Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/42

32 MERCURY This metal does not appear to have been known to the ancient Jews, nor is it mentioned by the earlier Greek writers. Theophrastus (about 300 B.C.) mentions it as a derivative of cinnabar. With the alchemists it was a substance of great consequence. Being ignorant of its susceptibility of freezing into a compact solid, they did not recognize it as a true metal, and yet, on the authority of Geber, they held that mercury (meaning the predominating element in this metal) enters into the composition of all metals, and is the very cause of their metallicity. When, about the beginning of the 16th century, chemistry and scientific medicine came to merge into one, this same mysterious element of "mercury" played a great part in the theories of pathology ; and the metal, in the free as in certain combined states, came to be looked upon as a powerful medicinal agent, which position, on purely em pirical grounds, it continues to hold to the present day. Mercury occurs in nature chiefly in the form of a red sulphide, HgS, called cinnabar, which, as a rule, is accom panied by more or less of the reguline metal, the latter being probably derived from the former by some secondary reaction. The most important mercury mines in Europe are those of Almaden in Spain and of Idria in Illyria; these until lately furnished the bulk of the mercury of commerce, but they are now almost eclipsed by the rich deposits of New Almaden in California. Considerable quantities of mercury are said to be produced in China and in Japan ; minor deposits are being worked in the Bavarian Palatinate, in Hungary, Transylvania, Bohemia, and Peru. At Almaden the ore forms mighty veins traversing micaceous schists of the older transition period ; in Illyria it is dis seminated in beds of bituminous schists or compact lime stone of more recent date. Chemically speaking, the extraction of mercury from its ores is a simple matter. Metallic mercury is easily vola tilized, and separated from the gangue, at temperatures far below redness, and cinnabar at a red heat is readily reduced to the metallic state by the action of iron or lime or atmospheric oxygen, the sulphur being eliminated, in the first case as sulphide of iron, in the second as sulphide and sulphate of calcium, in the third as sulphurous acid gas. To the chemical mind a close iron retort would suggest itself as the proper kind of apparatus for carrying out these operations, but this idea is acted upon only in a few small establishments, for instance, in that of Zwei- briicken in the Palatinate, where lime is used as a decom posing agent. In all the large works the decomposition of the cinnabar is effected by the direct exposure of the ore to the oxidizing flame of a furnace, and the mercury vapour, which of course gets diffused through an immense mass of combustion gases, is sought to be recovered in more or less imperfect condensers. At Almaden this roasting distillation is effected in prismatic furnaces, which, by a second upper (brick) grating are divided into two flats, the lower one serving for the generation of a wood fire, while the upper accommodates the ore, which is introduced through an opening in the dome-shaped roof. To avoid an excessive dilu tion of the mercury vapour with combustion gases, part of these are led out laterally into a chimney and the rest allowed to strike up through the heap of ore. The large mass of metalliferous vapour produced passes out through a system of pipes inserted laterally into the dome and so arranged that they follow first a descending and then an ascending plane, to lead ultimately into a condensation chamber which communicates in its turn with a chimney. The pipes are formed each of a large number of elongated pear-shaped earthenware adapters (called aludels), which are telescoped into one another as in the case of the iodine-distillation apparatus, the joints being luted with clay. The lowest row of aludels, which lie in the line of intersection of the two inclined planes, are pierced with holes below, so that what arrives as liquid mercury there runs out into a gutter leading to a reservoir. What of mercury vapour remains uncondensed in the aludels passes into the chamber, the intention being to have it condensed there ; in reality a large proportion of the mercury passes out through the chimney (and through the numerous leaks in the aludels) into the atmosphere to poison the surrounding vegetation and the workmen. Similar furnaces to the Almaden ones are used in Idria and at New Almaden ; only the condensation apparatus are a little less im perfect. But in all three places the loss of metal is very consider able ; at New Almaden it is said to amount to close upon 40 per cent. The mercury obtained is purified mechanically by straining it through dense linen bags, and then sent out into commerce in leather bags, or in wrought-iron bottles provided with screw plugs, each holding about 75 tt> avoirdupois. According to Balling s Metallurgische Chemie (Bonn, 1882), the production of mercury in the years named was as follows : Austria, exclusive of Hungary (1880) 369 tons. Hungary (1879) 180 ,, Italy (1877) 55 Spain (1873) 929 ,, United States (1875) 2054 ,, Assuming the amount to be the same from year to year, this gives a total of 3587 tons. The price of the metal is subject to immense fluctuations ; it generally ranges from 2s. to 7s. 6d. a pound avoirdupois. Commercial mercury, as a rule, is very pure chemically, so that it needs only to be forced through chamois leather to become fit for all ordinary applications ; but the metal, having the power of dissolving most ordinary other metals, is very liable to get contaminated with these in the labora tory or workshop, and requires then to be purified. For this purpose a great many chemical methods have been proposed, which, however, all come to this, that the base admixtures are sought to be removed by treatment with nitric acid, oil of vitriol, or other agents which act preferably on the impurities. The best of these methods is that of Briihl, who shakes the metal with a solution of 5 grammes of bichromate of potash and a few cubic centi metres of sulphuric acid in one litre of water, until the red chromate of mercury, first produced, has disappeared, and its place been taken by green chromic sulphates. Tho supernatant liquor and chromic scum are washed away by a powerful jet of water, and the clean metal is dried and filtered through a perforated paper filter. The only really exhaustive method is redistillation out of a glass apparatus. Unfortunately the operation is difficult of execution, as mercury " bumps " badly on boiling ; but this can be avoided by distilling the metal in a perfect vacuum. An ingenious apparatus for this purpose, in which the distilled metal itself is made to keep up the vacuum, was con structed lately by Leonhard Weber. A U-tube, the limbs of which are longer than the height of the barometer, is filled with pure mercury, and inverted, the one limb being made to dip into a vessel with pure, the other into another containing the impure, mercury. This second limb is inflated above so that the meniscus is about the middle of the bulb. This bulb is heated, and the conse quence is that the metal there distils over into the first limb to add to the supply of pure metal, the impure rising up in the second by itself to maintain a constant level. Dewar has modified the apparatus so that there is no need of a supply of pure metal to start with. Absolutely pure mercury does not at all adhere to any surface which does not consist of a metal soluble in mercury. Hence the least quantity of it, when placed on a sheet of paper, forms a neatly rounded-off globule, which retains its form on being rolled about, and, when subdivided, breaks up into a number of equally perfect globules. The presence in it of the minutest trace of lead or tin causes it to " draw tails." A very impure metal may adhere even to glass, and in a glass vessel, instead of the normal convex, form an irregular flat meniscus. Properties. The pure metal is silver- white, and retains its strong lustre even on long exposure to ordinary air.

At - 38-8 C., i.e., - 37 9 F. (Balfour Stewart), it freezes,