428 Mr. T. Graham on the Molecular Mobility of Gases. 



portion of nitrogen observed in atmospheric air being 78*99 per 

 cent., the air drawn into the diffusiometer was as follows : — 



Proportion of nitrogen 

 per cent. 



In air entering over column of 330-200 millims.l yq.oA 



mercury J 



In air entering over column 508 millims. . . . 79*32 

 In air entering over column 761-685 millims . . 79*53 

 In air entering over column 761-685 millims . . 79*69 



The separation is sufficiently decided, and is certainly remark- 

 able considering the comparatively loose texture of the stucco 

 plate. The gas entered in the two last experiments in about one 

 minute, which appears too rapid a passage, and not to be attended 

 with increased separation, compared with the immediately pre- 

 ceding experiment, in which the pressure was less and the pas- 

 sage of the gas proportionally slower. In all such highly porous 

 plates, we have always to apprehend the passage of a large pro- 

 portion of the gas in the manner of capillary transpiration, where 

 no separation takes place. 



It may be concluded that all porous masses, however loose 

 their texture, will have some effect in separating mixed gases 

 moving through them under pressure. The air entering a room 

 by percolating through a wail of brick or a coat of plaster will 

 thus become richer in nitrogen, in a certain small measure, than 

 the external atmosphere. 



The Tube Atmolyser. 



In the application of diffusion through a porous septum to 

 separate mixed gases, as a practical analytical method, it is desi- 

 rable that the process should be more rapid than it can be made 

 with the use of graphite and other diffusing-plates of small size, 

 and also that the process should if possible be a continuous one. 

 Both objects are attained in a considerable degree by adapting a 

 tube of porous earthenware to the purpose. Nothing has been 

 found to answer better than the long stalk of a Dutch tobacco- 

 pipe used as the porous tube. A tube of this description, about 

 2 feet long and having an internal diameter of 2*5 millims., is 

 fixed by means of perforated corks within a glass or metallic tube, a 

 few inches less in length and about 1 J inch in diameter (e,i, fig. 8), 

 as in the construction of a Liebig condenser. A second quill 

 tube (v) is inserted in one of the end corks, and affords the means 

 of communication between the annular space and the vacuum 

 of an air-pump. The external surface of the corks, and of those 

 portions of the pipe- stalk which project beyond the enclosing 



