Dialytic Separation of Gases by Colloid Septa. 411 



Oxygen 21 x 2-556 = 53-676 . . 40-46 

 Nitrogen 79x1 =79 . . 59-54 



100-00 



Hence air dialyzed by the rubber septum should consist of 

 40-46 oxygen and 59*54 nitrogen in 100 volumes. Now air from 

 the atmosphere was found to enter the vacuum of the 48-inch 

 diffusion! eter-tube, through a disk of rubber 22 millims. in dia- 

 meter, to the amount of 3*48 cub. centims. in twenty-one hours, 

 under the pressure of the atmosphere; therm. 23° to 24° C. 

 Of the 3'48 cub. centims. of gas so collected, 2 cub. centims. 

 were absorbed by pyrogallic acid and potash, representing 42-53 

 per cent, of oxygen in the dialyzed air. Here the gas was trans- 

 ferred from the diffusiometer for examination by depressing the 

 diffusiometer in mercury, and using a very narrow tube of rub- 

 ber as a gas-siphon communicating between the gas in the dif- 

 fusiometer and a jar inverted in the mercurial trough. The 

 elastic tube is first filled with mercury, and, being of considerable 

 length, a portion of it is drawn repeatedly through the fingers 

 so as to throw the mercury and aspirated gas into the collecting 

 receiver. The transference of gases in such circumstances may 

 also be effected with much advantage by means of the vacuum- 

 tube invented by Dr. Hermann Sprengel, as will immediately be 

 shown. 



The process of dialytic separation by means of a rubber sep- 

 tum may be varied in three points, — (1) in the condition of the 

 rubber septum, which may be a film of rubber formed from 

 caoutchouc varnish, as well as from distended sheet rubber; 

 (2) in the nature of the support given to the septum, which may 

 be a backing of cotton cloth or of silk (common waterproof 

 cloth prepared by means of caoutchouc varnish, in short), as well 

 as a plate of stucco, earthenware, or wood; and (3) in the means 

 had recourse to for sustaining a vacuum, or at least a consider- 

 able degree of exhaustion, on one side of the dialytic septum, 

 while atmospheric air, or any other gaseous mixture to be dia- 

 lyzed, has access to the other side of the same septum. Or the 

 air to be dialyzed may be compressed on one side of the septum, 

 and left of the usual tension on the other side — inequality of 

 tension on the two sides of the septum being all that is required 

 to induce penetration. 



The pneumatic instrument of Dr. Sprengel (fig. 1) is pecu- 

 liarly applicable to researches of the present kind. Indeed 

 without the use of his invention some parts of the inquiry would 

 have been practically impossible*. The instrument was origi- 



* • f Researches on the Vacuum," by Hermann Sprengel, Ph.D., Che- 

 mical Society's Journal, ser. 2, vol. iii. p. 9 (January 1865). 



