616 [June 18, 



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 artifi- 

 cial graphite. 



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

 cally impenetrable 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 com- 

 paring the time required for the passage of equal volumes of different 

 gases under a constant pressure. Of the following 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 follows : 



Time of capillary 

 transpiration. 



Oxygen 1 



Carbonic acid 0*72 



Hydrogen 0'44 



Through a plate of graphite, of half a millimetre in thickness, 

 the same gases were now observed to pass, under a constant pressure 

 of a column of mercury of 100 millimetres in height, in times which 

 are as follows : 



Time of molecular Square root of density 



passage. (oxygen 1). 



Oxygen 1 .... 1 



Hydrogen 0-2472 .... 0-2502 



Carbonic acid 1-1886 1-1760 



It appears then that the times of passage through the graphite 

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

 same gases first quoted above. The new times in question, however, 

 show a close relation to the square roots of the densities of the re- 

 spective gases, as is seen in the last Table ; and so far they agree 

 with theoretical times of diffusion usually ascribed to the same gases. 



The experiments were varied by causing the gases to pass into a 

 Torricellian vacuum, and consequently under the full pressure of the 



