Boilers for all purposes were formerly made of metal (usually copper or iron), and were exposed directly to the fire intended to heat their contents, but since the properties of steam have been more fully recognised, it is now very frequently employed for heating boilers--especially where a heat at or below the boiling point of water is required. There are great advantages arising from this plan, one of which consists in doing away with the risk of the materials in the boiler being burnt. Some boilers are now made of wood, having steam-pipes running through them, and in those cases in which the admixture of water is no detriment steam--in the form of jets--is thrown directly into the fluid to be heated, which very quickly raises it to the boiling point. Boilers of cast-iron, lined with platinum or enamel, are also used for various purposes, as the condensation of acid substances, &c., which would act on most metals. Glass and glazed pans, too, can be used with a steam apparatus, without any danger arising from breakage, which would frequently occur if they were directly applied to the fire. FURNACES. Illustration: FIG. 1. (‡ REVERBERATORY FURNACE.) Illustration: FIG. 2. (‡ BLAST FURNACE.) Illustration: FIG. 3. (‡ CUTAWAY DIAGRAM OF BLAST FURNACE.) Furnaces are fire-places constructed to serve particular purposes, and are chiefly of two kinds, “Wind furnaces” and “Blast furnaces.” Of the first kind the common house grate is an instance, of the second the blacksmith’s forge. The fire in a wind furnace is more or less shut up, so that the draught of air entering it shall pass from the ash-pit right up into the fire, and through it into the flue or chimney--the latter being tall, and of certain proportions, so as to ensure the requisite draught. These furnaces are used where heat of the very highest degree is not required, as in glass-houses, pottery-kilns, &c. The “Reverberatory furnace” is a modification of the wind furnace, and is used to throw heat on to the surface of substances, as in roasting ores of metals, to drive off the sulphur, arsenic, &c., or in the making of soda, litharge, and other processes where the admission of hot air with the flame is either beneficial, or at least not detrimental; fig. 1 shows the construction of this kind of furnace. Blast furnaces are for the production of the very highest degrees of temperature, and in these the air is forced into the fire by blowing machines or bellows, often worked by steam-engines; such furnaces are used for the smelting and casting of iron, &c. (fig. 2). A good blast furnace for small purposes may be made by two crucibles--those made of coarse blacklead and clay, and called “Blue pots,” are the best--one placed inside the other, the outer one having a hole at the lower part for the nose of the bellows, the inner one having the bottom cut off and a grating of iron put in to lodge just above the lowest part; the space between