quantity of oxygen gas, which it gives back again in the course of becoming solid. The quantity of oxygen thus absorbed is sometimes 20 times or more the volume of the metal. The only pure acids known to act on silver, are the sulphuric, with the aid of heat, and the nitric. Nitrohydrochloric acid likewise acts on silver, but feebly.

The density of silver is from 10.474 to 10.542, and its combining weight is reckoned 108.3. It unites with oxygen, chlorine, sulphur, and other substances, and it forms alloys with the bodies of its own class.

Its protoxide combines with ammonia, forming a body of vast explosive powers. This dangerous substance, if even slightly pressed by a hard body, nay, touched by a feather when dry, will explode with frightful violence. The explosion is attributed to the reduction of the metal, by the combination of its oxygen with the hydrogen of the ammonia, and the evolution of nitrogen gas. But this scarcely accounts for the effect, and it may be doubted whether other decompositions do not take place. Compounds of the same nature are formed by combinations of the oxides of gold and platinum with ammonia.

Chloride of silver is remarkable for its susceptibility to the action of light, on which account it is employed to fix images on paper. This mineral is found in veins, and sometimes in beds.

Sulphuret of silver, formed by single equivalents of sulphur or silver, is isomorphous with a sulphuret of copper, and the two substances replace one another in certain native sulphurets. The sulphuret is the most important of the ores of silver. It is found almost always in veins in the older formations, the matter of the adjoining rock being frequently impregnated with the sulphuret.

Silver is very generally associated in the natural state with copper, antimony, and arsenic, members of the sulphur

group of bodies. It is frequently found along with lead, in the ores of which it is seen to be diffused in fine threads. It is found likewise along with mercury.

If the compositions indicated by the table were correct, silver would approach in composition to both mercury and lead. Thus,

But, however this be, it is difficult to conceive that these bodies have not all been produced by similar agencies, causing chemical changes of the matter of the siliceous and other mineral substances in which they are found.

XXXII. MERCURY.

Mercury, associated in popular language with silver, as argentum vivum, or quicksilver, has been known from the earliest ages. It is found in several species of ore; but that from which nearly all the mercury of commerce is derived, is the sulphuret, termed likewise cinnabar, from which the metal is obtained by easy means. The cinnabar is mixed with lime, or iron turnings, and exposed to heat in a species of oven or large iron retort. The sulphur combines with the lime or the iron, forming with them a sulphuret, and the mercury itself, which is volatile, then passes off, and is received under water.

Mercury is distinguished from all the metals by being liquid at common temperatures. It has a silver-white colour and great lustre. Its density is about 13.3. At somewhat less than 40° below the zero of F. it becomes solid, crystallizing in regular octahedrons. At the moment of congelation it contracts, its specific gravity increasing to about 14.5. When solid, it is malleable, and may be cut with a knife. It boils at the temperature of 662°, and by a continuance of the heat, may be all evaporated, and condensed again on a cool surface. The vapour of mercury is invisible like air, and highly elastic, and, like other gases, it is expansible in an indefinite degree by heat. If enclosed in a strong vessel, and exposed to a high temperature, it will burst the vessel with tremendous force. Mercury, if entirely pure, does not seem to be acted upon by the moisture of the air at common temperatures; but if heated to near the boiling point, it absorbs oxygen, forming scales of red oxide, and, if exposed in the state of vapour to air or oxygen gas, it slowly combines with oxygen. It is readily dissolved by nitric

acid, but is not affected by hydrochloric acid, in which respect it approaches to the characters of silver.

Mercury, although obtained in the liquid form, may likewise be procured in the state of a fine powder, but it again assumes the liquid form on becoming dry. To obtain mercury in this form, equal weights of the crystallized hydrate of protochloride of tin, and chloride of mercury, are dissolved, the former in diluted hydrochloric acid, and the latter in hot water, and the solutions are mixed and stirred together. The mercury is liberated, the whole of the chlorine combining with the protochloride of tin, and forming bichloride, which remains in solution. So fine is the division, that the liberated mercury takes several hours to subside. Mercury then assumes the form of minute division, which silver and other metals under certain conditions as

sume.

Mercury combines with oxygen and chlorine, sulphur, phosphorus, and other bodies, and with the substances of its own class, forming the alloys termed amalgams. Its combining weight is generally reckoned 101.43, though many chemists double this number.

Mercury is almost equally related to the noble metals, gold and platinum, as silver, but, like silver, it exhibits relations with copper and lead. Like silver, copper, and lead, it has a strong affinity for sulphur, and it is chiefly in combination with this substance that it is found in the mineral state. The sulphuret exists in gneiss, and in beds and veins in later deposites, though in none later, it is believed, than in some shales of the older secondary formations. Mercury is thus one of those rare and beautiful substances, produced in the earlier periods of the globe, when conditions existed of heat and other agencies, which may well account for chemical changes, which we cannot now produce by art.

XXXIII. COPPER.

This metal exists abundantly and in various species of ore. It is often found native, but the ores which produce it most largely are the sulphurets, and in an especial manner, the sulphuret termed glance copper, and copper pyrites, which last is the most important of the ores of copper, and is a compound of sulphuret of copper and sesqui-sulphuret of iron. The smelting of copper is for the most part complicated and laborious. When the ore which is acted upon is copper pyrites, the end aimed at is to drive away the sulphur, and oxidate the iron. The latter oxide combining with silica, which is supplied from siliceous substances during the process, forms a silicate which can be removed, while the copper, by a complication of processes, is freed from the various substances with which it is associated, and at length obtained in a state of sufficient purity for ordinary uses.

Copper is of a fine red tint, and is the only one of the metals, except titanium, which possesses this colour. By burnishing, it acquires a high degree of lustre. Its taste is nauseous and styptic, and by rubbing or heating, a peculiar odour is produced. It is a very malleable metal, and may be hammered into leaves of considerable fineness. In tenacity it surpasses gold, platinum, and silver, and is inferior only to iron. A wire of .078 of an inch in diameter has been found to support a weight of 302.26 lb. avoirdupois. It is hard and elastic, and this property being possessed by some of its alloys, they are sonorous, and used as bells. Its crystalline form seems to be the octahedral. It fuses at a temperature of about 1996° F., and hence it is but a little less fusible than gold; and if the heat be increased, it is dissipated in white fumes. When exposed to a stream of the oxyhydrogen blowpipe, or to voltaic action, it takes fire, and burns brightly, emitting a lively green light. When exposed to the air of the atmosphere, it becomes gra