§ 39. filtration; bunsen's process. 35 



plaster casts, and retains its form when removed from the 

 mould ; if found necessary, it may be ignited once more 

 and shaped in the mould with the cone. It may be sol- 

 dered at its upper edge by a grain of gold and borax, so 

 that it will be less liable to get out of shape, but this is 

 not necessary. If properly made, the light should not be 

 visible through tlie point of tins platinum funnel when it 

 is held before the window. 



With the platinum funnel in the throat of the glass 

 funnel, adjust the paper filter, which may be much small- 

 er than would be used in the ordinary way of filtering, in 

 the usual manner, with special care to secure perfect con- 

 tact between the filter and the funnel at all points. Con- 

 nect the tube of the funnel with a large, strong glass 

 flask, by means of a rubber cork pierced with two holes, 

 so that the tube extends about G cm. beyond the cork ; 

 through the other hole pass a short glass tube so that it 

 extends just to the lower surface of the cork ; this tube 

 should be bent once at a right angle outside of the flask ; 

 it may be connected with a small brass stop-cock by 

 means of a short rubber tube with a small bore and very 

 thick walls ; all the rubber tubing used in the apparatus 

 should be of this kind. 



Kow, pour the liquid to be filtered on the filter, rarefy 

 the air in the flask, and keep the former full as long as 

 any of the liquid remains. The precipitate may be al- 

 lowed to come within 1 mm. of the edge of the filter. 



In washing the precipitate, pour the water from a flask, 

 fill up to about a centimetre above the rim of the filter, 

 with care not to disturb the precipitate, and let each por- 

 tion of water drain ofi" completely before adding a fresh 

 quantity ; thus the w^ashing may be thoroughly cflected 

 in a wonderfully short time ; if the vacuum in the flask 

 is nearly perfect, or the pressure on the filter is nearly an 

 atmosphere, three or" four washings sufiice, even in the 

 case of precipitates that are the most difticult to wash. 



