For obtaining the copper in this way, it is necessary that we have the metal dissolved in muriatic or sulphu ric acid, for the nitrate of iron generated by the decom position of the nitric acid solution is itself liable to be decomposed by heat. lf, therefore, the copper be dis solved in nitric acid, the solution must be evaporated to dryness, and the residue, dissolved in muriatic acid, again evaporated and dissolved in water ; or the metallic ingredients of the nitric acid solution may he precipi tated by potassa, and the precipitate dissolved in muria tic acid. Into either of these solutions, the plate of iron is immersed. Zinc is sometimes employed to precipi tate copper, but as this separates iron also if present, it is liable to fallacy. Even if the solution do not contain iron, yet if there be an excess of acid, and the zinc itself contain that metal, it will first be dissolved by the acid, and then precipitated by the zinc ; for this reason its use is improper. Occasionally the precipitated copper, be fore it is weighed, is mixed with oil and borax, and sub jected to heat in a crucible, by which it is freed from impurities, and the metal is thus obtained in its pure state.
Assay of Cofilzer Alloys. Perhaps there is no metal, the alloys of which are more numerous and more useful than those of copper. It is of consequence, therefore, to be able by analysis to ascertain the proportions of the ingredients which they contain. It must be remarked, however, that though, by the aid of chemistry, we can ascertain with precision, not only the ingredients, but the proportions of the substances contained in these alloys, yet we often fail in forming an alloy possessed of all the properties of that subjected to analysis. This depends, in a great mcasuse, on the difference in the purity of the metals which we employ ; a slight difference in these causing an alteration in the properties of the alloys which they form. The assay of these ores is also useful, as it enables us to procure from them the metals in their se parate state. This is chiefly practised with the view of obtaining the copper.
The most important of the alloys of copper arc those with tin. Tin, when added to copper, renders it harder, more sonorous, and more fusible : hence it is employed in the formation of hell-metal, and the other useful alloys of copper. When copper is alloyed with tin in the pro portion of 100 of the former to about 8 or 12 of the lat ter, it forms the metal employed in the manufacture of ordnance. Bronze, and bell-metal, are composed of about 100 copper and from 10 to 20 of tin, to which occasionally a little zinc, and sometimes also antimony is added.
When the tin is in larger proportion, as about 30 of copper to about 15 of tin, the alloy is speculum metal, which is very hard, and admits of a fine polish. With
this, a little zinc, silver, and arsenic, are mixed.
When the alloy consists only of copper and tin, we have an easy way of separating these metals. Tin not only is more easily oxidated by heat and air than cop per, but the protoxide of the former metal has the pro perty of depriving the protoxide of the latter of its oxy gen, by which it is reduced. We have only therefore to subject the alloy to heat to obtain the copper in its metallic form. In this process we do not procure the whole of the copper existing in the alloy ; besides, if we apply the heat by which the metals are oxidated too long, the tin attracts more oxygen from the air, and does not thus deprive the protoxide of copper of its oxygen. Thus Fourcroy found, that when he exposed an alloy of 80 of copper and 20 of tin to heat and air, till they amounted to 104, 54 parts of copper were obtained by afterwards subjecting the whole in a covered vessel to a high temperature. When, however, 100 parts of the same alloy were heated with access of air till they in creased to 117, a very minute quantity of copper was ob tained from them by the subsequent heating. Accord ing to Fourcroy, when copper is to be obtained by the method just mentioned, the alloy, supposing it com posed of 80 copper and 20 tin, should be heated in the air till it gain about 6 or 7 parts in weight, end then subjected to a high temperature in close vessels. When the alloy has been too much oxidated, it must be mixed with the due proportion of alloy, and then exposed to a high temperature, by which a large quantity of metallic copper will be procured.
Occasionally a little nitre, or black oxide of manga nese, is mixed with the alloy, by which the oxidation is more speedily accomplished. Some glass or salt should also be added, to increase the fusibility of the oxide of tin formed, and thus allow the metallic copper to fall to the bottom. The most accurate experiments on this subject on a large scale, are those of Pelletier and Dar eel, which were done by exposing the alloy to heat and air, and by the addition of black oxide of manganese.
In one of these, 400 lb. of alloy, known to be com posed of 80 copper and 40 tin, were heated, and con stantly stirred till they increased to 425 lb. 2 oz. These were added to 800 lb. of alloy brought to a state of fusion in a reverberatory furnace, and the mixture constantly stirred during 20 minutes, and occasionally afterwards for 9 hours, The fused metal was then T drawn off, and amounted to 761 lb. 12 oz.; 7 lb. 4 oz. of metal were also obtained during the trials to ascertain its purity. and the scoria yielded 64 lb. more, making in all 833 lb. ; that is, very nearly 70 parts of copper from the. 100 of alloy, which contained 80 of this metal.