i6 



PRACTICAL ORGANIC AND BIO-CHEMISTRY 



completely as possible, the solid being pressed down with a spatula 

 or flat piece of glass. The solid is washed two or three times with 

 solvent which is added in small portions. The whole of the solid must 

 be wetted and the solvent drained off completely before more is added. 



Fio. n. 



(f) By Filter Presses. 



Large quantities of solid matter with small amounts of liquid are separated 

 by means of a filter press. The material is pressed either between layers ol 

 cloth or in a sheet of cloth which can be folded to make a sort of bag. 

 Squeezing by hand in the latter method will remove some of the liquid and 

 the greater pressure from a press will remove nearly all. The dry residue can 

 be taken out, stirred up with solvent and again pressed out. 



The greatest pressure usually attainable is from a Buchner press or a hy- 

 draulic press. * The solid matter must be in a fairly dry condition and, if moist, 

 is mixed with some absorbent, generally siliceous earth or " Kieselguhr ". 

 Liquid is squeezed out of this mass by the high pressure. 



Subsequent filtration through a fluted paper is usually necessary after fil- 

 tration by a filter press as particles come through the pores in the cloth. 



(d) Through a Layer of Neutral Material. 



Solutions containing colloidal particles are generally most difficult to filter 

 as the particles either come through fluted filter paper or soon clog the pores. 



Osborne has used filter paper pulp as a medium for filtering solutions of pro- 

 teins. A sufficient quantity of pulp is nv'xed with the liquid and this is poured 

 up >n the paper on a Buchner funnel, where it forms a layer exposing a large sur- 

 face and prevents the underneath paper from breaking and becoming clogged. 



A layer of siliceous earth upon a Uuchner funnel has al.^o been found very 

 efleci ufficient quantity of the siliceous earth is mixed in a small mor- 



tar with the liquid and poured upon the funnel so as to forma layer from 1-2 cm. 

 thick, and t h .e liquid is then filtered through this. 



The first poitions of filtrate may be cloudy, and must be returned to the 

 filter and filtered again. 



