276 



this kind should never be exposed to the direct fire, but only to 

 the heat of the watei^-bath, to prevent the metal from fusing and 

 the contents from getting charred. The water-bath consists of a 

 copper boiler, in which the still is fitted so as to reach nearly to 

 the bottom, while resting rather lightly on the prominent edges of 

 the contracted mouth of the boiler. 



The tin still serves for extractions and distillations {see F.) on a 

 larger scale. 



E. — Straining Apparatus. 



For larger quantities dishes or deep vessels of porcelain or stone- 

 ware are used with a tenacidum (square framework, constructed of 

 four narrow pieces of wood) of proper size to be placed on the 

 dishes, and carrying on its four prominent points a linen cloth or 

 a linen bag-filter, through which to strain the substance under 

 investigation. The latter operation is accelerated by occasionally 

 stirring or pressing the contents with a strong glass rod or with a 

 porcelaiu spa,tida, or instead of these a sjxitida, made of pine or 

 beech and previously cleaned by boiling with water, suffices for 

 most pur})oses. 



Thick, slimy masses let the liquid pass so slowly that it is 

 necessary to strain through a cloth, the meshes of which do not 

 contract by moisture. This is miller s gauze, or bolting cloth of 

 silk, which is sold in numerous gradations as to the width of the 

 meshes, and may be therefore selected according to circumstances. 



After the dripping has ceased, the contents of the cloth or of the 

 bag-filter are submitted to the action of a screv)-j)ress or of an 

 hydraidic press. Those parts of the press, which are in direct 

 contact with the cloth or with the liquid passing from it, must be 

 made of tin. — For pressing smaller quantities, a small self-acting 

 'press, similar in construction to a bookbinder's press, is to be 

 employed, but modified that it may be fastened to a table, and its 

 sides must be covered with thick jilates of glass. 



Still smaller quantities are poured into paper-filters, spread out 

 va. funnels of glass or jjorcelain, the latter resting on the edges of 

 glass-jars, and, if necessary, supported by means o{ filter-plates or 

 other appliances. If the filtering process is slow, the funnel must 

 be covered with a glass-plate in order to screen ofi" air and dust. 

 This is not less necessary -with very volatile liquids, for instance, 

 alcohol, ethei-, &c., in order to prevent losses. 



More economically, larger quantities or slowly filtering alcoholic 

 liquids are filtered in a displacement apparatus of glass, the top of 

 which can be shut nearly air-tight. 



For the filtration of liquids, which i-equire warming either 

 because they are then only of sufficient fluidity, or because in lower 

 temperatures the solved parts would become solid, a water-bath- 

 funnel is employed, i.e. a water-bath of funnel-shape and exactly 



