326 



Mr. J. A. Craw. On the Filtration of [Dec. 1, 



Conclusion. — Solutions of salts such as sodium chloride, or colloids such as 

 ferric hydrate, will not appreciably modify the rate of filtration through 

 gelatine unless in concentrated solution. Amylic alcohol and butyric acid 

 accelerate the rate of filtration. 



It seemed possible that this effect might be due to the condensation of 

 amylic alcohol and butyric acid on the walls of the gelatine pores, and that 

 the change in viscosity might account for the increased rate. This, however, 

 does not seem probable, for a toluene emulsion did not markedly affect the 

 rate of filtration, and as the viscosities of the toluene and amylic alcohol used 

 were found to be 0'49 and 4*4 respectively when compared with distilled 

 water as unity, one might conclude that the toluene would accelerate, and the 

 amylic alcohol retard, the rate of filtration. 



On the other hand, the action of amylic alcohol and butyric acid is 

 consonant with the view that the surface forces between gelatine and saline 

 are modified. The surface tension of saline against air diminishes with 

 increasing concentration of both amylic alcohol and butyric acid. Both of 

 these substances markedly influence the rates of sedimentation of solid 

 suspensions through water,* and this fact seems to have a close connection 

 with their influence on the rate at which water passes through a porous solid 

 or jelly such as gelatine. 



The effect of continued pressure on gelatine seems to be a tightening of the 

 pores, which is nearly complete at 100 atmospheres in 40 minutes, and 

 decompression apparently allows the pores to resume their original dimen- 

 sions in about 10 minutes at atmospheric pressure. 



On the Expression of Water and Gelatine from a Wet Filter. 



In agreement with E. W. Eeid, the filtrates were found to contain gelatine, 

 but in variable amounts. Thus the first fraction of about 4 c.c. usually con- 

 tained most, and after 20 c.c. had passed the gelatine was only present in 

 traces. Rough estimations with tannic acid indicated about - 5 per cent, 

 gelatine in the first, and less than - l per cent, in the fifth fraction on filtering 

 distilled water. These contents of gelatine were not found to influence the 

 titrations given above. 



It has been hitherto assumed that the water in the pores of a wet filter is 

 largely expressed before the liquid undergoing filtration appears in the filtrate 

 undiluted by the same. For this reason Martinf discarded the first 10 c.c. of 

 filtrate, and E. W. EeidJ concluded that the concentration of even a second 

 fraction of 25 c.c. might be considerably reduced by this dilution. It seems 

 to me, however, that the effect of the water in the gelatine on the concen- 

 * Cf. Craw (1904). t Loc. cit. % Loc. ext. 



