1863.] 615 



elongated, when the length exceeds the diameter at least 4000 times. 

 These new proportions of efflux are the rates of the " Capillary 

 Transpiration of Gases"*. The rates were found to be the same in 

 a capillary tube composed of copper as they are in a tube of glass, 

 and appear to be independent of the material of the capillary. A 

 film of gas no doubt adheres to the inner surface of the tube, and the 

 friction is really that of gas upon gas, and is consequently unaffected 

 by the nature of the tube-substance. The rates of transpiration are 

 not governed by specific gravity, and are indeed singularly unlike 

 the rates of effusion. 



The transpiration-velocity of oxygen being 1, that of chlorine is 

 1*5, that of hydrogen 2*26, of ether vapour at low temperatures the 

 same or nearly the same number as hydrogen, of nitrogen and 

 carbonic oxide half the velocity of hydrogen, of olefiant gas, am- 

 monia, and cyanogen 2 (double or nearly double that of oxygen), of 

 carbonic acid 1*376, and of the gas of marshes 1*815. In the same 

 gas the transpirability of equal volumes increases with density, 

 whether occasioned by cold or pressure. The transpiration-ratios of 

 gases appear to be in constant relation with no other known property 

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

 isolated from all else at present known of gases. 



There is one property 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, pro- 

 portional 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 constant. The resistance to the passage 

 of a liquid through a capillary was observed by Poiseuille to be 

 nearly as the fourth power of the diameter 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 therefore have a mass of capil- 

 laries of which the passages form a large aggregate, but which are 

 individually too small to permit a sensible flow of gas under pressure. 

 A porous solid mass may possess the same reduced penetrability as 

 * Phil. Trans- 1846, p. 591, and 1849, p. 349. 



2x2 



