540 
Notes. 
diaphragm a system of density shells similar to those outside, but 
with the gradient of density centrifugally instead of centripetally 
arranged. This system of shells is termed negative, and is as 
effective as the outer positive system in regulating the flow according 
to the ‘ diameter law/ so that this law will still hold good even 
if the outer air currents are sufficient to sweep away the external 
positive shells altogether. 
All the known facts of diffusion through circular apertures in 
a diaphragm are in complete accord with the above explanation, 
which is fully elaborated in the original paper. 
By diffusing colouring matter through apertures in a septum, 
under such conditions as to prevent convection currents, the * density 
shells' have been rendered visible, and it has been shown that their 
ellipsoidal form is exactly that which is demanded by the above 
hypothesis. Moreover, this method gives an experimental demonstra¬ 
tion of the more rapid projection of the diffusing particles from the 
edges of the 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: (i) for cases where the thickness 
of the diaphragm 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 determina¬ 
tion of the diffusivity constants of atmospheric C 0 2 , of the vapours 
of liquids, and of substances 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 
