3841. Chemical Washing

3841.    Chemical Washing. When precipitation takes place, the deposit requires to undergo edulcoration, or cleansing from the liquid from which it was precipitated. With heavy and bulky precipitates, this is done by repeated washing, and, after the deposit ha3 again settled, decantation of the supernatant liquid (see No. 3847 (Edulcoration)); but when the powder is light, and separates with less facility from the liquid, the washing is better performed by a continuous stream of water passing through a filter on which the precipitate has been previously collected. The apparatus employed for a self - feeding filter (see No. 3840 (Self-Feeding Filter)) is admirably adapted for this purpose. Lixiviation, or the separation of soluble matter from an insoluble powder, can bo performed in the same way. (See Nos. 14 (Elutriation), 23, and 32.)

3842. Chemical Drying

3842.    Chemical Drying. In order to deprive chemical substances of water or moisture, the simplest means is evaporation. This may be performed either by merely exposure in open shallow vessels to the natural action of a dry atmosphere, called spontaneous evaporation; or by the application of heat, either directly or by a water-bath, etc.. (see No. 12 (Desiccation)); this is not always advisable or necessary, as some substances undergo change by heat, and must be dried by other means. By enclosing the substance to bo dried in a box or drying-chamber in which is placed an open vessel containing strong sulphuric acid or chloride of calcium, the strong affinity for water that these substances possess keeps the air perfectly dry, and absorbs the moisture from it as fast as the water evaporates from the material which is being dried. Tho water of crystalline bodies is usually driven out by exposing the crystals in a capsule or evaporating dish to heat, only just sufficient being applied to effect the purpose. Some crystals part with their water of crystallization spontaneously by exposure to the air, crumbling into powder; such crystals are called efflorescent, to distinguish them from those deliquescent crystalline bodies which spontaneously liquefy or dissolve in their own water of crystallization. Others will yield their water in an artificially dried atmosphere, as above stated; while many have sufficient affinity for water to retain it until driven off by heat, more or less intense. Crystalline substances which have been deprived of the water of crystallization, that is, have undergone desiccation, are said to bo dry.

3843. Decarbonization

3843.    Decarbonization. This operation is performed on cast iron, to convert it into steel or soft iron. The articles to bo decarbonized are packed in finely-powdered hematite, or native oxide of iron, to which iron filings are often added, and exposed for some time to a strong red heat, by which the excess of carbon is abstracted or burnt out. The process somewhat resembles annealing or cementation.