Notes . 
539 
These two sets of phenomena may be explained as follows :— 
In the case of the absorbing disk in perfectly still air, the con¬ 
vergent streams of carbon dioxide creep through the air towards the 
absorbing disk, 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 disk and terminating 
in the rim. The state of things is exactly an^k)gous to the electric 
field in the neighbourhood of a conductor of the same shape and 
dimensions as the absorbent disk \ In the case of the gas, the curves 
or ‘ shells ’ of equal density are the analogues of the similarly curved 
surfaces of equipotential above the electrified disk, whilst the con¬ 
verging lines of creep or flux of the gas are the analogues of the 
lines or tubes of force which bend round into the disk as they 
approach it. 
If we consider two such absorbent disks of different diameters, the 
curved surfaces in each system corresponding to a given density will 
be found at actual distances from the disks which are in the same 
proportion to each other as are the diameters of the disks. In other 
words, the gradient of density on which the rate of flow depends will 
be proportional to the diameters of the disks, which is exactly what 
is found experimentally. 
This case of an absorbent disk is the exact converse of one which 
has been theoretically investigated by Stefan, viz. the conditions of 
evaporation of a liquid from a circular surface. He found that the 
lines of flux of the vapour proceeding from the surface of the liquid 
must be hyperbolas, whilst the curved surfaces of equal pressure of 
the vapour must form an orthogonal system of ellipsoids, having their 
foci, like the hyperbolas, in the bounding edges of the disk. This 
was a purely mathematical deduction which has never been verified 
experimentally, but it will be seen that the exactly converse phenomena 
of diffusion are in complete agreement with it. 
In the other case of a diffusive flow through a circular aperture 
in a diaphragm, the lines of flow, which are convergent as they 
approach the aperture, bend round their foci situated in the edges 
of the disk and form a divergent system on the other side. If the 
chamber into which they pass is a perfectly absorbent one, and 
is sufficiently large, there will be formed on the inner side of the 
1 The authors are indebted to Dr. Larmor for this suggestion of the electrostatic 
analogy. 
