322 



Mr. J. A. Craw. On the Filtration of [Dec. I, 



trace could be found at 100 atmospheres filtration pressure. On suddenly 

 diminishing the pressure to 20 atmospheres the liquid percolating through 

 was very intensely coloured and gave the value (250). Ke-establishing the 

 pressure of 100 atmospheres, the filtrates became less and less intensely 

 coloured (-±'75) and (0'9). A drop in the pressure to 50 atmospheres did not 

 materially change the concentration, but a further drop to 15 atmospheres 

 gave a filtrate with the value (66"6). Increasing the pressure to 50 

 atmospheres caused the filter to tighten once more with respect to the 

 hydrate, and on gradually decreasing the pressure to 10 atmospheres the 

 filter remained tight. In this condition sudden variations of pressure from 

 100 to 15 atmospheres had but a slight effect on the permeability of the 

 filter. 



Eesidual Fluid : The colour was more intense than that of the original 

 fluid and its content was (180) Fe. 



Conclusions. — The gelatine fifter is slightly permeable to the typical 

 colloid ferric hydrate, but at constant pressure the permeability decreases as 

 the filtration proceeds. The permeability is increased enormously by 

 suddenly diminishing the pressure, but is not much affected by a gradual 

 diminution. After gradual diminution of pressure a filter is obtained which 

 does not markedly change in permeability on suddenly varying the pressure. 

 The high concentration of the residual fluid is probably due to the water 

 penetrating the gelatine easily, whereas the ferric hydrate is largely left on 

 the surface of the gelatine, where it forms a concentrated solution of higher 

 specific gravity than the rest of the fluid and so gives rise to convection 

 currents which cause it partly to mix with the remainder of the fluid in the 

 filter case. The candle retains a skin of colloidal ferric hydrate, and the filter 

 most probably at the beginning of the filtration acted as a simple 

 gelatine filter, but subsequently as a compound ferric hydrate gelatine filter. 



Filtration of a Staining Colloid, Neutral Bed, through 11 per cent. Gelatine 

 under Varying Pressure. 



Preliminary experiments with horse serum and soluble starch showed 

 qualitatively similar effects to those obtained with colloidal ferric hydrate, 

 and it seemed probable that the majority of colloidal solutions would behave 

 in a similar way on filtration through gelatine. 



It seemed probable, however, that those colloidal solutions which stain 

 gelatine would show considerable difference in behaviour. 



Method. — A - 5-per-cent. solution of neutral red in distilled water was 

 pressed through 11 per cent, dialysed gelatine and the content of the fractions 

 of filtrate, etc., determined colorimetrically. 



