Imágenes de páginas
PDF
EPUB
[blocks in formation]

WITH the exception of the rare element beryllium, these metals were first obtained (although not in the pure state) by Davy, who, soon after his discovery of the metals potassium and sodium, showed that the so-called earths were not elementary bodies as had been supposed, but were compounds of different metals with

oxygen.

The element beryllium is of later discovery, for although as early as 1798 it had been shown by Vanquelin that the particular “earth” in the mineral beryl was different from any other known earth, it was not until 1827 that the metal it contained was isolated by Wöhler. In a state approaching to purity, beryllium was first prepared by Humpidge, 1885.

None of the elements of this family occurs in nature in the uncombined condition; and, with the exception of magnesium, the metals themselves, in their isolated condition, are at present little more than chemical curiosities. In the case of beryllium this is due to the comparative rarity of its compounds; but with calcium, strontium, and barium, whose compounds are extremely abundant, it is owing partly to the difficulty of isolating the metals in a pure state, and also to the fact that hitherto they have received no useful application. Beryllium and magnesium are white metals, which retain their lustre in the air. Calcium, strontium, and barium on exposure to air quickly become converted into oxide.

The metals calcium and strontium, as obtained by earlier experimenters, presented a pale yellow colour (it is doubtful whether the metal barium was actually obtained by these chemists). But

the calcium which has recently been obtained by Moissan is described by him as a silver-white metal.*

Beryllium oxide

All these metals form an oxide of the type RO. is insoluble in water; magnesium oxide is very slightly soluble (1 part in 55,000 or 100,000 parts of water), but the solution shows a feeble alkaline reaction. The calcium, strontium, and barium oxides show increasing solubility, and stronger alkalinity and causticity. On this account these elements are known as the metals of the alkaline earths. These three elements also form peroxides of the type RO

All the monoxides are basic, and combine with acids to form salts of the types RC, RSO4, R(NO3).

The element beryllium (the typical element) stands apart from the others of this family in many of its chemical relations. Thus the oxide BeO, unlike the corresponding compounds of the other elements, does not combine with water to form the hydroxide. The hydroxide Be(HO), is soluble in sodium and potassium hydroxide. In this respect beryllium exhibits its resemblance to zinc. The chloride also differs from the other chlorides in being volatile.

In its permanence in air, its colour, its high melting-point, the solubility of its sulphate, and the readiness with which its hydroxide is converted by heat into the oxide, beryllium exhibits a close similarity to magnesium. In the solubility of its hydroxide in potassium hydroxide, and in its inability to decompose water, beryllium also shows a marked resemblance to zinc.

The three elements, calcium, strontium, and barium, exhibit a closer resemblance to each other in most of their physical and chemical relations, than to either magnesium or beryllium.

They are readily distinguished by their different spectra. Barium salts, when heated in a non-luminous flame, impart to it a green colour. Calcium and strontium, under the same circumstances, each give a red colour; but the red imparted by strontium compounds is more brilliant, and less orange, than that of calcium salts. When the flames are examined by the spectroscope, the most characteristic lines given by barium are two in the bright green (Baa and Baẞ). These are accompanied by a number of less brilliant lines. The spectrum of strontium consists of four

* Perhaps nothing could be more eloquent as to the truth of the above statement that these elements in the isolated state are mere chemical curiosities, than the fact that even their colour can scarcely be stated with certainty.

specially prominent lines, one in the bright blue (Srd), one in the orange (Sra), and two in the red (Srẞ and Sry), with others less pronounced; while that of calcium contains one brilliant green line (Caß) and one equally brilliant orange line (Caa), besides a large number of less prominent lines

Occurrence.

BERYLLIUM.

Symbol Be. Atomic weight=9.1.

This element occurs principally in the mineral beryl, a double silicate of the composition 3BeO,Al2O3,6SiO2. The transparent varieties are used as gems, the transparent green beryl being the precious emerald.

Phenacite is beryllium silicate BeSiO4, while chrysoberyl has the composition BeO,Al2O3.

Formation. The element is obtained by heating sodium in the vapour of beryllium chloride, all air having been previously replaced by hydrogen. The product is afterwards melted beneath fused sodium chloride, when it is obtained as a coherent solid metal. It may also be obtained by the electrolysis of the fused mixed chlorides of beryllium and potassium.

Properties. -Beryllium is a white metal resembling magnesium. It has a specific gravity of 2.1, and is moderately malleable. It does not readily tarnish in the air at ordinary temperatures, but when strongly heated, becomes coated with a protecting film of oxide. The powdered metal, when heated, takes fire, and burns with a bright light. It has no action upon water, even at the boiling temperature.

Beryllium is easily dissolved by dilute hydrochloric acid, with evolution of hydrogen. Cold dilute sulphuric acid is without action, but when heated slowly dissolves it. Nitric acid slowly attacks it when concentrated and hot. It readily dissolves in potassium hydroxide, with evolution of hydrogen.

Beryllium Compounds.-The best known are the oxide (berylla), BeO, a white infusible powder, insoluble in water, soluble in acids; the chloride, BeCl2, obtained by heating the oxide with charcoal in a stream of chlorine, a white crystalline solid, readily fused and volatilised.

Beryllium compounds do not impart any colour to a Bunsen flame. They are characterised by possessing a sweet taste, hence the name of glucinum originally given to this element.

MAGNESIUM.

Symbol, Mg, Atomic weight=24.36.

Occurrence.-Magnesium is not found in the uncombined state. In combination it is widely distributed, and is extremely abundant. In the mineral dolomite, associated with lime as carbonate, it occurs in mountainous masses.

Magnesite, MgCO3; kieserite, MgSO,,H,O; carnallite, MgCl2,

KC1,6H2O, are amongst the commoner naturally occurring magnesium compounds. It is also a constituent of asbestos, meerschaum, serpentine, tale, and a large number of other silicates. As sulphate and chloride it is met with in sea-water and many saline springs.

Modes of Formation.-Magnesium was obtained by Bunsen by the electrolysis of fused magnesium chloride; and later by Matthiessen by electrolysing the fused double chloride of magnesium and potassium (carnallite).

On a manufacturing scale it was later produced by the reduction of magnesium chloride by means of sodium. A mixture of anhydrous magnesium chloride (or fused mixed chlorides of magnesium and sodium, or potassium), powdered cryolite, and sodium is thrown into a red-hot crucible, which is quickly closed. A violent reaction takes place, at the conclusion of which the melted mixture is stirred with an iron rod to cause the globules of magnesium to run together.

The crude metal is afterwards purified by distillation.

At the present time magnesium is manufactured by a process which is practically that formerly employed by Matthiessen on a small scale, but modified in detail to suit modern electrical resources. An iron crucible or melting pot is used, which is made the cathode, and the double magnesium potassium chloride (carnallite) is maintained at a temperature about 700°--i.e. a dull red heat-by means of gaseous fuel. The anode consists of a stout carbon rod which dips into the molten material, and is surrounded by a porcelain cylinder which conveys away the chlorine.

Properties.-Magnesium is a silvery-white metal, which does not tarnish in dry air, but becomes coated with a film of oxide when exposed to air and moisture. At a red heat it melts, and at higher temperatures may be distilled. When heated in the air it takes fire, and burns with a dazzling white light, which is extremely rich in the chemically active rays. The flash of light, obtained by projecting a small quantity of magnesium filings into a spirit flame, is used for photographic purposes. Magnesium is only moderately malleable, and is only ductile at high temperatures; it is readily pressed into the form of wire at a temperature slightly below its melting-point. Magnesium only slightly decomposes water even at the boiling-point; but when strongly heated in a current of steam, the metal takes fire (p. 173). Magnesium is rapidly dissolved by dilute acids, with brisk evolution of hydrogen, but solutions of caustic

alkalies are unacted upon by it (compare Zinc). When heated with aqueous solutions of ammonium salts, hydrogen is evolved, and a double salt of magnesium and ammonium is found in the solution. Magnesium combines directly with nitrogen, when strongly heated in that gas, forming magnesium nitride, N,Mgs (p. 232).

On account of the brilliant light emitted by burning magnesium, it is employed for signalling purposes, and also in pyrotechny.

Magnesium Oxide (magnesia), MgO, is found native as the mineral periclase. It is formed when magnesium burns in the air, or when magnesium carbonate is submitted to prolonged gentle calcination, when it is obtained as a white bulky powder, known in commerce as calcined magnesia or magnesia usta.

Magnesia is extensively manufactured from the magnesium chloride occurring in the Stassfurt deposits, by first converting the chloride into carbonate and subjecting this to calcination. Magnesia has been obtained in the crystalline form, identical with that of periclase, by heating the amorphous compound in a stream of gaseous hydrochloric acid. It may be fused in the oxyhydrogen flame, and on cooling it solidifies to a vitreous mass which is sufficiently hard to cut glass. On account of its extreme refractoriness, magnesia is used for a variety of metallurgical purposes, such as the manufacture of crucibles, cupels, &c.

Magnesium Hydroxide, Mg(HO), is found in nature as the mineral brucite. It is prepared by precipitating a magnesium salt by sodium or potassium hydroxide. At a dull red heat it loses water, and is converted into the oxide, and the magnesia so obtained has the property of rehydrating itself in contact with water, with evolution of heat.

Magnesium hydroxide slowly absorbs carbon dioxide, forming the carbonate; owing to this fact, and to the property it possesses of rehydration, magnesia that has been prepared by calcination at a low temperature can be employed as a cement. Thus, if calcined magnesite be made into a paste with water, the mixture is found to harden in about twelve hours, and ultimately to acquire a hardness equal to that of Portland cement.

Magnesium Chloride, MgCl2.-This salt is formed when magnesia, or magnesium carbonate, or the metal itself, is dissolved in hydrochloric acid. From this solution monosymmetric crystals of the composition MgCl2,6HO are deposited. When this salt is heated it loses water, and at the same time is partially decomposed into hydrochloric acid and magnesia; in order, therefore, to pre

« AnteriorContinuar »