324 



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



iodide. Ee-establishing the pressure of 100 atmospheres the iodine content 

 rapidly increased, whereas the iodide in the first few drops was abnormally 

 'high (93 - 5), but decreased in the second fraction of 6 c.c. to (86 - 0). Once 

 more removing the pressure for three hours the iodine diminished to (1*0) 

 and the iodide to (34 - 4), confirming the former result. The pressure was now 

 slightly raised, viz., to 10 atmospheres, when the iodine became (25) and the 

 iodide reached the highest value so far (94 - 6). A further rise in pressure to 

 100 atmospheres increased the iodine to (30) and diminished the iodide to 

 '(92 - 5). Variation between 100 and 50 atmospheres did not further affect the 

 iiodine content but did influence the iodide. Finally, a gradual fall in pressure 

 lto 25 atmospheres caused both iodine and iodide to diminish in concentration. 



Eesidual Fluid : The concentration of iodine was only (30), whereas the 

 iodide was (1097). 



Conclusions. — The gelatine absorbs a large amount of iodine, but is easily 

 permeable to the same and the permeability rises and falls, as in the case of 

 meutral red, with rising and falling pressure. The increasing concentration of 

 iodine in the gelatine increases the power of the latter in retaining potassium 

 iodide. On entirely removing the applied pressure the gelatine robs the 

 liquid percolating through almost entirely of its iodine and the iodised 

 •gelatine abstracts about two-thirds of the iodide. 



The filtrates, after a drop in the pressure, show an increased amount of 

 iodide on recompression similar to the effect observed in the filtration of 

 sodium chloride. 



■ On the Influence of the Nature of the Solution and of Varying Pressure on the 



Bate of Filtration. 



A few preliminary measurements of the rates of filtration of various 

 solutions were made to elucidate some of the physical properties governing 

 the action of the gelatine during filtration. 



Method. — As a rough approximation the number of drops per minute 

 falling from the nozzle of the filter case was taken as a measure of the rate 



■ of filtration. The rate was determined after the first 2 c.c. had passed, and 

 again after about 50 c.c. in all had been collected. 



Bates of Filtration. — Table III shows that distilled water, 0'8 per cent, 

 ■sodium chloride and megatherium lysin pass 9 per cent, gelatine in the initial 

 stage more rapidly than in the final under constant pressure. Solutions 

 containing 0-214 per cent, iodine, with 1"54 per cent, potassium iodide, and 

 5 per cent, colloidal ferric hydrate pass 11 per cent, gelatine with similar 

 variations in rate. For all these solutions the decrement in rate is practically 



