to maintain the fusion, and at the same time by the bubbling of the carbonic acid mixes the molten iron thoroughly.

During the bubbling up of the whole mass of iron, and the extreme elevation of temperature caused by the union of the carbon of the impure iron with the oxygen of the air, the oxide of iron is formed, and as fast as it forms fuses into a sort of glass; this unites with the earthy matters of the “impure” iron, and floats on the upper part as a flux, thus ridding the “cast iron” of all its impurities, with no other fuel than that contained in the metal itself, and in the air used. When the flame issuing from the “converter” contracts and changes its colour, then the time is known to have arrived when the iron is “ de-carbonized.” The amount of carbon necessary is artificially added, ebullition takes place, a flame of carbonic oxide comes out, and the metal is then run into ingots.

The compounds of iron which are soluble in water have a peculiar taste called chalybeate (like ink). Many mineral springs are so flavoured, and taste, as the immortal Samuel Weller put it, “like warm flat-irons.” Iron is frequently used as a medicine to renew the blood globules.

Protoxide of Iron is known only in combination.

Sesqui-Oxide of Iron is “red ironstone.” Powdered it is called English rouge, a pigment not altogether foreign to our use. In a pure state it is a remedy for arsenical poisoning, and is really the “rust” upon iron.

Bisulphide of Iron is iron pyrites, and is crystalline.

Chloride of Iron is dissolved from iron with hydrochloric acid. It is used in medicine.

Cyanide of Iron makes, with cyanide of potassium, the well-known prussiate of potash (ferro-cyanide of potassium), which, when heated, precipitates Prussian blue (cyanogen and iron).

The Sulphate of the Protoxide is known as copperas, or green vitriol, and is applied to the preparation of Prussian blue.

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Manganese is found extensively, but not in any large quantities, in one place; iron ore contains it. It is very hard to fuse, and is easily oxidised. The binoxide is used to obtain oxygen, and when treated with potassium and diluted, it becomes the permanganate of potassium, and is used as “Condy’s fluid.” It readily oxides organic matters, and is thus a disinfectant. It crystallizes in long, deep, red needles, which are dissolved in water. It is a standard laboratory test. There are other compounds, but in these pages we need not detail them.

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Cobalt and Nickel occur together. They are hard, brittle, and fusible. The salts of cobalt produce beautiful colours, and the chloride yields an “invisible” or sympathetic ink. The oxide of cobalt forms a blue pigment for staining glass which is called “smalt.” Nickel is chiefly used in the preparation of German silver and electro-plating. The salts of nickel are green. Nickel is difficult to melt, and always is one of the constituents of meteoric iron, which falls from the sky in aërolites. It is magnetic like cobalt, and is extracted from the ore called kupfer-nickel. A small United-States coin is termed a “nickel.”

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Copper is the next metal we have to notice. It has been known for centuries. It is encountered native in many places. The Cornish copper ore is the copper pyrites. The fumes of the smelting works are very injurious, containing, as they do, arsenic and sulphur. The ground near the mines is usually bare of vegetation in consequence of the “smoke.” Sheet copper is worked into many domestic utensils, and the alloy with zinc, termed Brass, is both useful and ornamental. Red brass is beaten into thin leaves, and is by some supposed to be “gold leaf”; it is used in decorative work. Bronze is also an alloy of copper, as are gun-metal, bell-metal, etc.

Next to silver, copper is the best conductor of electricity we have. It is very hard and tough, yet elastic, and possesses malleability and ductility in a high degree. It forms two oxides, and there are several sulphides; the principal of the latter are found native, and worked as ores. The sulphate of copper is termed blue vitriol, and is used in calico-printing, and from it all the (copper) pigments are derived. It is also used in solution by agriculturists to protect wheat from insects. When copper or its alloys are exposed to air and water, a carbonate of copper forms, which is termed verdigris. All copper salts are poisonous; white of eggs is an excellent remedy in such cases of poisoning.

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Lead is obtained from galena, a sulphide of lead. It is a soft and easily-worked metal. When freshly cut it has quite a bright appearance, which is quickly tarnished. Silver is often present in lead ore, and is extracted by Pattison’s process, which consists in the adaptation of the knowledge that lead containing silver becomes solid, after melting, at a lower temperature than lead does when pure. Pure lead therefore solidifies sooner.

One great use of lead is for our domestic water-pipes, which remind us in winter of their presence so disagreeably. Shot is made from lead, and bullets are cast from the same metal. Shot-making is very simple, and before the days of breech-loading guns and cartridges, no doubt many readers have cast bullets in the kitchen and run them into the mould over a basin of water or a box of sand. For sporting purposes lead is mixed with arsenic, and when it is melted it is poured through a sort of sieve (as in the cut) at the top of a high tower. (See figs. 413 and 414). The latter illustration gives the section of the shot tower; A is the furnace, B is the tank for melting the lead, and the metal is permitted by the workman at C to run through the sieve in fine streams. As the lead falls it congeals into drops, which are received in water below to cool them. They are, of course, not all round, and must be sorted. This operation is performed by placing them on a board tilted up, and under which are two boxes. The round shot rush over the first holes and drop into the second box, but the uneven ones are caught lagging, and drop into box No. 1. They are accordingly sent to the furnace again.

The next process is to sort the good shot for size. This is done by sieves—one having holes a little larger than the size of shot required. This sieve passes through it all of the right size and smaller, and keeps the bigger ones. Those that have passed this examination are then put into another sieve, which has holes in it a little smaller than the size of shot wanted. This sieve retains the right shot, and lets the smaller sizes pass, and so on. The shot are sized and numbered, glazed by rolling them in a barrel with graphite, and then they are ready for use. Bullets are made by machinery by the thousand, and made up into cartridges with great speed.

The compounds of lead are also poisonous, and produce “colic,” to which painters are subject. Red lead, or minium, is a compound of the protoxide and the binoxide, and may be found native. The former oxide is litharge; white lead, or the carbonate of lead, is a paint, and is easily obtained by passing a stream of carbonic acid into a solution of acetate of lead. It is used as a basis of many paints.

Tin is another well-known metal. It is mentioned by Moses. It possesses a silver-like lustre, and is not liable to be oxidised. The only really important ore is called Tinstone, from which the oxygen is separated, and the metal remains. Cornwall has extensive tin mines. Tin is malleable and ductile, and can be beaten into foil or “silver leaf,” or drawn into wire. It prevents oxidation of iron if the latter be covered with it, and for tinning copper vessels for culinary purposes. The Romans found tin in Cornwall, and the term “Stanneries” was applied to the courts of justice among the tin miners in Edward the First’s time. We have already mentioned the alloys of tin. The oxides of tin, “Stannous” and “Stannic,” are useful to dyers. The latter is the tinstone (SnO₂). Sulphide of tin is called “Mosaic gold,” and is much used for decorative purposes.

Zinc is procured from calamine, or carbonate of zinc, and blende, or sulphide of zinc. It has for some years been used for many purposes for which lead was once employed, as it is cheap and light. Zinc is a hard metal of a greyish colour, not easily bent, and rather brittle; but when made nearly red-hot, it can be rolled out into sheets or beaten into form by the hammer. Zinc is about six-and-three-quarter times heavier than water. Like many other metals, it is volatile (when heated to a certain extent it passes off into vapour), and the probable reason that it was not known or used of old is that it was lost in the attempt to smelt its ores. Zinc is now obtained by a sort of distillation; the ores are mixed with the flux in a large earthen crucible or pot.

We have already noticed the alloy of zinc with copper (brass), and the use of zinc to galvanize iron by covering the latter with a coating of zinc in a bath is somewhat analogous to electro-plating. The metal is largely used as the positive element in galvanic batteries, and for the production of hydrogen in the laboratory. Zinc forms one oxide (ZnO), used for zinc-white. The sulphate of zinc is white vitriol, and the chloride of zinc is an “antiseptic.” Certain preparations of the metal are used in medicine as “ointments” or “washes,” and are of use in inflammation of the eyelids.

Chromium. This “metallic element” is almost unknown in the metallic state. But although little known, the beautiful colours of its compounds make it a very interesting study. The very name leads one to expect something different to the other metals—chroma, colour. The metal is procured from what is known as chrome-ironstone, a combination of protoxide of iron and sesqui-oxide of chromium (FeOCr₂O₃). By ignition with potassium we get chromic acid and chromate of potassium, a yellow salt which is used to make the other compounds of chromium. The metal is by no means easy to fuse.

Sesqui-Oxide of Chromium is a fine green powder employed in painting porcelain.

Chromate of Lead is termed “chrome yellow,” and in its varieties is employed as a paint.

Chromate of Mercury is a beautiful vermilion. There are numerous other combinations which need not be mentioned here.