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Potassium is a brilliant metal of bright silvery lustre; at the temperature of the air it may be cut with a knife, but at the freezing-point it is brittle; at 570.7 C. it melts, and at a low red heat distils. Its density is only 0.865, the density of water being considered as 1.000.

II. SODIUM=Na. The metal sodium bears a remarkable resemblance to potassium. If placed upon water it acts upon it with the utmost energy, fuses, and rolls, as a red-hot globule, rapidly from side to side of the containing vessel ; the heat, however, does not rise sufficiently to inflame the hydrogen evolved unless the movement of the metal be impeded or confined, as by placing it upon a moistened piece of paper, to a smaller area ; the hydrogen is then kindled and burns with an orange-yellow flame, owing to particles of sodium being vaporized with it. Like potassium, sodium cannot be exposed to the air, and must be protected from it in a similar manner.

The lustre of sodium is as brilliantly white as that of silver ; the metal is as soft as potassium at the ordinary temperature of the air, melts at 90° C., and distils at a lower temperature than potassium. Its specific gravity is 0.972.

III. Lithium=Li. This metal is far rarer than either of the two preceding; like them it decomposes water, but not with their extreme energy. If exposed to the air it combines with the oxygen, and is therefore kept out of contact with that element.

Lithium has the colour and brightness of polished silver; at common temperatures it is harder than potassium or sodium, but softer than lead; it is also capable of welding, i. e. of junction by pressure. It melts at 180° C., and cannot be distilled. It is the lightest of all solid

bodies, its specific gravity being only 0:5936. Metals which are lighter than Water, and decompose it at common

temperatures.

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SUBDIVISION II. BARIUM, STRONTIUM, CALCIUM, AND MAGNESIUM.

These metals are for the most part of a colour closely resembling that of gold: they are of greater specific gravity than water. To a powerful affinity for oxygen is superadded an incapability of volatilization by heat, which renders them still more difficult of preparation than the members of the preceding class. They tarnish in the air from formation of oxide, and decompose water, although not with the extreme energy of potassium, sodium, and lithium.

I. BARIUM=Ba. The metal barium decomposes water at the common temperature. It has a yellow colour.

II. STRONTIUM=Sr. The metal strontium decomposes water at the ordinary temperature. It has a golden colour, and a specific gravity of 2:5418.

III. CALCIUM=Ca. The metal calcium decomposes water at the ordinary temperature. It is of a pale yellow colour, and its specific gravity is 1.5778. Its hardness approaches that of gold.

IV. MAGNESIUM=Mg. The metal magnesium does not decompose water at the common temperature, but is oxidized by hot water. It is white, and has the specific gravity of 1:87.

Metals which are heavier than Water, and decompose it at common

temperatures.

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SUBDIVISION III.

YTTRIUM, ERBIUM, TERBIUM, CERIUM, LANTHANIUM, DIDYMIUM,

THORINUM, ZIRCONIUM, GLUCINUM, URANIUM, ALUMINIUM. CHROMIUM, MANGANESE, IRON, COBALT, NICKEL, ZINC.

These metals are either dull white, greyish white, or bluish white. Their specific gravity varies from 2:56 to 8.8. The majority of them are produced with as much difficulty as those of the second class; this additional disadvantage also attaches to some of them, that the substances of which they form part are themselves found in very small quantity, Iron and zinc are the most easily obtained, then nickel, cobalt and aluminium, next chromium and manganese, whilst the remainder are of extremely rare occurrence. Many among them are tarnished by exposure to the air, moist air exerting a far more powerful effect than air when dry. Some of these metals decompose water slowly at ordinary temperatures, expelling its hydrogen; but most of them evolve hydrogen from hydrochloric acid, combining with its chlorine. A new characteristic is to be remarked in this class of metals, viz. the power which some of them possess of being attracted by the magnet.

I. The first ten metals are almost unknown, and are not likely to fall into the hands of the student. II. ALUMINIUM=Al. The metal aluminium does not decompose water even in the slightest degree at the boiling temperature, nor does it tarnish in dry or moist air; if, however, it is heated in the air, it burns and becomes wholly converted into its oxide (A1,02). This metal does not decompose water to which nitric or sulphuric acid has been added, but it acts readily upon hydrochloric acid, expelling the hydrogen and combining with the chlorine.

Aluminium is a white metal, but has a slightly bluish tinge ; it is somewhat less brilliant than silver; it is malleable; it melts at a red heat. Its density is 2:56. It is not magnetic.

III. CHROMIUM=Cr. The metal chromium behaves with water as aluminium ; it does not oxidize in the air at ordinary temperatures, but at a red heat it absorbs oxygen readily from the air, and is converted into its oxide (Cr, 02). The accounts of its action on hydrochloric acid vary.

Chromium is a greyish-white metal, very brittle, and requiring the heat of the most powerful furnace for fusion. Its density is 8.9. It appears to become magnetic below the freezingpoint.

IV. MANGANESE=Mn. The metal manganese decomposes water at the ordinary temperature, therein differing from the other members of the subdivision. It may naturally be inferred that it readily absorbs oxygen from the air also, and must for this reason be preserved under coal-tar naphtha. It of course expels the hydrogen from hydrochloric acid.

This metal is of a grey colour, but with little metallic lustre ; it is brittle, and requires the heat of the wind-furnace to effect its fusion. Its density is 8.01.

V. Iron=Fe. The metal iron exerts a very slight action on water at a temperature of 50° or 60° C., but when exposed to the joint action of air and moisture it absorbs oxygen with great avidity, forming the oxide well known as rust (Fe, 03). On account of its tendency to combine with oxygen when moisture is present, it is always necessary to cover this metal when exposed to the weather with paint or some such substance, which may form a protective coat between it and the atmosphere. It decomposes hydrochloric acid with great readiness.

Iron has a peculiar grey colour and lustre, and a fibrous structure which confers great toughness. It has the property of welding at a white heat. It melts at 1550° C., and at a higher temperature volatilizes slightly. Its density is 7.87. This metal is powerfully magnetic.

VI. COBALT=Co. The metal cobalt decomposes water above the ordinary temperature, but not when cold, nor does it absorb oxygen from the air except when heated. It decomposes hydrochloric acid slowly.

It has a reddish-grey lustre, and, like iron, a somewhat fibrous structure. It melts at about the same temperature as iron. Its

density is about 8.6. It is generally stated to be magnetic, although some doubts are entertained upon this point.

VII. NICKEL=Ni. The metal nickel bears a strong resemblance to cobalt. It does not oxidize in water or moist air at ordinary temperatures. It decomposes hydrochloric acid slowly.

It has a greyish-white lustre, and in structure resembles iron and cobalt. It melts at a temperature rather higher than the melting point of iron. The density is 8.82. It is magnetic, but less powerfully so than iron.

VIII. Zinc=Zn. The metal zinc is a peculiar one, having but little in common with the four preceding, which all present a strong family likeness. It does not decompose water at the ordinary temperature, or remove oxygen from dry air. In moist air it becomes covered with a thin film of oxide, which protects the metal beneath. If heated in the air it burns with a greenish light, forming the oxide Zn, O. It decomposes hydrochloric acid readily.

Its lustre is bluish white. It melts at 412° C., and volatilizes at a red heat. Its density is 6.9 after fusion, and 7.19 after compression by rolling. It is not magnetic.

Metals, the majority of which expel Hydrogen from Hydrochloric

Acid at the common or boiling temperature.

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