Static .Diffusion of Gases and Liquids, &c., in Plants. 127 



aperture than from a point nearer its centre, a fact completely in 

 harmony with the deduction of Stefan regarding the evaporation of 

 liquids under analogous conditions. 



The various cases which present themselves in practice with regard 

 to the rate of diffusion through single apertures in a diaphragm are 

 then discussed from the above point of view, and simple formulae for 

 the determination of this rate for single and double systems of density 

 shells are established : (1) for cases where the thickness of the dia- 

 phragm is negligible, and (2) for other cases where the apertures 

 become more or less tubular. In a subsequent section of the paper it 

 is shown how closely the observed facts conform to these deductions, 

 and that in static diffusion through apertures in a septum we have a 

 new and accurate method for the determination of the diffusivity 

 constants of atmospheric COa, of the vapours of liquids, and of sub- 

 stances in a state of solution. 



Since the velocity of the diffusive flow through unit area of an 

 aperture in a diaphragm varies inversely with the diameter, it might 

 reasonably be expected that a diaphragm could be so perforated with a 

 series of very small holes arranged at suitable distances from each other,, 

 as to exercise little or no sensible obstruction when it was interposed in 

 a line of diffusive flow, although the aggregate area of the small holes 

 might represent only a small fraction of the total area of the septum. 

 Multiperforate diaphragms of this kind were found to possess all the: 

 remarkable properties which had been anticipated. 



The material used for the septa was very thin celluloid, which was. 

 perforated at regular intervals with holes of about 0*38 mm. in diameter. 

 Details of a number of experiments with such diaphragms are given,. 

 in which it is shown that they may be so arranged as to produce but 

 little obstructive influence on the diffusive flow of a gas when the 

 total area of the apertures amounts only to about 10 per cent, of the 

 area of the septum, and that nearly 40 per cent, of the full diffusive 

 flow may be maintained when the number of the apertures is so far 

 reduced as to represent an area of only 1'25 per cent, of the full area, 

 of the septum. 



The explanation is to be found in the local intensification of the 

 gradient of density in the immediate neighbourhood of the diaphragm,, 

 and which does not extend to the column away from the apertures.. 

 This disturbance of gradient is brought about by the rapid convergence 

 of the lines of flux, and their divergence on the other side, with the. 

 consequent formation of a system of " density shells " over each 

 aperture. A system of perforations of this kind may be compared with 

 a system of conductors electrified to a common potential, the density of 

 the diffusing substance above the apertures corresponding to electric 

 potential, and the non-absorbing portions of the diaphragm to a surface 

 formed by lines of electric force. Just as the electric capacity of a. 



