and in th ected whi it is to rushed to: ected with of potash sures, ich is De deprism, king, le for signs pient their e of the nich the best. Most crucibles are by this means eaten irregularly, showing the necessity of uniformity of grai resist perforation. All crucibles should be cautiously annealed before us placing them in an inverted position over the furnace, o wise they are liable to split when plunged into a red-hot I have noticed this tendency with the best plumbago cruc Plumbago or black-lead pots are made from va proportions of fire-clay mixed with powdered graphit coke dust. Good graphite is neither altered nor fuse exposure to the highest temperatures (air being absent that it is an admirable substance for crucibles. The gra is powdered, sifted, and mixed with sufficient clay to re it plastic. Good plumbago crucibles, after a careful liminary annealing, withstand the greatest change temperature without cracking, and may be heated times in succession. When an ordinary crucible requires to be protected the corrosive action of metallic oxides, or when small am of metallic compounds have to be reduced, the insid coated with a lining of charcoal. This is done by mixing the charcoal with sufficient starch, paste, or tr to make it adhere when pressed. The crucible is then lo filled with the brasque, and a cavity of the desired size by boring with a triangular-shaped piece of wood, and made smooth with a round elongated wooden tool, whose and shape are apportioned to the capacity of the cavity des or the brasque may be plastered on the inside of the cru by the hand. § 19. Fire-Bricks.—A fire-brick used to withstand temperatures must only contain small quantities of alkalies, which should not exceed 1 per cent. Glen Stourbridge, and Wortley (Leeds) are the leading br extensively used; these are safe, though of course ther many other fairly serviceable bricks made of brands les repute. Ganister bricks are exceedingly valuable the very highest temperatures for the crowns furnaces. They do not crack on cooling so composed almost entirely of silica. The made near Sheffield, has a very high reputati Ganister bricks should be set in thin ga Crowns are best put on dry, and just "slu top when finished. No fire-brick has a fair a clay inferior to itself; but however excell good furnace builder will use as little as possi Dinas bricks are practically infusible, almost wholly of silica. The fractured surf coarse, irregular structure of a light-brown col which is added exerts a fluxing action on t quartz, and so causes them to agglutinate (see Mr. James Dunnachie in a paper read bef Association said: "The great variety of purp fire-bricks are used, and the various qualities req make it impossible that one brick can answer tions of furnaces. They require to stand str changes of temperature; in some cases heavy hard knocks; in others they require to resist th chemical actions of various kinds. "A brick high in silica, containing a fair alumina, and nearly free from alkalies and oth is the one which combines, in the highest degre and freedom from cracking. To get a really g we must first procure the best materials for its but after that much depends upon how it is were as careful about the curves of our furnace about the lines of our ships, and as particula quality of the materials and workmanship empl importance of the subject demands, we might, in double the duration of our furnaces, without wai possible discoveries of the future." The following analysis will serve to show the of British materials used in furnace construction § 20. The mode of procedure in the p alloy will be largely influenced by the nat to be operated upon. Some metals are vol pass off as vapour when heated a few deg melting points. Others have little tende and may be raised to high temperatures volatilisation. When a volatile metal has to a non-volatile metal, and the fusing poin approximately the same, combination can effected by mixing the constituents and m gether in the same crucible or furnace. T seldom the case, and, as a general rule, the c alloy, one or all of which are volatile, have v melting points, and then it is requisite for the constituent to be melted first, and for the other the solid state. Again, an alloy may contain o metals and a volatile one, in which case th metal is added to the crucible, after the fixed have been fused, and raised to a temperatu melt the volatile constituent immediately it is that combination may be effected before an due to vaporisation, has occurred. Union components of an alloy is more perfectly secure of the contents with a stirring-rod, the mo many cases being a wooden or carbon rod, w admixture without the introduction of any su to contaminate the mixture, and modify its pr A thing to be guarded against in the melti metals, or alloys containing base metals as stituents, is oxidation. Various plans are ado loss of metal and injury to the alloy from this most common one is to cover the metals with not only excludes the air admitted to the furna to absorb any oxygen liberated from the n OYS ion of the me and read bove vapor t sens loyed both st read them howere nts of liverge fract added ore fire volatil fusion. The gas thus formed by union of carbon with oxy is termed carbonic oxide CO, and this gas being a reduc agent, is capable of taking up another atom of oxyg forming carbonic acid CO2. Thus, as long as the mixtur covered with carbon, the carbonic oxide formed effectua shields it from oxidation. In the method already referred of stirring metals with a carbon rod to promote mixture, same gas, carbonic oxide, is formed, and thus the rod only promotes union by mechanical agitation, but generate gas which protects the metals in a great measure fi oxidation. In some cases this is not admissible, as c mercial metals are impure, and it may be advisable to ad sufficient oxygen, either from the air or by means o special oxidising agent, added along with the flux, to conv the impurities into oxides, which do not alloy with metals, but either enter into combination with the flux form a slag, or rise to the surface as dross or scum. In m cases it is advisable that the covering body should not ex any influence on the metals beneath. These are Some manufacturers are in the habit of throwing fat resin on the heated metals before fusion. composed by the heat, liberating gases, and when well sti with the molten metal promote combination by the mechan agitation imparted by their escape. They also act chemica in removing oxygen, by the union of that element with carbon and hydrogen set free. When the evolution of has ceased, a quantity of carbon remains in a finely divi state, which covers the metals and protects them f oxidation. Borax is sometimes used to exclude the air, but i much more costly than carbon, and when it is not requi as a flux, its employment is accompanied with some ev Now borax is composed of the base soda in combination w boric acid, which is only partly saturated with the soda, the excess of acid unites with any metallic oxide pres forming double borates, of a glassy nature. Commer borax is often very impure, and is adulterated with comm F |