660 Mr. Horace T. Brown [March 22, 



The electrostatic analogy which has served so well in determin- 

 ing the form of the zones of equal density around single apertures 

 may also be used for predicting their distribution around a series of 

 apertures in a diaphragm. 



If we regard the individual holes in a multiperforate diaphragm 

 as so many minute discs, all electrified to a common potential, the 

 lines of equi-potential and the lines of force should take a form 

 something like that represented in the diagram (see Fig. 5), the 

 lines of equi-potential forming complete ellipses in the immediate 

 neighbourhood of the electrified discs, but gradually intersecting and 

 forming a series of wavy lines which become more and more hori- 

 zontal as the distance gets more remote. 



Could they be rendered visible, these are also the forms which 

 we should expect the lines of equal density of a substance to take 

 when it is diffusing through a series of small apertures. I am able 

 to give you a verification of this, by throwing on the screen a photo- 

 graph showing the result of intermittent diffusion through a series 

 of such apertures. (See Figs. 6 and 7.) The lines of equal density 

 are marked out by the alternate bands of sulphate and chromate 

 of barium, as they were in the last experiment. 



From the shape of these lines of equal density it is possible to 

 determine the form of the stream lines of the diffusing substance, and 

 to show that the tendency of a multiperforate septum of this kind, is 

 to locally increase the gradient of density in its neighbourhood and 

 so to accelerate the flow through the small apertures. We get, in 

 fact, a complete and satisfactory explanation of the small amount of 

 obstruction which such a diaphragm produces, when put in the way of 

 a diffusive flow of gas or liquid. 



Intermittent diffusion, such as I have described, may be used to 

 illustrate in a variety of ways the distribution of electrical potential 

 around electrified bodies which are within the sphere of each other's 

 action. 



It is generally a difficult and laborious task to work out the 

 distribution of the surfaces of equi-potential around electrified bodies 

 which are near enough to influence each other. By this system of 

 intermittent diffusion we may sometimes make nature work out the 

 problem for us. Here, for instance (see Fig. 8), is a figure copied 

 from Clerk Maxwell's ' Electricity and Magnetism,' representing 

 the form which is assumed by equi-potential surfaces around two 

 points, charged with quantities of electricity of the same kind in the 

 ratio of four to one. If the analogy is correct, diffusion through 

 apertures having their diameters in the ratio of two to one, ought to 

 give the same series of figures. Tou see from the photograph of an 

 actual experiment given in Fig. 9 that this supposition is correct. 



In Fig. 10 are given the calculated lines of force at the edges of 

 two parallel plates, one of which is insulated and electrified, the 

 other connected with the earth. These ought to correspond in shape 

 to the cqui-density lines of a substance undergoing steady diffusion 



