BY DR. LAUDER BRUNTON. 563 



should not be too strong, nor should it strike the filter or precipitate 

 perpendicularly, as it is then apt to scatter the precipitate or tear the 

 filter. When the filter is nearly full of water, the whole should be 

 allowed to run through, and the washing again repeated. 



Wtithing of Precipitates by Decantation. When a precipitate sub- 

 sides quickly, it is more readily washed by decantation than on a filter. 

 Granular and gelatinous precipitates are not easily washed completely 

 on a filter, and it is better to wash them as well as possible by decanta- 

 tion, and to finish the operation on a filter. In washing by decantation, 

 the precipitate is placed in a tall beaker, and stirred well with a quantity 

 of water, alcohol, or other washing liquid. It is then allowed to sub- 

 side, and the supernatant liquid carefully poured off or removed by a 

 syphon (see fig. 833) ; this is repeated till the washing is complete. In 

 order to prevent any of the precipitate being carried off in the washing 

 and lost, the liquid used for washing may be collected and passed 

 through a filter. Any part of the precipitate retained by the filter may 

 then lie washed, and the rest of the precipitate added to it. 



211. Filtration. Filtration is the separation of insoluble substances 

 from liquids bypassing the latter through a porous material which keeps 

 the former back. When the substance to be removed is in large pieces, 

 or when the liquid is thick and viscid, and will not pass easily through 

 paper, it may be filtered through linen or gauze. The linen may either 

 be stretched over the mouth of a beaker or placed in a porcelain strainer 

 in the form of a hollow cone, with numerous perforations near its apex. 

 The removal of the last portions of the liquid may generally be hastened 

 by squeezing the linen either with the hand or in a press (fig. 334). 

 Fine precipitates are usually separated by filters of unglazed porous 

 paper, made specially for the purpose. To make a filter, take a round 

 or square piece of paper of the proper size, and fold it twice at right 

 angles. If a square piece has been used, it must now be cut round. 

 Open it in the form of a cone, and place it in a funnel. If the funnel is 

 of proper form (its sides sloping at an angle of 30 to its axis), the filter 

 will fit it exactly. If it does not, the angle at the apex of the paper cone 

 must be modified. The filter should always be a little smaller than the 

 funnel, and never project above its edges. Before pouring in the liquid 

 to be filtered, the paper must be moistened with distilled water, alcohol, 

 or ether, according as the liquid is aqueous, alcoholic, or ethereal. If 

 this is not done, the first portions of the fluid which pass through are 

 apt to be muddy, but they may be cleared by pouring them back on the 

 filter and making them pass through a second time. To avoid breaking 

 the filter at the apex, the liquid should be poured on it so as to fall on 

 its sides, which are supported by the funnel, and the stream directed by 

 a. glass rod. The filtrate is generally collected in a beaker ; it is well 

 to let the end of the funnel touch the side of the glass, so that the liquid 

 may run down it without splashing If the filtrate only is wanted, fil- 

 tration may be quickened by using a ribbed or plaited filter. To make 

 this, take a circular piece of filter paper and fold it into quadrants, and 

 then into half quadrants, making all the folds towards one side. Then 

 make a fold towards the other side between each two of those already 

 made, and push the paper into the funnel, pressing the point down into 

 the neck of the funnel ; then pour in the liquid, Avhen it will open com- 

 pletely. Instead of this, three glass rods, bent at the top so as to hook 

 on to the edge of the funnel, may be laid inside it at equal distances 

 from each other. These are useful both in quickening the filtration and 

 in supporting the bottom of the filter, especially when the funnel is 

 badly made and its tube is too wide at its junction with the cone. When 

 albuminous liquids are filtered through paper, the pores become very 

 quickly choked up, and it is therefore better to use a number of small 



