Dialytic Separation of Gases by Colloid Septa. 521 



and external surface 0*0153 of a square metre. It was closed 

 by thick plates of platinum soldered at both ends, one of the 

 plates being perforated by a long small tube of platinum, by 

 which the cavity of the palladium tube could be exhausted of air. 



Now the closed palladium tube remained air-tight, when ex- 

 hausted by the Sprengel tube at the ordinary temperature, at 

 260°, and at a temperature verging on low redness, the gas 

 without being atmospheric air. Hydrogen being then substi- 

 tuted as the external gas, the walls of the palladium tube still 

 remained impermeable at a low temperature. No hydrogen 

 gained the interior in three hours at 100°. But the temperature 

 being gradually raised by means of an oil-bath to 240°, hydrogen 

 then began to come through, and at a gradually increasing rate 

 to 265°. The hydrogen then entered steadily at the rate of 8- 67 

 cub. centims. in five minutes. This gives a rate of 327 cub. 

 centims. for a square metre of surface per minute. Heated to 

 a temperature just short of redness, the passage of hydrogen was 

 increased to 11*2 cub. centims. in five minutes, or 423 cub. cen- 

 tims. for a square metre per minute. 



With coal-gas as the external atmosphere the penetration of 

 the palladium began about the same temperature, and was con- 

 tinued at 270° at the rate of 57 cub. centims. for a square metre 

 of surface per minute. The penetrating gas had no odour of 

 coal-gas, contained no trace of carbon, and appeared to be abso- 

 lutely pure hydrogen. The exact isolation of the latter gas by 

 septa of both platinum and palladium appears most extraordinary. 



A quantitative determination of the hydrogen in a gaseous 

 mixture could probably be effected by means of the hollow cy- 

 linder of palladium. 



Is the power to penetrate the metals in question confined to 

 hydrogen? It has been lately concluded by Dr. C. Wetherill 

 that the turgescence of the ammonium amalgam depends entirely 

 upon the retention of hydrogen gas -bubbles * ; hydrogen, then, 

 appears to exhibit an attraction of a peculiar kind for mercury. 

 The ready liquefaction of the same gas by the platinum metals 

 evinces also a powerful mutual attraction. The only other vola- 

 tile body which has been observed to pass, like hydrogen, through 

 a plate of palladium is common ether — and that at the atmo- 

 spheric temperature, while a passage was denied to hydrogen at 

 the same time. The palladium was in the form of foil. Although 

 thin foil of this metal is generally visibly porous and allows air 

 to pass through like a sieve, a tube diffusiometer, covered with 

 a disk of the selected palladium-foil, and standing over mercury, 

 retained a volume of 40*5 millims. of air over a vertical column 

 of 155 millims. of mercury for twenty-fours without depression 

 * American Journal of Science, vol. xlii. No, 124. 



