170 ANNUAL OP SCIENTIFIC DISCOVERT. 



sity, whether occasioned by cold or pressure. The transpiration ratios 

 of gases appear to be in constant relation with no other known prop- 

 erty of the same gases, and they form a class of phenomena remarkably 

 isolated from all else at present known of gases. There is one prop- 

 erty of transpiration immediately bearing upon the penetration of the 

 graphite plate by gases. The capillary offers to the passage of gas a 

 resistance analogous to that of friction, proportional to the surface, and 

 consequently increasing as the tube or tubes are multiplied in number 

 and diminished in diameter, with the area of discharge preserved con- 

 stant. The resistance to the passage of a liquid through a capillary 

 was observed by Poiseuille to be nearly as the fourth power of the di- 

 ameter 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 be- 

 yond any assignable limit, so the flow may be retarded indefinitely, 

 and caused at last to become too small to be sensible. We may, there- 

 fore, have a mass of capillaries of which the passages form a large ag- 

 gregate, but which are individually too small to permit a sensible flow 

 of gas under pressure. A porous, solid mass may possess the same re- 

 duced penetrability as the congeries of capillary tubes. Indeed, the 

 state of porosity described appears to be more or less closely ap- 

 proached by all loosely aggregated mineral masses, such as lime-plas- 

 ter, stucco, chalk, baked clay, noncrystalline earthy powders, like hy- 

 drate 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 impenetrable to gas by either of 

 the two modes of passage previously described, is readily penetrated 

 by the agency of the molncular or diffusive movement of gases. This 

 appears on comparing the time required for the passage of equal vol- 

 umes 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 for- 

 merly observed to be as follows : 



Time of capillary 

 traDspi ration. 



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. (oxygenl). 



Oxygen 1 . 1 



Hydrogen G.2472 . 0.2502 



Carbonic acid 1.18SG . 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. This penetration of the graphite plate by gases ap- 

 pears to be entirely due to their own proper molecular motion, quite 

 unaided by transpiration. It seems to offer the simplest possible ex- 

 hibition of the molecular or diffusive movement. This pure result is 

 to be ascribed to the wonderfully fine porosity of the graphite. The 



