Mackenzie Cupola
The Mackenzie cupola has a continuous tuyere, which allows the blast to enter the fuel at all points. Each size is proportioned to melt a given quantity of iron in a certain time. Above one ton per hour melting capacity they are made oval in form. This construction brings the blast to the centre of the furnace with the least possible resistance and, it is claimed, the smallest amount of power, causing a complete diffusion of air and a uniform temperature. The sizes of the cupolas indicate the melting capacity per hour; that is, a No. 1 melts 1 ton per hour; No. 6, 6 tons; and so on with all sizes. See CUPOLA; BLOWER.
Magnesium is a very brilliant metal of almost silvery whiteness. It is more brittle than silver at an ordinary temperature, but becomes malleable at something below a red heat. Its specific gravity is 1.74. It melts at a bright-red heat, and volatilizes at nearly the same temperature as zinc. In dry air its lustre is retained, but a crust of magnesia forms on its surface when subjected to a moist air. Magnesium in the form of a wire or ribbon takes fire at a red heat, burning with a dazzling bluish-white light. The pure oxide of magnesium (magnesia) is obtained by igniting the carbonate, but this is both a difficult and expensive means of obtaining it, and recourse is had to the impure magnesian limestones found in Thuringia and in some parts of England. It is a white powder, varying in density according to the source from whence it is obtained. It is unalterable by heat, and has never been fused; and on account of its refractoriness is valuable as an ingredient for the manufacture of crucibles for metallurgical purposes. See DOLOMITE.