126 Dr. H. T. Brown anl Mr. E. Escombe. 



dimensions as the absorbent disc.* 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 disc, 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 disc as they approach it. 



If we consider two such absorbent discs of different diameters, the 

 curved surfaces in each system corresponding to a given density will 

 be found at actual distances from the discs which are in the same pro- 

 portion to each other as are the diameters of the discs. In other 

 words, the gradient of density on which the rate of flow depends will 

 be proportional to the diameters of the discs, which is exactly what is 

 found experimentally. 



This case of an absorbent disc is the exact converse of one which has 

 been theoretically investigated by Stefan, viz., the conditions of evapo- 

 ration 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 disc. 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 disc 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 diaphragm a system of density 

 shells similar to those outside, but with the gradient of density centri- 

 fugally 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 demonstration of the 

 more rapid projection of the diffusing particles from the edges of the 



* The authors are indebted to Dr. Larmor for this suggestion of the electrostatic 

 analogy. 



