† Jameson has not inserted the Nadelstein, nor has Brochart the Rutile or the Iserine. I have prefixed an asterisk* to the substances inserted from Jameson.

*XX. OR SILVAN GENUS. Graphic

Native sylvan

Yellow
Black

ore

III. The metals unite with oxygen. The most important property, common to the whole class of metals, is their susceptibility of union with oxygen. The combination of oxygen with a metal forms a metallic oxyd, which was formerly called a calx. The oxyds of metals are of various colours, according to the metal and the treatment. They may be reduced by subjecting them with charcoal powder, as well as oil, tallow, &c. in a crucible to the action of heat. The oxygen is thus carried off, and the metal resumes its original state. According to the Stahlian theory, a metal, in the act of oxydizement, loses its phlogiston, and is resolved into an earth; for a metal according to that doctrine is a compound of carth and plogiston; and that in the reduction of an oxyd, by employing a carbonaceous substance, phlogiston was imputed to the metal. The Lavoiserian theory explains it, by abolishing the hypothetical element of phlogiston, and asserting that in the formation of an oxyd, oxygen was absorbed, and the metal loses its metallic splendour, and that, in the reduction of an oxyd, the oxygen was disengaged, leaving the metal in its original state.

1. Mere exposure to the atmosphere, at the ordinary temperature, is sufficient to oxydize some metals, such as arsenic and manganese.

2. By an increase of temperature, and exposure to air, other metals undergo this change, but the former in these cases is absolutely necessary. Of the metals, of this kind, we may mention iron, zinc, copper, tin, &c. When made red hot they lose their metallic splendour, and are changed into oxyds of various colours. Very frequently this process takes place with an emission of heat and light, or flame. Combustion, therefore, takes place. Arsenic and zinc burn with a brilliant flame.

3. Other metals, on the contrary, as gold and platinum, are not oxydized even by the combined operation of air and an increased temperature.

By the electrical or galvanic influence, metals may be made to combine readily with oxygen. A wire of gold, silver, or platinum, for instance, is volatilized by the discharge of an electrical battery; and the metal, in this state, is oxydized by the air of the atmosphere. When beat into very thin leaves, these, and other metals burn with great brilliancy, producing at the same time different colours. These effects are produced by

the discharge of a powerful galvanic combination.

4. By oxydizement, whether by the combined operation of heat and atmospheric air, or which effects the same change more readily in oxygen gas, all metals acquire weight.

5. Water will oxydize some metals, both at the ordinary temperature of the air, and in high temperatures. Thus it is, that iron filings in a moist atmosphere, or when wetted with water, become oxydized, in consequence of the decomposition of that fluid. When the vapour of water is passed through a gun barrel, heated to redness, hydrogen gas is emitted, and the metal is oxydized, the iron gaining 28 per cent. of oxygen. See Hydrogen Gas.

6. Those metals, as silver, gold, and platinum, which do not attract oxygen either from oxygen gas or atmospheric air, as well as the readily oxydizable metals are capable of taking it from acids, and thereby becoming oxydized. Thus, nitro-muriatic acid, or oxymuriatic acid, first oxydizes gold or platinum, and then dissolves the oxyd; silver, copper, &c. decompose nitric acid, in the same manner, and produce metallic salts by the subsequent solution of the oxyd. All metals before solution must be oxydized, or combined with oxygen, which must take place before, or in the act of solution.

7. The solution of a metal in an acid, is accompanied with a decomposition of the acid. Thus, nitric acid and copper, when brought into contact, occasions a disengagement of nitric oxyd, or nitrous gas. See Nitrous Gas. This evolution of gas is called effervescence. Again, if iron filings be added to concentrated sulphuric acid, no effect ensues; but if water be added, a rapid decomposition takes place, the water is decom

posed, hydrogen gas is evolved, and the metal is oxydized at the expense of the water. See Hydrogen Gas. If, on the contrary, the metal be already combined with oxygen, and then added to an acid, the solution is effected without effervescence; for if the red oxyd of iron be added to sulphuric acid, this effect will take place.

8. Metals have different affinities for oxygen, and are capable of uniting with it in different proportions. These oxyds differ in their external and chemical characters, according to the degree of oxydizement. Twenty-eight parts of oxygen, for instance, and seventy-two of iron produce a black oxyd, whereas if forty-eight per cent. of oxygen be absorbed, a red ore is formed. Manganese united with one fifth of oxygen, affords a white oxyd, with one fourth, a red ore; and with two fifths, ore of a deep black colour, being the manganese of the shops. In proportion to the quantity of oxygen in these compounds, the more feeble is the affinity by which it is retained. In order to form a nomenclature for the metallic oxyds, to express the degree of oxydizement, Dr. Thomson has adopted the following method:

Prot-oxyd denotes the lowest degree, the minimum of oxydizement, or the first oxyd which the metal is capable of forming. Deut-oxyd denotes the second oxyd of a metal, or the metal combined with two doses of oxygen. He also employs the terms trit-oxyd, tetoxyd, pent-oxyd, and hect-oxyd, to express the third, fourth, fifth, and sixth degrees of oxydizement. The Jast term, per oxyd, signifies the largest dose of oxygen, and is used to express the highest degree, or the maximum, of oxydizement.

9. In proportion to the degree of oxydizement, the metal forms very different compounds with a given acid. Iron with 27 per cent. of oxygen gives with sulphuric, nitric, or muriatic acid, a green salt, insoluble in alcohol, and precipitated of a white colour by prussiate of potash; while the same metal with 46 per cent. of oxygen forms a reddish salt, soluble in

alcohol, and precipitated by prussiate of potash of a deep blue colour.

10. Metals retain oxygen with different degrees of force. Some metallic oxyds, as of mercury, are reduced to a metallic state by the mere application of heat, and others, as that of iron, require the addition of some substance possessing a strong affinity for that principle, as charcoal. See Carbonic Acid Gas.

The affinities of the metals for oxygen, may be shewn by exposing a mixture of oxyd of mercury and iron filings to heat; the latter will take the oxygen from the former, and become oxydized, and the mercury will re-appear in a metallic state. Or, if a glass which contains oxyd of lead be melted with metallic iron, the latter metal attracts oxygen from the former, and the lead assumes the metallic state. The separation of oxygen from a metallic oxyd, is termed the revival or reduction of a metal.

11. Metals held in solution in acids, are separated in the state of oxyds and hydrates, by the addition of alkalies. See Water. The alkali takes the place of the metal, forming a peculiar salt, whilst the latter is thrown down or precipitated. Owing to the different degrees of affinity of metals for oxygen, some of them are separated from the solution when another metal is presented. They are not separated, however, in the state of oxyds, but in a metallic form. See Affinity. Thus for instance, when a piece of iron is suspended in a solution of copper, the oxygen of the oxyd of copper, which is held in solution, goes to the iron, which becomes oxydized; and as fast as it is dissolved, the copper is separated, or precipitated on the iron. This precipitate is, therefore, metallic copper.

12. By the union of oxygen with metals, we are furnished with oxyds or acids. Thus, some metals when oxygenated to the minimum afford only oxyds, but if oxygenated to the maximum, produce acids. Those metals, that possess the latter property, are acidifiable. Such as chromium, arsenic, molybdenum, &c.