Mr. T. Graham on the Molecular* Mobility of Gases. 413 



resistance to the passage of liquid through a capillary was ob- 

 served by Poiseuille to be nearly as the fourth power of the dia- 

 meter of the tube. In gases the resistance also rapidly increases ; 

 but in what ratio, has not been observed. The consequence, 

 however, is certain, that as the diameter of the capillaries may 

 be diminished beyond any assignable limit, so the flow may be 

 retarded indefinitely, and caused at last to become too small to 

 be sensible. We may then have a mass of capillaries of which 

 the passages form a large aggregate, but are individually too 

 small to allow a sensible flow of gas under pressure. A porous 

 solid mass may possess the same reduced permeability as the 

 congeries of capillary tubes. Indeed the state of porosity de- 

 scribed appears to be more or less closely approached by all loosely 

 aggregated mineral masses, such as lime-plaster, stucco, chalk, 

 baked clay, non-crystalline earthy powders like hydrate of lime 

 or magnesia compacted by pressure, and in the highest degree 

 perhaps by artificial graphite. 



3. A plate of artificial graphite, although it appears to be 

 practically impermeable to gas by either of the two modes of 

 passage previously described, is readily penetrated by the agency 

 of the molecular or diffusive movement of gases. This appears 

 on comparing the time required for the passage through the 

 plate of equal volumes of different gases under a constant pres- 

 sure. Of the three gases, oxygen, hydrogen, and carbonic acid, 

 the time required for the passage of an equal volume of each 

 through a capillary glass tube, in similar circumstances as to 

 pressure and temperature, was formerly observed to be as fol- 

 lows : — 



Time of capillary transpiration 

 of equal volumes. 



Oxygen 1 



Hydrogen 0-44 



Carbonic acid .... 0*72 



Now through a plate of graphite, half a millimetre in thick- 

 ness, the same gases were observed to pass, under a constant 

 pressure of a column of mercury of 100 millims. in height, in 

 times which are as follows : — 



Time of molecular Square root of density 

 passage. (oxygen 1). 



Oxygen ... 1 1 



Hydrogen . . 0-2472 02502 



Carbonic acid . 1-1886 1*1760 



It appears that the times of passage through the graphite 

 plate have no relation to the capillary transpiration-times of the 

 same gases as first quoted. The new times in question, how- 



