DIALTTIC SEPAEATION OF GASES BY COLLOID SEPTA. 417 



within the platinum tube, the gas delivered by the Sprengel pump, in the cold, during 

 a period of forty minutes, amounted to no more than a bubble of the size of a pin-head, 

 showing the tightness of the apparatus. The Sprengel pump being constantly kept in 

 action, the tubes were now heated to redness, and then gradually to a temperature 

 approaching a white heat. The gas delivered each five minutes was found to be 13, 

 15"5, 17"4, 16*9, 18'6 cub. centims. as the temperature rose. These volumes are referred 

 to a temperature of 20° and barometer of 760 millims. The last observation gives a 

 passage of 3 "72 cub. centims. of hydrogen per minute. The platinum tube employed 

 here was joined without solder, having been drawn from a mass of platinum which had 

 been aggregated by fusion. It was similar in this respect to the tube employed by M. 

 Deville. The tube was 0-812 metre in length (32 inches) and 1-1 millim. in thickness, 

 with an internal diameter of 12 millims. But only a portion of about 200 millims. (8 

 inches) of the tube were heated to redness in the furnace experiment. The inner surface 

 of the heated portion has therefore an area of 0076 square metre. Hence one square 

 metre of heated platinum delivers 489'2 cub. centims. of hydrogen per minute. This 

 result admits of comparison with the passage of gases through a septum of rubber. In 

 the most favourable circumstances, when the thin membrane of a rubber balloon was 

 employed, the passage of air into a vacuum was at the rate of 2 6 "5 cub. centims. per 

 square metre in one minute. The passage of hydrogen may be taken as 4"8 times as 

 rapid as that of atmospheric air, or at 127*2 cub. centims. per minute. But while 

 the thickness of the platinum septum was 1*1 millim., that of the rubber film was only 

 one-seventieth part of a millimetre. Hence we have the ultimate comparison : — 



Passage of hydrogen gas in one minute through a septum of 1 square metre : 



Through rubber 0*014 millim. in thickness, 127*2 cub. centims. at 20° C. ; 



Through platinum 1*1 millim. in thickness, 489*2 cub. centims. at bright red heat. 



If the permeation of hydrogen is due to the same agency in both septa, can the vast 

 superiority of the platinum septum be connected with its greatly higher temperature ^ 



It was interesting now to turn from hydrogen to the passage of other gases through 

 heated platinum. The experiments were all made in the same way, and at a full red 

 heat. The temperature, it will be observed, was short of that at which the elements of 

 water and carbonic acid are partially dissociated. 



Oxygen and nitrogen. — Atmospheric air, which may be taken to represent both of 

 these gases, was now allowed to flow through the annular space between the tubes, the 

 interior platinum tube being kept vacuous as usual. In one hour the gas collected by 

 the constant action of a Sprengel pump amounted only to 0*3 cub. centim. Hydrogen 

 in the same time would have given 211 cub. centims. It is very doubtful, too, whether 

 the trifling fraction of a centimetre of gas collected had all passed through the platinum ; 

 a part (or the whole of it) may have entered by the joints of the apparatus. Platinum, 

 then, cannot be said to be sensibly permeable to either oxygen or nitrogen, even at a full 

 red heat. 



Carbonic acid. — This gas was supplied from a bottle containing marble, by the action 



3m 2 



