« PreviousContinue »
All the metals are considered as simple bodies, none having been decomposed or resolved into other principles; also, at one time, they were supposed to be formed of a peculiar basis and an imaginary inflammable principle called phlogiston. This theory was very favourable to the idea of forming metals, and transmuting or changing them into each other. The existence of phlogiston is no longer believed in, and the science of alchemy is only remembered as affording an instance of the dangers of false theories, and of the great credulity of persons in many respects well informed.
The oxide of a metal was formerly called a calx, and its conversion was supposed to be owing to the loss of the phlogiston; but it was observed that the metal gained instead of losing weight by this change; in fact, it acquires just the weight of the oxygen it combines with. When the oxides of metals are made to part with their oxygen, hey are reduced to the metallic state, and upon this depends the art of reducing metals from their
This metal was unknown in Europe before 1748, and is still chiefly found in South America: it has been also found in Estremedura, in Old Spain.
In colour it is nearly as white as silver. It is very difficult of fusion, and can only be melted by the assistance of oxygen gas or by galvanic elec. tricity. From its refractory quality, it is employed for crucibles and other chemical utensils exposed to heat, for which it is admirably adapted.
It is also extremely ductile and malleable, and may be drawn into very thin wire, and hammered into thin plates.
Platina does not tarnish on exposure to the atmosphere, and takes an excellent polish, on which account it is used in making specula for telescopes. It is also capable of being welded, a property only possessed by it and iron. It is the heaviest of the metals; its specific gravity being nearly 22.
Platina is readily dissolved by the nitro-muriatic acid and by chlorine, but is not acted upon by any other acid. It also combines with sulphur and phosphorus.
Platina is brought to Europe in small flattened grains, which, however, are not pure platina, but contain a mixture also of nine other metals. metals, osmium, iridium, rhodium, and palladium, were unknown till they were discovered in these grains.
Gold is found in nature in a metallic state. It is generally met with in grains called gold-dust, mixed with the sands of rivers, either being carried away by them from the rocks which contain it, or having been deposited in ancient alluvium. It is chiefly found in Africa, also in Brazil and Peru. There are mines of it also in Hungary; and it is met with in quantities too small to be worth working, in the sands of many rivers of Europe.
Lately some was found in the county of Wicklow, in Ireland; one grain weighed 22 ounces, and considerable expectations were formed; but, notwithstanding, the works were abandoned as unproductive.
Gold is the heaviest of the metals except platina. It is of a rich yellow colour, and not very hard when pure.
It melts at a bright red heat, but cannot be oxidated by any furnace, though it may by electricity and galvanism. It does not oxidate in the air; hence it is so useful in gilding, its beautiful lustre remaining untarnished.
It is the most ductile and malleable of the metals, and may be drawn into the finest wire for goldlace and other purposes, and may also be hammered into leaves of extreme thinness for gilding.
Gold is not acted on by any acid except the nitro-muriatic acid and chlorine. From this property the former was named aqua-regia, gold being called by the alchemists the king of the metals.
The solution of gold, called muriate of gold, yields by evaporation crystals of a beautiful yellow colour, which, when dissolved in water and precipitated by a solution of tin, afford the beautiful powder called the purple precipitate of cassius, much used in enamelling. This consists of oxide of gold mixed with oxide of tin.
If any substance, as a piece of ribband, be dipped into the muriate of gold, and then exposed to a stream of hydrogen gas, the gold will be revived, and the substance covered with it. Some combustible bodies attract the oxygen from the solution of gold, and cause it re-appear in its metallic state. Thus, if a piece of charcoal be put into a glass-jar containing a diluted solution of gold, and exposed to the direct rays of the sun, it will soon appear gilt. When ammonia is added to a solution of gold, a yellow precipitate is formed, called fulminating gold, because it has the property of exploding when exposed to heat.
If to a solution of muriate of gold, sulphuric ether be added, the gold will combine with the ether,
leaving the acid, and will float on the surface of the fluid. If polished steel be dipped into this, it will be covered with a coating of metallic gold. This process is employed for gilding lancets, and other surgical instruments, to defend them from rust.
Gold easily alloys with mercury, which is, therefore, much employed for extracting it from the substances with which it is mixed in its natural state. The mercury, being volatile, is driven off by heat, and the gold remains free.
Gold in its purest state is too soft to be used as coin; it is, therefore, alloyed with of copper. Jeweller's gold generally contains considerably
Gold seems to have been one of the earliest known of the metals. The ancients were lavish in its use, and it is still frequently used in orna ments among savage tribes.
Silver is often found native, or in the metallic state, but it is most usually combined with other metals, or sulphur. In its native state it frequently assumes an arborescent form. The richest silver mines are in Mexico and Peru; but others exist in many countries. Lead ore very frequently contains a quantity of silver, and sometimes it is worth extracting.
Silver is of a brilliant white colour. It is very ductile and malleable; may be drawn into fine wire, and beaten into thin leaves; but it is inferior to gold in these qualities.
Silver fuses when heated red hot, and may be cast into moulds, but is not thus converted into oxide
by any continuance of heat: it is oxidized by common and galvanic electricity.
It is not oxidized by the air; but it is tarnished by exposure, because the sulphurous vapours form with the metal a sulphuret of silver.
Oxide of silver is of a dark olive colour, and is obtained by precipitating it from the nitrate of silver by lime-water, this metal being soluble in the nitric acid.
Nitric acid can dissolve more than half its weight of silver, the solution depositing crystals. When these are fused by a gentle heat, they may be poured into moulds, and form the substance called lunar caustic, used in surgery.
Nitrate of silver is used by chemists as a test for muriatic acid; for if it be dropped into any liquid containing muriatic acid, a white precipitate will appear, owing to the superior affinity of silver to muriatic acid, and to the insolubility of muriate of silver. Nitrate of silver is very caustic, staining animal and vegetable substances of a black colour, and hence it is employed as a permanent marking ink for linen, and also for staining hair; though for this last purpose it should be used with great caution, and much diluted.
If a few drops of the nitrate of silver be put upon a piece of glass, and a copper wire be placed in it, a beautiful metallic precipitation of the silver will appear in an arborescent form.
When silver is precipitated from its solution in nitric acid by ammonia, it forms fulminating silver, which is a dangerous preparation; for it explodes by the slight contact of a body.
When mercury is added to the nitric solution of silver, a precipitation of metallic silver is formed