derate heat is required; for stone ware or porcelain the kiln must be provided with six or eight furnaces for sending up flame. The glass-house furnace must be constructed in such a manner as to allow the workmen to have free access to it without diminishing the heat. The reason why brick furnaces often crack is, their internal parts being more heated, and consequently more expanded than the external: to prevent this the furnace should be heated and cooled gradually, at least till properly seasoned. The furnaces employed in heating retorts only differ in a contrivance for heating the vessel slowly and equally over the surface. These probably have got the name of balnea from water being mostly used, which is safest when no great heat is required; and this is called balneum maria. When sand is used, it is called balneum arena. Some have recommended steel filings, and Dr. Lewis quicksilver; but as quicksilver soon evaporates, I would prefer the softer metals, as bismuth, &c. It is often found useful to make use of air, which is called distilling in copella vacua. The vessel is to be placed in the middle of an iron vessel, suspended by iron hooks, and you must close the iron vessel so as to exclude the external air. The lamp furnace has been used both for the purposes of evaporation and digestion. For various hints on furnaces, see Lewis's Commerce of Arts, and Mr. Nicholson's Dictionary of Chemistry. Lutes are commonly used for joining two vessels together, and stopping the cavities. The ordinary lutes deserve the name less than others, having no clay in their composition: they are used to stop the junctures of vessels when the heat is gentle; but it is better to leave a hole made with a pin than to shut the juncture quite close. Flour and water with gum arabic is good for distilling; also linseed meal (from which the oil has been expressed) makes a remarkably tight luting. The most convenient linings are bladders cut into slips, and moistened; the inside of the bladder must be applied next_tne vessel*. When the fumes are corrosive, fiery lutes must be used; the best of which is pipe clay mixed with seven times its quantity of white sand. The clay only serves to combine the particles of sand together, for sand resists the most violent heat. A good luting for lining the internal part of a furnace is made of charcoal dust and water with one-fourth of clay, over which spread a covering of sand and clay, and beat them well with a hammer. The charcoal will last two or three years; the clay is mixed with the charcoal to prevent its cracking. * When a bladder is used with the retort and receiver, a hole may be made in it with a pin, by which the operator may judge of the contents by the smell. Mr. Nicholson* most judiciously remarks that the ingenious student in chemistry, when he has acquainted himself with the first principles, will easily perceive that there are few philosophical inquiries which require a large apparatus of furnaces or vessels. A tobacco pipe (the bore of which must be stopped with good clay) is a very useful crucible, in which many operations may be performed with a good pair of double bellows. An earthen pot, or an iron ladle, will contain a sand bath, and common phials, or Florence flasks, serve very well for matrasses. Chafing-dishes, or small iron stoves, will produce a great heat if well managed; and the blow-pipe and spirit lamp, with a set of small retorts and receivers, may be applied to the performance of almost every part of experi mental chemistry. See his Dictionary of Chemistry. |