DIALTTIC SEPAEATION OF GASES BT COLLOID SEPTA. 



401 



column in the tube. In order to compare the penetration of different gases, a hood of 

 thick vulcanized rubber, provided with a small entrance- and exit-tube for gas (such as 

 is often used in gas experiments), is placed over the upper end of the diffusiometer 

 described, and cemented to it by means of fused gutta percha. The gas to be operated 

 upon can thus be conveyed from the apparatus in which it is generated, or from a gaso- 

 meter in which the gas is stored, into the hood or upper chamber of the diffusiometer, 

 and the excess of gas supplied be allowed to escape into the atmosphere by the exit- 

 tube of the hood. The stucco plate used as a support to the film of rubber is so highly 

 porous as not to add sensibly to the resistance experienced by the gases in passing 

 through the rubber, and, having no absorbent power of its own, may be left entirely out 

 of consideration. 



A comparison was made of the passage through the rubber film, on the same day, of 

 carbonic acid, hydrogen, oxygen, and nitrogen; barom. 773 millims., therm. 23° to 23°-5 C. 

 The time during which the mercurial column fell in the diffusiometer from 748 to 723 

 mUlims. was noted in seconds, and also from 723 to 698 millims. The gases were all care- 

 fully dried. 



Table I. — Passage of Carbonic Acid in seconds. 



The passage of carbonic acid thus exhibited will be found to be considerably more 

 rapid than those of hydrogen and the two other gases which follow : — 



Table II. 



A single experiment, made at the same time on the passage of atmospheric air, gave 

 times of 1318" and 1524" for the two stages, or 2842" for the whole fall. The time of 

 penetration of air is therefore intermediate between that of oxygen and nitrogen entering 

 singly. 



Although such numbers do not possess the close uniformity which appears in diffu^ion- 



3£2 



