Three modes of gas-firing are to be noticed, each of which is adapted to special local conditions, (a) The gas is made from the fuel in a detached fireplace and conducted while hot into the combustion chamber of the furnace, and the air for complete combustion IS heated by the products of combustion on their way to the chimney (6) Both the producer gas and the air are heated before they enter the combustion chamber, ^s in the Siemens system of regenerative finng. (c) Natural gas is piped to the furnace, where it meets air heated by the chimney gases. The primary advantages of gas firing are that less fuel is required, that there is better control of the heat in the furnace, and that larger and more accessible furnaces can be built. In Silesia the introduction of gas-firing has led to the use of furnaces containing eighty muffles. In the United States, Belgian furnaces of type (a) are built to contain 864 retorts; °f type (0), to contain 300 to 400 retorts; and of type (c), preferably about 600 retorts. The use of gas-fired furnaces greatly simplifies manual labour. On a direct-fired furnace at least one man, the brigadier, must be an expert in all the operations involved; but with a gas furnace a division of labour is possible. One man who understands the use of gaseous fuel can regulate the heat of a thousand or more retorts. The men who charge and empty th» retorts, those who draw and cast the metal, and those who keep the furnace in repair, need not know anything about the making or using of gas, and the men who make the gas need not know anything about a zinc furnace. Again, in direct-fired furnaces there are commonly seven or eight rows of retorts, one above another, so that to ser'e the upper rows the workman must stand upon a table, where he is exposed to the full heat of the furnace and requires a helper to wait upon him. With gas-firing the retons can be arranged in four horizontal rows, all within reach of a man on the furnace-room floor. Furthermore, with the large furnaces which gas-firing makes possible mechanical appliances may be substituted for manual labour in many operations, such as removing and replacing broken retorts, mixing and conveying the charge drawing arid casting the metal, charging and emptying the retorts and removing the residues and products.

Refining.— The specific effects of different impurities on the physical properties of zinc have only been imperfectly studied fortunately, however, the small amounts of any of them that are likely to be found in commercial zinc are not for most purposes very deleterious. It is generally recognized that the purest ores produce the purest metal. Grades of commercial zinc are usually based on selected ores, and brands, when they mean anything usually mean that the metal is made from certain ores. Chemical control of the metai purchased is not nearly as common as it should be, and the refining of zinc is at best an imperfect operation, lo obtain the metal chemically pure a specially prepared pure oxide or salt of zinc is distilled. A re distilled zinc, from an ordinar-ily pure commercial zinc, is often called chemically pure but re distillation is seldom practised except for the recovery of zinc from |-lvanizcrs dross and from the skimming's and bottoms of the melting lurnaces of zinc rolling mills. The only other method ol rehning is by oxidizing and settling. A bath, even of very impure zinc, is allowed to stand at about the temperature of the melting-point of the metal for forty-eight or more hours, whereupon the more easily oxidizable impurities can be largely removed in the dross at the top, the heavier metals such as lead and iron settling towards the bottom.

This method is rarely practised

except by the rollers of zinc.

A certain amount of refined zinc

can be dipped from the furnace; a further amount, nearly free Irom iron, can be liquated out of the ingots cast from the bottom 01 the bath in a subsequent slow remelting, and it is sometimes possible to eliminate a zinciferous lead which collects in the sump of the furnace Owing to the fact that at temperatures between Its me ting and boiling point zinc has a strong affinity for iron. IS often contaminated by the scraper while being drawn from the condenser, as is shown by the fact that the scraper wears away rapidly. As each retort in a furnace is in all essentials a separate crucible, and as the metal from only a few of them goes into a single ingot, there can be no uniformity either in the ingots made from the same farnace during a day's run or in those made from several furnaces treating the same ore. Some brass founders break from a single ingot the quantity of zinc required to produce the amount of brass they wish to compound in one crucible, but when perfect uniformity is desired the importance of remelting the zinon a large scale cannot be too strongly emphasized

Electrolytic Separation of Zinc.—The deposition cf pure zinc is beset with many difficulties. Zinc being more electro-positive even than nickel, all the heavy meuls mu*t be removed before its deposition IS attempted Moreover, unless the conditions are closely watched, it is lable to be thrown down in a spongy form. M Kiliani found that (he sponge was produced chiefly when a weak solution, or a low cunent-dcnsity, was used, and that hydrogen was usually evolved simultaneously; sound deposits resulted from the use of a current-density of 200 amperes, or more, per sq ft. evnI.'in?H^ solutions. The cause of the spongy deposit is variously explained, some (Siemens and Halske) ascribing it to the existence tfr°"]S7"'^ of zinc and hydrogen, and others, among whom are G., Niabnsen. F . Mylius and A. Froinm. F . Foerster and Vy. Borchers. trace it to the presence of oxide, produced, for examole either by the use of a solution containing a trace of basic aTof zinc (to prevent which the bath should be kept just-almost imrer ceptibly—acid). or by the presence of a more elertTo-reS The zinc -deposited sets up local action at the expend ses have been patented, the ore being acted upon by acid, and the resulting solution treated, by eithe? chemical or electrolytic means, for the successive removal of the other heavy metals. The pure solution of zinc is then electro" sLd tod a process of dealing with complex^^w of the w-ell-know'n Broken Hill t>-pe, containing sulphides of silvcV lead and zinc but the system was abandone^d after a long trial the process 82) has been published by the inventor, describing the practical trial at the Cockle Creek The ore was crushed roasted, and leached with sulphuric acid (with or without ferric sulphate): the solution was ur^ed and then electrolysed for zinc with lead anodes and itfi a current density of 5 amperes per sq. ft . at 275 volts when diaphragms were used, or 2-5 volts when they were dispensed with, or wfth ft 3 or 2-5 volts respectively, the e ectrolyte conta ning 1-2 ft of zinc in the form of sulphate and i to ! oz of sulphuric acid, per gallon.

The current efficiency was about s diaphragms were used to prevent the acid S with the cathode

liquor and so hindering deposition. C. Hoepfner has patented

several processes, in one of which (No. l3,7-i6 of iSoI) a ranirilv rotating cathode is used in a chloride soluHoi. a porous part^fon separating the tank into ancde and cathode compartmems, nd Hn. f

"^ generated by electrolysis at the anode l5eing recovered Hoepfner s processes have been employed both in England and process, with an electrolyte containing alkah-metal sulphate and zinc sulphate, has been used in Germany, and a process invented by Dicffenbach has also been tried in thit Halske have proposed the addition of oxidizing agents such as free halogens, to prevent the formation of zinc hydride, to which they attribute the formation of zinc sponge. Borchers and others deposit zinc from the fused chloride In Borchers process the chloride is heated partly by external firing, partly by the heat generated owing to the use of a current density of 90 to 100 amperes per sq. ft.

frarnirrd metal showing a high lustre when freshly fractured fuses at 415? C. and under ordinary atmospheric pressure boils at 1040° C. Its vapour density shows that'^it is deposits crystals belonging to the hexagonal system, and freezes into a compact crystalline solid, which may be brittle or ductile according to cPcumstances. If zmc be cast into a mould at a red heat, the ingot produced IS laminar and brittle; if cast at just the fusing-point it IS granular and sufficiently ductile to be rolled into sheet at the According to some authorities, pure zinc always yields ductile ingots. Commercial "spelter" always breaks 50° C. it is susceptible of being rolled out into even a very thin sheet. Such a sheet, if once produced, remains flexible when cold. At about 200° C the metal becomes so brittle that it can be pounded in a mortar."' The specihc gravity of zinc cannot be expected to be perfectly constant- is 6-915, and rises to 7 191 aUer rolling. The coefficient of linear expansion is o-oo2, qos for 100 from 0° upwards (Fizeau). The specific heat is oog^sS (-Regnault) Compact zinc is bluish white; it does not tarnish much in the air. It is fairly soft, and clogs the file. If zinc be heated to near its boiling-point, it catches fire and burns with a brilliant light into its powdery white oxide, which forms a reek in the air {laua pinlosopluca. " philosopher's wool ). Boiling water attacks It appreciably, but slightly, with evolution of hydrogen and formation of the hydroxide, Zn(OH), . A rod of perfectly pure zinc, when immersed in dilute sulphuric acid, is so very slowly attacked that there is no visible evolution of gas; but, if a piece pi pfatinum, copper or other more electro-positive metal be brouehr into contact with the zinc, it dissolves readily, with evolution of hydrogen and formation of the sulphate. The ordinary impure metal dissolves at once, the more readily the less pure it is. Cold dilute nitric acid dissolves zinc as nitrate, with evolution of nitrous oxKle. At higher temperatures, or with stronger acid, nitric oxide. lU, is produced besides or instead of nitrous. Zinc is also soluble in soda and potash solutions, but not in ammonia.

Applications—Zinc is largely used for "galvanizing" iron, sheets ot clean iron being immersed in a bath of the molten metal and then removed, so that a coat of zinc remains on the iron, which IS thereby protected from atmospheric corrosion. It is also a constituent of many valuable alloys; brass, Muntz-metal. pinch beck tombac, are examples. In technological chemistry it finds application as a reducing agent, e.g . in the production of aniline from nitrobenzene, but the use of iron is generally preferable in view of the cheapness of this metal.