314 



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



" dried " filter are not subject to the same conditions of filtration and are 

 not, therefore, comparable. Wet filters, on the other hand, show a much 

 greater constancy in rate of filtration, and are, therefore, to be preferred. 



On the Preparation of the Filters. — To overcome the difficulty of dilution 

 of the filtrates by the water of wet filters, the gelatine to be used in filling 

 the candles was in several cases dissolved in the fluid to be filtered. 

 Thus the filtration of - 8 per cent, sodium chloride and of megatherium 

 lysin took place through 9 per cent, gelatine which had been dissolved in 

 0'8 per cent, sodium chloride, and l - 54 per cent, potassium iodide was 

 filtered through 11 per cent, gelatine in T54 per cent, iodide. A Pasteur- 

 Chamberland candle of size B, which had in the majority of cases been 

 heated for 10 hours at least in a muffle furnace to remove organic matter, 

 was fixed into a brass socket with " Faraday " cement and fitted con- 

 centrically into the internally tinned gunmetal jacket or " filter case." 

 After thoroughly washing through with about 400 c.c. of hot water and 

 250 c.c. of the solution of French gelatine to be used in forming the filter, 

 at a temperature of 37° C, the gelatine at 30° C. was slowly passed through 

 the cooling filter at an air pressure just sufficient to cause about 1*5 c.c. or 

 15 drops to filter per minute. The filtration was continued until no trace 

 of air bubbles was visible in the drops, after which a further 50 c.c. were 

 allowed to pass. During the final filtration the upper surface of the gelatine 

 solution in the filter case was not allowed to descend below a level 3 cm. 

 above the crown of the candle. The filter was then removed, drained, 

 placed in the neck of a flask containing a little water, and kept in the ice 

 chest at 6° to 8° C. until required. If the filter so prepared had an 

 obviously thick skin of gelatine upon its surface, this was removed by 

 immersing in the gelatine solution at 30° C. and draining as before. In the 

 comparative experiments with different concentrations of gelatine, the filters 

 were treated in the manner last described to get membranes of approxi- 

 mately equal thickness. Only those filters were used which allowed less 

 than 2 c.c. or about 20 drops per minute to pass at 100 atmospheres pressure 

 and 10° to 15° C, and the majority of filters allowed only about - 5 c.c. per 

 minute to pass under these conditions. After use the candles could be 

 washed out with water at 50° C, dried, heated in a muffle furnace, and on 

 refilling with gelatine again gave reliable filters. 



Filtration of a Typical Crystalloid, Sodium Chloride, through 9 per cent. 



Gelatine. 



Sodium chloride was chosen on account of its characteristic crystalloidal 

 properties and the important rSle it plays in physiological fluids. Further, 



