METHODS OF COLLOID RESEARCH 



95 



Dialysis is considerably hastened by agitating the dialyzing fluid. 

 I have never found it suggested that the contents of the dialyzer 

 should be stirred but this is a useful procedure with non-foaming 

 solutions. F. HOFMMSTER fastens all the dialyzing thimbles to a 

 common rod which he rocks up and 

 down with a motor. R. KOHLER places 

 fish bladders in wide-mouthed bottles 

 which he closes with a rubber cork and a 

 rubber cap, and then places it in a shak- 

 ing machine; to prevent twisting off the 

 fish bladder at its neck, he inserts sev- 

 eral glass rods as shown in Fig. 14a. 



Ultrafiltration. 



PUBBER CAP 



RUBBER STOPPER 



FISH BLADDER 



..-GLASS RODS 



H. BECHHOLD defines ultrafiltration 

 as filtration through jelly filters. They FlG 14a 



serve to separate colloid solutions from 



crystalloids and for the separation of colloid mixtures having parti- 

 cles of different size. If we know the size of the pores in an ultra- 

 filter, ultrafiltration affords information as to the size of the particles 

 in the colloid under investigation. 



Ultrafilter. For ultrafiltration, sac-like membranes may be em- 

 ployed prepared as for diffusion experiments (see p. 91) and mounted 

 as shown in Fig. 11. 



A. SCHOEP * increased the permeability of membranes by adding 

 glycerin and castor oil to the collodion. This is of great importance 

 in filtering inorganic colloids. 



This sort of ultrafiltration is used especially in France (G. MAL- 

 FITANO, J. DUCLAUX), but it is of limited utility. Filtration occurs 

 very slowly (a few cubic centimeters per hour) and the filters can- 

 not withstand much pressure, so that their usefulness is very 

 limited. 



For ultrafiltration, H. BECHHOLD* 4 used pieces of filter paper im- 

 pregnated with jellies. By means of this paper support the filters 

 acquire great strength and may at times sustain in BECHHOLD 's ultra- 

 filtration apparatus, pressures of 20 atmospheres or more. Since 

 H. BECHHOLD discovered that the premeability or tightness of the 

 ultrafilters depended on the concentration of the jellies used in pre- 

 paring them, it is possible to make filters with pores of any desired 

 size. The filters may be purchased ready-made. 1 



1 Schleicher and Schiill, in Duren, market Bechhold's ultrafilters in aluminium 

 boxes which contain 10 filters filled with water and sealed with a rubber ring, 

 (diam. 9 cm.). This firm keeps in stock six kinds, of different porosity. 



