Static Diffusion of Gases and Liquids, &c., in Plants. 125 



air column which may be quantitatively investigated by the same 

 simple mathematical treatment as the " flow " of heat in a bar when 

 the permanent state has been reached, or the " flow " of electricity 

 between any two regions of a conductor maintained at a constant 

 difference of potential. 



By a long series of experiments of this nature it was found that the 

 diffusivity constant, k, for very dilute CO* does not materially depart 

 from the value assigned to it by Loschmidt and others, when experi- 

 menting with much higher ratios of mixture, and that the difference is 

 certainly not of sufficient magnitude to be taken into serious account 

 in the study of the natural processes of gaseous exchange in the 

 assimilating organs of plants. 



In the static diffusion of a gas, vapour, or solute, as the case may 

 be, the amount of substance diffusing in a given time, all other condi- 

 tions being the same, is directly proportional to the sectional area of 

 the column. It is found, however, that if the flow is partially 

 obstructed by interposing at any point in the line of flow a thin 

 septum pierced with a circular aperture, the rate of flow across unit 

 area of the aperture is greater than it would be across an equal area of 

 the unobstructed cross-section of the column at this point. If the 

 margin around the aperture has a width of at least three or four times 

 its diameter, the rate of flow is now found to be directly proportional 

 to the linear dimensions of the aperture and not to its area, so that the 

 velocity of flow through unit area varies inversely as the diameter. 



A large number of experiments on the diffusion of carbon dioxide, 

 water-vapour, and sodium chloride in solution, are given in support of 

 this proposition. All these show that the rate of diffusion across such 

 a septum, all other conditions being the same, is directly proportional 

 to the diameter of the aperture, and not, as might have been expected, 

 to its area. 



Exactly the same result is obtained when small circular discs of an 

 absorbent, such as a solution of caustic alkali, are surrounded by a 

 wide rim and exposed to perfectly still air, the amount of carbon 

 dioxide absorbed under these conditions being proportional to the 

 diameters of the discs. 



If, however, there are any sensible air currents the absorption 

 becomes proportional to the areas. 



These two sets of phenomena may be explained as follows : 



In the case of the absorbing disc in perfectly still air, the con- 

 vergent streams of carbon dioxide creep through the air towards the 

 absorbing disc, establishing a steady gradient of density, and this 

 creep will be a flux perpendicular to the lines of equal density, which 

 form curved surfaces or " shells " surrounding the disc and terminating 

 in the rim. The state of things is exactly analogous to the electric 

 field in the neighbourhood of a conductor of the same shape and 



