« PoprzedniaDalej »
13. The union of sulphur with metals, are called sulphurets. All the metals, except gold and one of two others, are capable of combining with sulphur. The latter may be made to unite with sulphur in different proportions. Thus iron and copper, in particular, furnishes a sulphuret, or super-sulphuret. Some of the metals, such as tin, zipc, and manganese, even when oxydized are susceptible of union with sulphur, forming sulphuretted oxyds. Some are precipitated in this state by hydro-sulphurets. See Hydro-sulphurets.
14. With phosphorus, the metals have an affinity, forming metallic phosphurets.
15. With carbon some of them are capable of combining, forming carburets.
16. The metals are for the most part capable of uniting with each other, and the compounds that are thus formed are termed alloys.
17. As metals are seldom found native, but in com. bination with oxygen, sulphur, arsenic, &c. forming ores, before they can be employed for use, they are torrified or roasted, and exposed with some carbonaceous matter, as a flux, in order to deprive them of the mineralizing substances. The volatile ingredients, are separated by roasting, and the oxygen is carried off by the carbon.
18. The ores of metals, we have remarked, may be distinguished from other mineral substances by their specific gravity, which greatly exceeds that of stony substances. The specific gravity of ores, in general, are five, six, seven, or more times, heavier than water. To find the specific gravity of ores and stones, an operation is essential, which is termed weighing them hydrostatically. To accomplish this object with accuracy, it is indispensably necessary to be provided with a balance properly adjusted, and accompanied with a set of weights, above and below the grain. Mr. Nicholson's weights, for hydrostatic purposes, consist in the number of a thousand grains, and the deci. mal fractions of a grain. See Nicholson's Chemistry.
Such accuracy, as is of the first importance in nice experiments, is scarcely necessary for common pur
poses. In general, when the specific gravity is determined in whole numbers, without using the decimal fractions of a grain, it will be sufficient for ores.
Procure a specimen of the mineral, weighing a few drams, freed as much as possible from the matrix, or stony part, and suspend it from the scale of a fine ballance. The mineral must be suspended by a horse hair, or thread of silk attached to the scale, and fastened to the mineral with a loop knot, and its weight ascertained in air, an allowance being made for the weight of the thread or hair. The weight is then to be noted down. The mineral, still suspended from the scale, is to be immersed in a glass of water (which for very accurate purposes should be distilled); a diminution of weight will now ensue; then determine the number of grains necessary to restore the equilibrium. The latter will indicate the magnitude or bulk of the mineral, whilst the former, or the weight in air, will shew the absolute weight. Or, to determine the magnitude, and of course the diminution of weight in water, note down the weight in air, and ascertain its weight in water, subtract the latter from the former; and the magnitude, or quantity of water it displaces will be known. After this, the next object is to determine the specific gravity. The following general rule must now be observed. By the specific gravity of any substance is understood, the quotient of its absolute weight, divided by its magnitude. The calculation, therefore, may be readily made by knowing these two circumstances, viz.
1. The absolute weight, or weight in air.
2. The magnitude, or the quantity of water displaced at a given temperature.
According to the rule, let the sum of the weight in air, be divided by the sum of the weight which the body has lost during its immersion in water, and the quotient will shew the specific gravity.
Suppose a piece of mineral weighsin air 360 grains, but when immersed in water of a stated temperature loses 60% grains, its specific gravity would be nearly 6.
The following is a summary of the average specific gravities of different metallic ores. The specific gravity of Crude platina reaches from
20.6 to 22. Gold
5.7 to 11.8 Silver ores
5.8 to 10.6 Copper ores
5.2 to 6.5 Lead ores
5.1 to 7.18 Tin ores
5,3 to 6.97 Iron ores
5.6 to 8.5 Zinc ores
5.0 to 5.5 Mercury ores
5.4 to 7.75 Antimony ores
5.1 to 6. Bismuth ores
5.8 to 8.6 Nickel ores
5.5 to 6,5 Cobalt ores
5.3 to 7.2 Arsenic ores
5.1 to 6.8 Tungsten ores
5.8 to 7.5 Tellurium ores
5.4 to 6. Uranium ores
5.2 to 5.5 Molybdena ores
5.2 to 6.2 Chrome ores
5.3 to 6.5 Titanium ores
5.1 to 6. Manganese ores
5.1 to 6.8 Columbium ores
5.918 Tantalium ores, not yet ascertained.
Experiment 1. If the auriferous sand of certain rivers be washed with water, in order to separate those bodies of an inferior specific gravity, by which means the gold is obtained at the bottom of the vessel in the state of gold dust; or if native gold, freed as much as possible from its matrix, be reduced to a fine
* Accum's Analysis.
powder and digested, by the assistance of heat, in four parts of nitro-muriatic acid, renewing the acid as it evaporates, a solution of the gold will be formed. The same process may be repeated until all the gold is extracted. Evaporate the solution to dryness, and pour upon the dry mass as much boiling distilled water as is sufficient to dissolve it. Filter the solution through paper, and add to it a solution of sulphate of iron, pre, pared by dissolving one part of that salt in eight or ten of boiling water, until no further change of colour ensues. Suffer the mixture to stand for a few elays, and the gold will be precipitated in the form of a brown powder.
Collect the powder, which is metallic gold, intro. duce it into a crucible, and fuse it into a button or mass.
Rationale, The nitro-muriatic, acid dissolves the gold, the solution is décomposed by sulphate of iron, and the gold is precipitated in a metallic state; the theory of which will be presently noticed.
Remark. The filamentous, dendritical, lamellated, arborescent, wire shaped gold, &c. may be analysed in the foregoing manner. Should silver exist with the gold, a white powder will be seen floating in the nitromuriatic solution; as this is a muriate, 100 parts would indicate 75 parts of silver,
Experiment 2. If to auriferous sand, reduced by washing, or gold dust, one tenth part of mercury be added, and the mixture triturated in an iron or copper vessel containing water, the gold will be dissolved by the mercury, leaving the foreign matter behind. After some preparatory steps, the mercury is separated from the gold and silver, if any) by exposing the alloy in an earthen retort to a heat sufficient to distil the mercury. The gold will remain in the retort.
Rationale. The mercury dissolves the gold, and, by distillation, is volatilized, leaving the gold which is not volatile in the fire, in the retort.
Remark. The gold may be further purified by cupellation, a process which will presently be described. Should silver be present, the gold may be reduced into very fine laminæ, and treated with nitric acid,
which only dissolves the silver. The silver inay be separated from this acid, by muriatic acid, and the muriate of silver thus formed, may be decomposed by soda in a crucible, by which means the silver is separated. Should sulphur and arsenic occur in a mixture with gold, they may be disengaged by torrefication, and then melted with iron, which unites with what remains. Metallic substances may be separated from gold by different fluxes. After having separated the sulphur, a mixture of super-tartrate of potash and nitrate of potash may be used. The gold left is afterwards purified by cupellation. Or, if the gold be first freed from sulphur, it may be melted with one and a half parts of litharge, and three parts of glass, in a crucible covered with common salt. All the heterogeneous metals will thus scorify, and set the gold free. Gold, before the blow pipe, with different fluxes, exhibits singular phenomena.*
Experiment 3. If to an ore, supposed to contain gold, nitro-muriatic acid be added, in the manner described in Experiment 1, and afterwards mixed with a small quantity of muriate of tin; and if a purple precipitate is formed, the presence of gold may be inferred.
Remark. Gold has been known from time immemorial. When pure it is of an orange red, or reddish yellow colour. It possesses a considerable lustre. Its hardness is 61. Its specific gravity is 19.3: Gold is so malleable, that, according to Magellan, its surface may be extended-by the hammer 159.092 times. One grain of gold will cover 501 square inches, and the leaf, thus formed, is only 2007 of an inch thick. The quantity of gold with which silver wire is covered, is above th of an inch thick. An ounce of
* See professor Bergman's Treatise on the Blow pipe, and it use in the examination of bodies, particularly minerals.
+ Or that degree, according to Mr. Kirwan, which yields most difficulty to the knife.