« PreviousContinue »
curious as specimens, but of little value as sources of this useful metal, which is almost exclusively drawn from the sulphuret or galena.
This ore is distinguished by its metallic lustre, and its foliated fracture. It is soft, heavy, and brittle; its specific gravity 7.5. It is very commonly crystallized in cubes, octohedra, and prisms. Before the blow-pipe it exhales sulphur, and gives a globule, which by the joint action of heat and air disappears, provided the lead be pure. It contains 87 of lead and 13 of sulphur.
This ore of lead occurs in primary as well as in secondary rocks; in this country our richest mines are in slate and limestone. And such is the recent deposition of lead in some cases, that it is actually found within the cavities of shells. Great Britain presents us with the richest lead mines of the known world; they furnish more than half of the lead annually raised in Europe which amounts to about 500,000 quintals.
The mines of Northumberland, Durham, Derbyshire, and Somersetshire are chiefly in limestone. In Devonshire it is found in slate; as at Beeralston upon the Tamar. Fluor spar is a very common accompaniment, of which some interesting mineralogical varieties occur at Beeralston. Lanark, Dumfries, and Aberdeen are the principal Scotch shires rich in lead. The mines of France, Westphalia, Austria, Spain, and Prussia are the next in importance. Those of Saxony, Bavaria, and Russia are of little consequence. It is probable that China is rich in lead.
The reduction of the sulphuret of lead to a metallic state is a sufficiently simple process. The ore is broken up and freed as much as possible from extraneous matters, or dressed; it is then ground or broken finer, and thrown in quantities of about a ton at a time into a reverberating furnace. The flame of pit coal is made to play upon it, it soon gets red hot and melts; the fire is then slackened till the mass gets dull red, and the sulphur then evaporates; quick lime is thrown in, and a separation is soon observed into lead and slag; the metal is then drawn out.
Galena often contains silver; and although there is no method of ascertaining this but by analysis, we observe it generally impairs the lustre of the ore. When the quantity of silver is extremely minute it is not worth separating, and hence, the lead of commerce often contains it; but when it amounts to about 12 or 14 ounces per ton, then the processes become more complicated and curious to obtain it. The richest argentiferous lead of this country, which I remember to have heard of, is that of Brunghill moor in Yorkshire, yielding=230 ounces per ton. In the reign of Charles I. the mines of Cardigan gave 80 ounces per ton. The leads of Durham and Westmorland about 17 ounces.
Mr. Brande proceeded to describe the process of reduction by cupellation, and the mode of separating the silver :—
The process of cupellation is often performed with a very different intent, as in the analysis of ingots of gold and silver, containing certain quantities of copper or other base alloy. This analysis is commonly called assaying, and rests upon the property which lead has of not merely being converted into a fusible glassy oxide itself, when exposed to heat, but when in that state of dissolving, and oxidizing the copper, or other base metals.
Pure lead has a bluish white colour, and much inherent lustre, but soon tarnishes. Its specific gravity is 11.3. It is malleable, but little tenacious, and melts below a red heat. It combines with oxygen in three proportions, forming massicot or yellow oxide, minium or red oxide, and the peroxide is of a brown colour.
In these combinations 97 parts of the metal are respectively united to 7,5-11,25 and 15 of oxygen. All the salts of lead contain the first oxide.
Copper and lead are metals much used for domestic and culinary purposes, and as it not unfrequently happens that articles of food, as well as water, are tainted by their combinations, and thus rendered pernicious to health, it will be right, in this place, to say a few words of the means by which they may be detected.
Copper vessels are easily acted upon by weak acids, or even
by the joint action of air and water; hence, any vegetable matter either sour, or having a tendency to become so, may become very deleteriously tainted. This not unfrequently occurs with pickles and sweetmeats, and will almost always happen when any kind of food is left for some time in a copper saucepan not perfectly tinned. In these cases the presence of copper may be detected by ammonia, and by a plate of iron.
Lead is by no means an unfrequent ingredient in water, which, according to the custom of this country, is preserved in leaden cisterns, and conducted through pipes of the same metal; although it does not at all follow that on this account the water should be necessarily tainted. Where the water contains carbonic acid in any quantity, it will be apt to have carbonate of lead mechanically suspended in it. It is very rarely the case that lead is in a state of solution in the water; so that in examining a suspected water, the sediment as well as the clear water must be attended to. The best tests for lead, are sulphuretted hydrogen, which gives a brownish black precipitate in solutions containing lead'; hydriodate of potash forms a yellow precipitate; sulphate of soda a white one, which heated before the blowpipe on charcoal affords a globule of the metal.
Much has been said concerning the addition of sugar of lead to wines, and an idea has often gone abroad that white wines are frequently adulterated with this pernicious compound; that it is used to counteract tartness. I have examined more than 100 samples of white wine of different kinds and from different sources, with a view of ascertaining the truth of this opinion; in two instances only I discovered lead: in one case it was in very large quantity, but there was good evidence of the wine having been put into a bottle which had previously contained goulard water; in the other case, lead was observed only by very nice tests and in very small quantity, and it arose from some shot having been left in the bottle, a very minute portion of which had been dissolved by the acid of the wine. So that the apprehension which some have entertained concerning the frequent and intentional adulteration of white wines with sugar of lead seems to be perfectly groundless.
VIII. On some Combinations of Ammonia with Chlorides. By M. Faraday, Chemical Assistant in the Royal In
It has been already shewn, particularly by Sir H. Davy, that
several of the binary compounds of chlorine, as those of phosphorus, tin, &c. exert a strong affinity for ammonia, condensing it when in the gaseous state, and neutralizing its alkaline properties. The combinations which will here be offered to notice, are of a different kind, and if they deserve any attention, it will be in consequence of the weakness of the power which is exerted in their formation, and the slight change of properties induced on the substances by union.
It has been frequently observed by chemists, that if well fused muriate of lime be placed in ammoniacal gas, there is a rapid absorption of the gas, and the chloride becomes covered with a white powder. If ammonia be repeatedly added until the absorption ceases, the mass of chloride swells, cracks, splits in all directions, and at last forms a white pulverulent substance.
Exposed to the atmosphere, it deliquesces, but not so rapidly as muriate of lime. Thrown into water it dissolves, forming a strong alkaline solution. Heated, it gives off ammonia, and the chloride remains unchanged. Placed in chlorine it inflames spontaneously, and burns with a pale yellow flame.
The fused chlorides of barium and strontium suffer a very slight change in ammoniacal gas in many days; after more than a fortnight the chloride of strontium, weighing about 30 grains, had absorbed only a cubical inch of gas, and a slight efflorescent appearance was seen on the broken edge.
A piece of fused chloride of silver, weighing about 30 grains, placed in ammoniacal gas, gradually absorbed more than 40 cubical inches. The action took place over the whole surface of the mass, but most speedily at the fractured edges. The chloride swelled considerably, and crumbled into powder. The substance formed was at first white, but it blackened by
exposure to light, though without liberating any gas. Thrown into water the ammonia was separated, forming a solution, and the chloride remained unchanged. Heated, the whole of the ammonia was given off. Placed in clorine it inflamed spontaneously, and the ammonia was decomposed.
Chloride of silver that had been well dried, but not fused, gave the same compounds with ammonia, but in a much shorter time.
A strong solution of chloride of silver in ammonia was left for some weeks in a bottle stopped only by a piece of paper. At the end of that time several perfectly colourless and transparent crystals had formed in it; some of them being as much as a quarter of an inch in width. Their general form was that of a flat rhomboid, but sometimes two acute angles of the rhomboid were wanting, and then the crystals looked like hexaedral prisms with oblique bases.
Exposed to the air, these crystals became opaque, gradually loosing the whole of the ammonia, and were then so friable as to fall into powder by a slight touch; the substance remaining was a dry chloride of silver. Placed in water, the same change occurred, but more readily, the water separated the ammonia, and they instantly become opaque. Heated, they gave off much ammoniacal gas, and the chloride remained unaltered. Exposed to light, they gradually blackened, though covered by the solution from which they were deposited.
If the ammoniacal solution be weak, other crystals are formed which are pure chloride of silver.
Dry corrosive sublimate placed in ammoniacal gas had suffered no change in fourteen days, nor had any action been exerted on the ammonia; there was a diminution of a quarter of a cubical inch of gas, probably owing to a little water being present. The corrosive sublimate heated gave out no ammonia, and the whole of the gas remaining was absorbed by
The precipitate obtained by adding ammonia to a solution of corrosive sublimate, appears to be a compound of the two bodies, but the alkali is neutralized in this case, and it is therefore more analagous to the combination of ammonia