of metallic oxides in the material operated upon, and in the ashes of the fuel, so that a crucible should be selected which is best adapted to the special purpose to which it is to be applied. The component parts of a crucible are first crushed to a fine powder, and passed through a sieve, the fineness of whose meshes will vary with the desired fineness of the grain in the pot (the plasticity being closely connected with the fineness of the particles; at any rate for small crucibles this closeness of grain appears to be indispensable), then the fine powder is mixed with water and kneaded to the right consistence for use. The best results are obtained by using a mixture of different fire-clays, the most infusible being those containing the largest amount of silica, and the minimum of oxides of iron and lime. The presence of potash and soda in small quantity sensibly increases the fusibility, but they act advantageously in soldering the particles together. Iron pyrites, which is frequently disseminated through clays, especially those from the coal measures, is perhaps the most injurious constituent. A crucible made from such clay will become indented with small cavities, and even holes, when exposed to a prolonged high temperature. It follows then that the most refractory crucibles are those made from pure clays, the nearest approach to which is presented by some French clays. The fitness of a clay for making crucibles may be determined by moulding a portion into the shape of a prism, or any form containing sharp edges, carefully drying, baking, and exposing to a high temperature in a covered crucible for some time. If very refractory, the test will show no signs of fusion. If the edges are rounded it is a proof of incipient fusion, and if melted, the clay is useless. Clay vessels of all kinds may be tested to ascertain their power of resisting corrosion by melting in them a mixture of litharge, red oxide of copper, and borax, and noticing the time this mixture will take to permeate them. Those which resist this destructive action the longest will of course be the best. Most crucibles are by this means eaten away irregularly, showing the necessity of uniformity of grain to resist perforation. All crucibles should be cautiously annealed before use by placing them in an inverted position over the furnace, otherwise they are liable to split when plunged into a red-hot fire. I have noticed this tendency with the best plumbago crucibles. Plumbago or black-lead pots are made from varying proportions of fire-clay mixed with powdered graphite or coke dust. Good graphite is neither altered nor fused by exposure to the highest temperatures (air being absent), so that it is an admirable substance for crucibles. The graphite is powdered, sifted, and mixed with sufficient clay to render it plastic. Good plumbago crucibles, after a careful preliminary annealing, withstand the greatest changes of temperature without cracking, and may be heated many times in succession. When an ordinary crucible requires to be protected from the corrosive action of metallic oxides, or when small amounts of metallic compounds have to be reduced, the inside is coated with a lining of charcoal. This is done by first mixing the charcoal with sufficient starch, paste, or treacle to make it adhere when pressed. The crucible is then loosely filled with the brasque, and a cavity of the desired size made by boring with a triangular-shaped piece of wood, and then made smooth with a round elongated wooden tool, whose size and shape are apportioned to the capacity of the cavity desired, or the brasque may be plastered on the inside of the crucible by the hand. § 19. Fire-Bricks.- -A fire-brick used to withstand high temperatures must only contain small quantities of the alkalies, which should not exceed 1 per cent. Glenboig, Stourbridge, and Wortley (Leeds) are the leading brands extensively used; these are safe, though of course there are many other fairly serviceable bricks made of brands less in repute. Ganister bricks are exceedingly valuable for withstanding the very highest temperatures for the crowns of reverberatory furnaces. They do not crack on cooling so much as bricks composed almost entirely of silica. The Lowood brick, made near Sheffield, has a very high reputation in this class. Ganister bricks should be set in thin ganister cement. Crowns are best put on dry, and just "slurried" over the top when finished. No fire-brick has a fair chance if set in a clay inferior to itself; but however excellent the clay, a good furnace builder will use as little as possible. Dinas bricks are practically infusible, and composed almost wholly of silica. The fractured surface presents a coarse, irregular structure of a light-brown colour. The lime which is added exerts a fluxing action on the particles of quartz, and so causes them to agglutinate (see also p. 58). Mr. James Dunnachie in a paper read before the British Association said: "The great variety of purposes for which fire-bricks are used, and the various qualities required in them, make it impossible that one brick can answer for all descriptions of furnaces. They require to stand strong heat and changes of temperature; in some cases heavy pressure and hard knocks; in others they require to resist the fluxing and chemical actions of various kinds. "A brick high in silica, containing a fair proportion of alumina, and nearly free from alkalies and other impurities, is the one which combines, in the highest degree, infusibility and freedom from cracking. To get a really good furnace, we must first procure the best materials for its construction; but after that much depends upon how it is built. If we were as careful about the curves of our furnaces as we are about the lines of our ships, and as particular about the quality of the materials and workmanship employed as the importance of the subject demands, we might, in many cases, double the duration of our furnaces, without waiting for the possible discoveries of the future." The following analysis will serve to show the composition of British materials used in furnace construction : : PREPARATION AND PROPERTIES OF ALLOYS § 20. The mode of procedure in the production of any alloy will be largely influenced by the nature of the metals to be operated upon. Some metals are volatile, and readily pass off as vapour when heated a few degrees above their melting points. Others have little tendency to vaporise, and may be raised to high temperatures without sensible volatilisation. When a volatile metal has to be alloyed with a non-volatile metal, and the fusing points of both are approximately the same, combination can be most readily effected by mixing the constituents and melting them together in the same crucible or furnace. This is, however, seldom the case, and, as a general rule, the components of an alloy, one or all of which are volatile, have widely divergent melting points, and then it is requisite for the most refractory constituent to be melted first, and for the others to be added in the solid state. Again, an alloy may contain one or more fixed metals and a volatile one, in which case the more volatile metal is added to the crucible, after the fixed metal or metals have been fused, and raised to a temperature necessary to melt the volatile constituent immediately it is introduced, so that combination may be effected before any serious loss, due to vaporisation, has occurred. Union between the components of an alloy is more perfectly secured by agitation of the contents with a stirring-rod, the most effective in many cases being a wooden or carbon rod, which promotes admixture without the introduction of any substance likely to contaminate the mixture, and modify its properties. A thing to be guarded against in the melting of all base metals, or alloys containing base metals as essential constituents, is oxidation. Various plans are adopted to avoid loss of metal and injury to the alloy from this cause. The most common one is to cover the metals with carbon, which not only excludes the air admitted to the furnace, but tends to absorb any oxygen liberated from the metals during |