The weight of a metal is increased by being con verted into an oxide.
illus. If 100 pounds by weight of lead be melted in the opun air, and kept in fusion, it gradually absorbs oxygen and is converted into red lead or minium. During this process, the lead acquires an accession to its weight of about 10 pounds to the 100, which is the weight of the oxygen, the metal had absorbed to convert it into an oxide.
2. Weigh a coil of iron wire accurately, and then burn it in oxygen gas, in the way formerly described. Then weigh the oxide formed by the burning, and it will be found to weigh more than the wire did. When the iron and gas are both weighed, the gas is found to have lost exactly what is gained by the iron.
176- Metals after being converted to oxides, may again be revived, or brought back to their metalic state. This process is termed reduction.
Illus. This depends on an operation opposite to that of oxidation, and is effected by the addition of substances which have a greater attraction for oxygen than the metals have. Thus carbon has a stronger attraction for oxygen than any of the metals. Mix therefore the metallic oxide to be reduced, say the red oxide of lead, with charcoal powder in a crucible, and on exposing them to a red heat the metal will be reduced, and appear in its metallic form.
Obs. In the reduction of the oxides of metals by charcoal, there is a quantity of carbonic acid gas formed equal in weight to what is lost by the oxide and the charcoal; because the materials of which the carbonic acid is formed are furnished by the two substances, the oxygen by the lead, and the carbon by the charcoal.
177- Some of the metals have so feeble an attraction for oxygen, that when they are converted into oxides, the action of heat alone is sufficient to drive off the oxygen and reduce them to the metallic state.
Illus. Mercury by a long and patient application of a moderate heat, is changed to the state of a red oxide. If this red oxide is exposed to a higher degree of heat, the oxygen abandons it and flies off in the form of oxygen gas, while the oxide is reduced to a metal.
Illus. It is on this principle that oxygen gas is obtained from black oxide of manganese, which being exposed to a red heat, in a gun-barrel or iron retort, yields it in abundance.
The metals unite with sulphur, phosphorus, and carbon, forming compounds called, sulphuret a, phosphurets, and carburets.
179. Of all the inflammable bases, sulphur appears to have the strongest affinity for metals, and its combinations with some of them are attended with remarkable phenomena.
Exp. Introduce into a Florence flask, three parts of iron filings or copper filings, and one part of flowers of sulphur mixed together. Then cork up the flask and place it over a lamp, jso as to heat it slowly. As soon as any redness appears, remove the flask from the fire, incandescence will go on, the mixture glowing with a red heat, which in the dark exhibits a very beautiful appearance.
Obs. As this experiment can be made without the presence of oxygen gas, it was formerly considered an objection to the doctrine, that oxygen is the sole supporter of combustion. But the experiment can be explained ou the doctrine of latent heat; for when the vessel is broken, it is found that the two bodies have united and formed a solid compound, viz : a sulphuret of iron or copper. The latent heat therefore, which the two substances contained, was given out in consequence of their passage into a more solid form.
180. When a metal enters into solution with an acid, a compound is formed, which differs entirely from either of these two substances. This compound is called a metallic salt.
Obs. When a metal enters into solution with an acid, the acid remains transparent as before. This circumstance serves to distinguish chemical solution from mechanical mixture, which is always turbid.
Exp. Pour concentrated nitric acid on some granulated zinc, taking care not to respire the vapour. The solution will be transparent.
Obi. As the metal dissolves, red fumes are emitted in abundance ; these are nih'ous ccid gas. The acid is decomposed aLid the metal is oxidated at ihe expense of that portion of oxygen which constituted nitric acid; the consequence is, that the loss of this portion of oxygen converts the nitric acid into nitrous acid gas which flies off as fast a; it is produced.
181. The metallic salts are much employed in the arts and in medicine.