Platinum obtained by the above process is not pure ; it contains arsenic, and the foreign ingredients of the grains, by which it is not so capable of standing an intense heat, or of resisting the action of chemical agents.
The other methods of obtaining platinum is by reducing its oxide obtained by dissolving the grains in nitro•mu iiatic acid, and precipitating by muriate of ammonia. For doing this, different processes are followed, though they all nearly agree with each other.
The best of these is, perhaps, that recommended by Mr. Cook.
It consists in exposing the precipitated oxide to heat, by which it is reduced, and the superfluous muriate of ammonia is expelled. About half an ounce of the spongy mass obtained is then put into an iron mould, and squeezed together by a wooden pestil. After this another half ounce is added, and in this way the operation is con tinued, till about six ounces are squeezed into the mould, which are still farther compressed by a strong iron screw, by which the whole of the air is expelled. It is then laid on burning charcoal, and exposed to a high tempe rature, and when hot beat on an anvil till it is of uniform density.. After this operation, it is coated with a reddish crust, which is removed by covering it with borax and exposing it to a white heat ; it is then washed with mu riatic acid, which dissolves the foreign matter, and leaves the metal resplendent.
Platinum thus obtained still retains Iridium, which was precipitated from the nitro•muriatic solution ; it is however sufficiently pure for the manufacture of uten sils.
Before subjecting Platinum to any of these processes, the grains must be spread on a table, and a current of air from a bellows must be passed obliquely over them. By this the lighter particles, which consist of quartz and iron ore, are removed.
Occasionally the grains, as brought to Europe, con tain a minute portion of gold. This may be got from them by treating them with a small quantity of nitro muriatic acid. To the solution green sulphate of iron must be added, and the precipitate thrown down, purified by fusion with nitre and borax.
Conn er.
The ores of copper generally employed for yielding the metal, are the sulphurets. These are wrought
principally in Cornwall, in Anglesea, and in Hun gary.
In Cornwall, the ores of copper are broken into small pieces, which are roasted in a furnace, somewhat similar to a reverberatory furnace, having a very long chimney to increase the heat, and to carry off the sulphur and arsenic with which the ore is roasted. During the roasting, which continues for about twelve hours, the ore is frequently stirred, so as to expose the whole of it to the flame. It is then put into a small furnace of the same form, and brought to a state of fusion, occasionally mixed with a little lime, to increase the fusibility. As the impurities collect at the top, they are raked out, and put into oblong moulds, in which they are allowed to cool. They then form a hard mass, which is used in building. The fused copper is drawn out through a hole in the lower part of the furnace, which was stopped by clay, mixed with a little coal, to prevent it from hard ening. Fresh quantities of the roasted ore are then put in, and the process is in this way carried on for a con siderable time.
The fused copper is conveyed into vessels suspended in a well, through which a stream of water runs. By this means the metal is reduced to the granular state. It is still, however, impure, being mixed with sulphur and arsenic.
To free it from these, the metal is repeatedly subject ed to heat in a reverberatory furnace, and each time put into the well. During these processes, the slag collects on the surface of the fused metal ; but as this contains a considerable quantity of copper, it is kept and mixed with the fresh ore, previous to its being put into the furnace.
The copper after this is kept at a low red heat for two days, and is then repeatedly fused, and cast into moulds about 14 inches in length. It is lastly put into the refining furnace, with a little charcoal, in which it is again fused. If after this it bear the hammer, it is fit for sale. When the fused copper is cast into the moulds, the purest part of it rises to the top, and may, when cold, be easily separated from the rest, by a blow of a hammer.