14. Elutriation

14. Elutriation. In chemistry, the operation of washing insoluble powders with water, to separate them from foreign matter, or the coarser portion. It is usually performed by grinding or triturating the mass with a little water, until reduced to a very fine powder, and this paste is suddenly diffused through a large quantity of water in a deep vessel, from which, after the subsidence of the grosser portion, the liquid is poured into another vessel, and allowed to deposit the fine powder it still holds in suspension. When this has taken place, the clear supernatant liquor is decanted, and the sediment drained and dried. The coarse sediment deposited in the first vessel is now submitted to a fresh grinding and diffusion through water, and the entire operation is repeated, until the whole of the pulverizable portion is washed over. The proper length of time for the liquid to remain in the first vessel, depends solely on the density of the powder, and the degree of fineness required in the product; heavy powders subsiding almost immediately, while light ones often take several minutes to deposit their coarser portion. Sometimes three or more vessels are employed, and the muddy liquor, after remaining a short time in the first, is poured into the next one, and this, in a short time longer, into the third, and so on, until the last vessel is filled, by which means, powders of different degrees of fineness are obtained; that deposited in the last vessel being in the minutest state of division.

15. Evaporation

15. Evaporation. The conversion of a fluid into vapor by means of heat, diminished atmospheric pressure, or exposure to a dry atmosphere. The process of evaporation is resorted to;- 1. For the vapor as a source of heat or power, as in steam boilers, etc.; - 2. To separate volatile fluids from other bodies which are either fixed or less volatile; - 3. To recover solid bodies from their solutions; - 4. To concentrate or strengthen a solution by expelling a portion of the liquid;- 5. To purify liquids by expelling any volatile matters which they may contain. As evaporation is, under ordinary circumstances, confined to the surface of the liquid, wide shallow vessels are the best for the purpose; the proce33 is greatly facilitated by exposing the surface to a current of dry air, especially if the air be heated. On a small scale, shallow capsules of glass, wedgwood ware, porcelain or metal, are commonly employed, and are exposed to heat by placing them over a lamp, open fire, or in a water or sand-bath. {See No. 44 (To Prepare Extracts).)

16. Fermentation

16.   Fermentation. Chemists distinguish fermentation into five kinds, viz:

The saccharine fermentation, by which starch and gum are converted into sugar.

The alcoholic or vinous fermentation, by which sugar is converted into alcohol.

The viscous or mucilaginous fermentation, which converts sugar into slime or mucilage, instead of alcohol.

The acetous fermentation, by which alcohol is converted into vinegar.

The putrid fermentation, or putrefaction, which is exhibited in its most marked form in the putrefaction of animal substances.