a common impurity of this mineral. Thallium presents the external characters of lead; it is heavier than lead, having a specific gravity of 11.85, and it is so soft that the thumb-nail can indent it; it is very malleable, and ductile enough to be drawn into wire; it melts at 200° and volatilizes at redness; its freshly cut surface has a bluish-white lustre; but it quickly tarnishes and is gradually oxidized in the air, so that it is best preserved under water. Water is not decomposed by it even at 100°. When strongly heated in oxygen, it takes fire and burns with a bright green flame.

Thallium dissolves in dilute acids, with evolution of hydrogen. There are several oxides of this metal, of which the most important is the oxide Tl,O, corresponding in composition, and, to some extent, in properties, with the oxide of sodium Na,O. This oxide is somewhat soluble in water, and yields a caustic alkaline solution, which absorbs carbonic acid from the air, and forms a welldefined series of salts. The sulphate, Tl,SO,, is a soluble salt, which forms an alum with sulphate of aluminum; the chloride, TICI, is only slightly soluble in water, resembling, in this respect, the chloride of lead, and being quite unlike the soluble chlorides of sodium, potassium, rubidium, and cæsium. The carbonate of thallium is a soluble salt; but the sulphide of thallium, Tl,S, is an insoluble black powder, which resembles the sulphide of lead, but is entirely unlike the sulphides of the alkali-metals. The soluble salts of thallium are very poisonous. In general, the properties of thallium are intermediate between those of lead and those of sodium and potassium. Like the alkali-metals, it replaces hydrogen atom for atom; its atomic weight is 204.

CHAPTER XXVII.

SILVER THE ALKALI-METALS-QUANTIVALEN CE.

532. Silver is a widely diffused and quite abundant element, but in its mode of occurrence it differs widely from the alkalimetals which we have just been studying. In the first place, it

OCCURRENCE OF SILVER.

449

frequently occurs native, both pure and alloyed with mercury, copper, and gold,-a mode of occurrence quite impossible for the alkali-metals, because of their readiness to combine with the elements of air and water. Native silver is found in various forms, sometimes crystallized in cubes or octahedrons, sometimes in filaments, both coarse and fine, and sometimes in shapeless masses. The metal more commonly occurs in combination with sulphur, mixed with sulphides of lead, antimony, copper, and iron. It is from argentiferous sulphides that the larger part of the silver of commerce is extracted; among ores of this kind the argentiferous sulphide of lead (galena) is the most abundant. Combinations of silver with selenium, tellurium, chlorine, bromine, and iodine are also to be enumerated among silver-containing minerals; of these the chloride (horn-silver) occurs in quantities large enough to make it valuable as an ore of the metal. It is noticeable that the only elements which are extracted in any quantity from their chlorides as ores are sodium, potassium, and silver. The chlorides of copper, mercury, and lead do, indeed, occur as natural minerals; but as sources of those metals they have no significance. A small proportion of silver exists in sea-water (about 1 milligramme in 100 litres), and its presence has been recognized in common salt, in chemical products in the making of which salt is used, in various sea-weeds, in the ashes of landplants, in the ash of ox-blood, and probably also in coal. In seawater it exists, as sodium and potassium do, in the form of chloride.

When silver is extracted from argentiferous sulphide of lead, the ore is first treated for lead, precisely as it would be if it contained no silver. The lead, so reduced, contains all the silver originally present in the quantity of ore treated. The subsequent separation of the metallic silver from the metallic lead depends upon the chemical properties of lead rather than of silver, for the silver remains unaltered during the whole process; this separation will therefore be described in the next chapter.

The mixed sulphides which contain silver have been heretofore generally reduced by a complicated process which depends ultimately on an amalgamation of the silver with mercury. The ore, after thorough washing and grinding, is mixed with a portion of common salt, and roasted for several hours; during this roasting, white fumes of arsenious and antimonious acids are expelled, the sulphides of copper and iron

450

EXTRACTION OF SILVER.

are partially converted into oxides, chlorides, and sulphates, and chlo ride of silver and sulphate of sodium are formed. The roasted product is then reduced to a very fine powder, and agitated in revolving casks with water and iron filings, or scraps, to which mercury is soon added. This operation lasts about 20 hours; during it, the iron decomposes the chloride of silver, and the mercury dissolves the silver to an amalgam; from this amalgam the excess of mercury is first squeezed out through leather or cloth filters, and the remainder is driven off by distillation. The residual spongy mass is silver, alloyed with a variable proportion of copper, derived from the ore and reduced to the metallic state by the same steps which have reduced the silver.

This process is European; the process of amalgamation as practised in Mexico and South America is quite different, and the reactions which it depends upon are somewhat obscure. The ore is not roasted, but, after being ground to powder, moistened with water, and mixed with from 1 to 5 per cent. of common salt, it is suffered to lie undisturbed for some days. From to 1 per cent. of roasted copper pyrites is then added, together with a considerable proportion of metallic mercury, and the mass is worked together and commingled by the trampling of mules or horses. After an interval of two or three weeks, a second dose of mercury is given, and after a still longer interval a third. By this last addition, a fluid amalgam is obtained, which is separated by washing and filtering, and distilled for the recovery of a portion of the mercury employed, and the isolation of the silver. In this process there is a great waste of mercury, because much of it is converted into a chloride of mercury (calomel) and lost. The recommendations of the process are mainly these—that it requires no fuel, except for the distillation of the amalgam, and that it leaves the silver in a condition of great purity. The whole process, though far from economical from the point of view of the theoretical chemist, was doubtless a legitimate outgrowth of the conditions under which it took birth.

Various processes have been patented for the extraction of silver without the use of the costly mercury, some of which have been successfully practised on a large scale. They depend, for the most part, either on the comparative stability, in the fire, of sulphate of silver when once formed, as compared with the sulphates of iron and copper, and the consequent possibility of dissolving sulphate of silver out of the roasted ore, or upon the fact that the chloride of silver, which results from the roasting of the ore with chloride of sodium, may be dissolved in solutions of the alkaline chlorides, and, indeed, in aqueous solutions of a great many other soluble salts, though it is by itself insoluble in water. Any aqueous solution containing, among other things, a silver

salt (whether in the condition of chloride, sulphate, or nitrate is indifferent) may be decomposed by digestion with metallic copper; the silver-salt will be decomposed, the corresponding copper-salt formed and dissolved, and the metallic silver will be precipitated.

533. Silver (Ag).-The element, silver, is much more familiarly known than any of its compounds; known from the earliest ages, this metal has always been prized as much for its beauty as for its rarity. White, brilliantly lustrous, susceptible of an admirable polish, wonderfully malleable and ductile, the best known conductor of heat and electricity, fusible only at a very elevated temperature and permanent in the air, whether hot or cold, wet or dry, it represents and embodies in the completest sense all that is commonly understood by the term metal.

This word metal cannot be strictly defined; it is a conventional term, vaguely used because expressing a vague idea. Thus metals would all be solid were not mercury, and perhaps cæsium, fluid; they are generally heavy; but lithium, sodium, and potassium float upon water; they have all a peculiar lustre, called metallic; but this lustre does not characterize metals alone, for coke and graphite, galena, molybdenite, and many other minerals often exhibit a similar lustre ; they may all be said to be opaque; but gold may be beaten out so thin as to transmit a greenish light. While it is not possible to define the term metal with precision from chemical any more than from physical properties, one general chemical fact deserves attention in this connexion. We have seen that bodies which contain a large proportion of oxygen, such as SO,, P,O,, N2O,, and CO,, have a common tendency to unite with other bodies which are alike in that they contain a much smaller proportion of oxygen, such as KO, Na,O, PbO, and CaO, to form more or less stable saline substances. The first class of bodies, which are usually rich in oxygen, have been called acids; the second class, which are usually poor in oxygen, have been designated collectively as bases. Now those elements which unite with oxygen to form acids alone are, as a rule, nonmetallic, and those elements which unite with oxygen to form bases are, in the chemical sense of the term, the metals; but no sharp line of division between metallic and non-metallic elements can be established on this principle, inasmuch as some elements

452

PROPERTIES OF SILVER.

which possess the other characteristics of a metal form no basic oxide, while some metals, like antimony, form oxides which are at one time bases and at another time acids. The metal arsenic, for example, forms no basic oxide; and we shall hereafter meet with another illustration of the same difficulty of classification, in the element tungsten.

Melted silver possesses the curious property of absorbing a large volume of oxygen (twenty-two times its bulk), from the air, while it is liquid. This gas it gives out again on solidifying. When a globule of molten silver is cooled suddenly, the film of solid metal which forms upon its surface is burst open by the escaping gas, and the liquid silver within is apt to be projected outwards with the gas; this phenomenon is called spitting; it often occasions a loss in silver assays. When heated on lime before the oxyhydrogen blowpipe, silver gives off vapors which become oxidized if the blast of gas contain an excess of oxygen; a fine silver wire is dispersed in greenish vapors when a very powerful electric discharge is sent through it. Silver combines slowly with chlorine, bromine, and iodine, and promptly with sulphur. The tarnishing of silver is due to the formation of a thin film of the black sulphide over the metallic surface, by combination between the silver and the sulphur of the sulphuretted hydrogen which is often present in the air of towns and houses. The best solvent for silver is nitric acid diluted with two or three parts of water; nitric oxide is evolved, and nitrate of silver remains in solution :

:

2Ag + 2H SO

=

3Ag+ 4HNO, = 3AgNO, + NO + 2H2O. Chlorhydric acid acts upon it but slowly; for the chloride of silver is but slightly soluble in chlorhydric acid, whether strong or dilute. Boiling sulphuric acid dissolves silver, and forms the sulphate, sulphurous acid being evolved during the reaction:Ag2SO, 2H,O + SO,. Neither the alkalies nor their nitrates have much effect on silver, whether they are in solution or are fused by heat; hence a silver dish is used in concentrating the caustic alkalies, and a silver crucible for fusing refractory minerals with the hydrate of sodium or potassium. The specific gravity of silver is 10-5, and its atomic weight 108.