DIALYTIC SEPAEATION OF GASES BY COLLOID SEPTA. 411 



to have a power to charge itself gradually from atmospheric air with about half per 

 cent, of its volume of carbonic acid. This carbonic acid, accumulated in thick sheet 

 rubber, appears again to be carried on by the other gases imbibed in a dialytic experi- 

 ment. 



4. Thin Balloons of india-rubber. 



These little balloons were made available for the dialytic passage of air into a vacuum 

 by filling them with sifted sawdust through a funnel, an operation which requires 

 some address. The balloon collapsed upon the sawdust, which formed an interior ball, 

 the sides of rubber still retaining a thickness of about one-fiftieth of a millimetre. The 

 rubber is not vulcanized. Such a ball, of which the original rubber weighed 0-76 grm., 

 still remained 95 millims. in diameter after the air was exhausted. It was found, when 

 exhausted, to admit 19'6 cub. centims. of dialyzed air in forty-one minutes; barom. 579 

 millims., and therm. 19° C. The same air possessed 41'32 per cent, of oxygen. The ball 

 had a surface of 0-0283 square metre, and it dialyzed 0'48 cub. centim. of air in one 

 minute. For a square metre of surface this is a passage of 16 '9 cub. centims. per 

 minute. The passage therefore is about fifty times as fast as through a sheet of rubber 

 of 1 millim. in thickness, while the high proportion of oxygen is sensibly the same. 

 Such a ball was found to dialyze air in the same manner for more than a month, if pro- 

 tected from mechanical injury. 



Three such balls, each containing twenty-three ounces of sifted sawdust, were made 

 to act together, by connecting them with three dependent branches from the same 

 horizontal glass tube. The horizontal tube was connected at one end with an ordinary 

 air-pump which produced a good vacuum by thirty or forty strokes of the piston. The 

 other end of the horizontal glass tube was attached to a good Sprengel apparatus of the 

 largest admissible size, constructed by Messrs. Elliot of the Strand. It was found, how- 

 ever, that the dialyzed air entered rather more rapidly than it could be extracted by 

 a single Sprengel apparatus. This was at the rate of 5 cub. centims. in one minute; 

 therm, about 20° C. The dialyzed air contained 40*5 per cent, of oxygen. 



The greatest amount of aerial dialysis per square metre was obtained by means of a 

 rubber bag, larger than usual, and weighing 1'55 grm. When filled with the sawdust 

 and exhausted, this bag still remained of 143 millims. in diameter, and with a surface 

 therefore of 0-0642 square metre. The air which passed through amounted to 1705 

 cubic centims. in ten minutes; therm, about 20°. This air gave 40'7 per cent, of 

 oxygen. For a square metre of surface, this is the passage of 26*5 cub. centims. per 

 minute, the highest which has as yet been observed. 



In the thin transparent envelope of the little balloon of rubber we have a colloid sub- 

 stance in the most favourable form yet applied to the dialysis of mixed gases. But 

 there is still much room for improvement in the mode of using the thin septum in question. 

 The balls are apt to contract considerably, owing to their elasticity, in the operation 

 referred to, of filling them with sawdust ; their walls become at the same time thicker and 

 less quickly pervious. A mode of destroying the elasticity of the membrane when in its 



