416 Mr. T. Graham on the Absorption and 



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 protected from mecha- 

 nical 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 admis- 

 sible size, constructed by Messrs. Elliot of the Strand. It was 

 found, however, 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 weigh- 

 ing 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 1 7*05 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 substance in the most favourable form yet ap- 

 plied 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 most attenuated condition, so that the 

 balloon might be cut open and the membrane spread out without 

 shrinking, would be very useful. Instead of depending upon the 

 interior support of sawdust, the membrane could then be stretched 

 over a more convenient frame to support it, of thin porous deal, 

 of unglazed earthenware, and even of a felted fabric, or several 

 thicknesses of unsized paper supported by a slight frame, so as 

 to form a hollow cavity that admitted of being exhausted of air. 

 The attention of manufacturers of rubber might be advantage- 

 ously directed to the preparation and proper support of the thin- 

 nest possible septa of that material. 



The varnish of rubber which appeared to offer the best septum 

 on drying, was a thin solution of rubber in 200 times its weight 



