FILTERS 49 



Bell-jars with ground rims and one or two tubules are 

 useful for evaporation in vacuo. They should stand on a 

 square of thick ground glass. To make an air-tight joint 

 the surface of the rim of the bell- jar, which must be quite 

 clean, should be well greased and pressed thoroughly 

 home on the ground-glass plate. A thick ridge of grease 

 should then be plastered all round the angle formed by the 

 rim of the bell- jar and the glass plate. Thick rubber 

 pressure tubing must be used for connections, and all 

 joints should be well greased. For evaporating large 

 quantities of fluid the writer devised a copper stand with 

 shelves, the shelves supporting glass dishes containing 

 alternately strong sulphuric acid and the fluid to be 

 evaporated, the whole being placed under a suitable 

 bell- jar. A mercurial gauge is a useful addition to show 

 the amount of exhaust and the occurrence of leakage. 

 The ordinary glass filter pumps used in chemical work 

 and actuated by a stream of water are also useful for many 

 purposes. 



Porous porcelain filters. The forms which are generally 

 employed are the Pasteur- Chamberland, the Doulton, and 

 the Berkefeld. These consist of " candles " composed in 

 the first two of unglazed porous porcelain, in the last of a 

 specially prepared diatomaceous earth. The filtration 

 through the Pasteur- Chamberland is much slower than 

 through the Berkefeld. All give a germ-free filtrate, but 

 the last should be employed if the fluid is thick or contains 

 many particles ; a preliminary filtration through paper 

 is an advantage. A useful method of conducting filtration 

 is the following : The filter " candle " (B, Fig. 5, p. 50) 

 is connected by a short length of pressure tubing with a 

 piece of glass tubing passing through a rubber cork in the 

 neck of an ordinary filtering flask c. The " candle " is 

 placed in a jar A, such as a glass measure or urine- jar, 

 which is filled up with the solution to be filtered. The 



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