DIALTTIC SEPARATION OF GASES BY COLLOID SEPTA. 409 



1. India-rubber between double cotton cloth vulcanized. 



This was a common elastic carriage-bag 18 inches by 15. The surface of both sides 

 amounted to 0-3482 square metre. The bag was pressed flat by the hands, and still 

 further exhausted by means of Sprengel's tube. After all the contents of the bag were 

 extracted and the collapse complete, the Sprengel tube began again to throw out air in 

 a slow but exceedingly regular manner. A small portion of sawdust, or of sand, intro- 

 duced beforehand into the bag, appeared to be useful in preventing the sides coming 

 together too closely, but was not essential. The air thus extracted from the bag in one 

 hour amounted to 15-65 cub. centims., or sensibly 1 cubic inch; therm. 23° to 24° C. 

 Such dialyzed air, from three successive experiments of one hour each, contained 38, 

 40-3, and 41-2 per cent, of oxygen, the inferior proportion of oxygen in the earlier 

 experiments being no doubt due to a small residue of undialyzed air remaining in the 

 bag before exhaustion. This dialyzed air rekindled glowing wood, so as to illustrate 

 the direct separation of oxygen gas from atmospheric air. For the purposes of combus- 

 tion, it may be viewed as air from which one-half of the inert nitrogen has been with- 

 drawn. 



It will be convenient to express the permeability of the colloid septum with uniform 

 reference to a square metre of area, and to an hour, or to a single minute of time. Here, 

 for a square metre of cloth, the passage of air amounted to 44-95 cub. centims. (3 cubic 

 inches nearly) per hour, or to 0-749 cub. centim. per minute. 



The view which the observation suggests of the nature of such an air-tight fabric is, 

 that it may be truly impenetrable to air when the composition and tension of the air 

 are the same on both sides of the cloth ; but it is penetrable when a vacuum or a 

 reduced state of tension is maintained on one side of the cloth and not on the other. 

 The compression of the air confined in a bag would no doubt have a similar effect, and 

 then the flow would be in an outward direction. But there is no evidence of a porous 

 structure in the vai-nished cloth. The gases of atmospheric air would pass through actual 

 openings according to the law of gaseous diffusion, which favours the nitrogen or lighter 

 gas, while it is the oxygen which is found to pass through the material most readily in 

 these experiments. The imbibition of the liquefied gas by the substance of the rubber, 

 with the subsequent evaporation of this liquid into the vacuum on the other side, is all 

 the explanation required. 



2. Vulcanized india-rubber tubing. 



A stout caoutchouc tube with an external diameter of 13 millims. (half an inch), an 

 internal diameter of 9 millims., thickness of 2 millims., and length of 3-658 metres 

 (4 yards) was exhausted, one end being closed and the other end connected with the 

 Sprengel pump. The gas collected in thirteen hours amounted to 11-25 cub. centims. ; 

 therm. 20° to 23° C. This gas contained 37-8 per cent, of oxygen. The gaseous pene- 

 tration is not great in so thick a tube, and there is reason to fear the influence of 

 gaseous diffusion to a small extent. The admission of air would be equally sensible if 



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