1901.] on Some Recent Work on Diffusion. 559 



thing connected with these curious facts of diffusion, it must be borno 



in mind that the reasoning on which it is based is in part deductive 



and in part dependent on an analogy. 



-—"Nearly 300 years ago it was said by Sir Thomas Roe that " many 



things hold well in discourse, and in the theorique, satisfie curious 



imaginations, but in practice and execution are found difficult and 



ayrie." 



Fortunately this does not apply to the present case, and I am able 

 to bring before you this evening for the first time an experimental 

 demonstration of the existence of zones of equal density in the 

 neighbourhood of an aperture through which diffusion is going on, and 

 to show you that they have the exact shape which the theory requires. 



I have here a rectangular glass cell divided horizontally by a 

 thin plate of celluloid having a circular hole punched through it. 

 The lower half of the cell is rilled with a solution of gelatine con- 

 taining a little barium chloride, and the upper half with a solution of 

 sodium sulphate. 



The relative strengths of the solutions are so adjusted that the 

 two salts, diffusing in opposite directions through the aperture, shall 

 meet somewhere in the gelatine where a precipitate of barium 

 sulphate is thrown down at the surfaces of contact of the two 

 opposing streams of diffusion. The result is that we get a slowly 

 growing spheroidal mass of precipitate, starting from the aperture, 

 and resembling in shape the head of an inverted mushroom. 



If we arrange for the diffusion of the sodium sulphate to be inter- 

 mittent, or better still if we alternate the diffusion of a sulphate with 

 that of a chromate, we get well marked zonings in the precipitate 

 forming the spheroid, zonings which correspond to the successive 

 forms which the spheroid has assumed during growth, and which 

 therefore must have been zones of equal density of the diffusing 

 substances. We can study the forms which these assume in relation 

 to the aperture by subsequently cutting sections through the gelatine, 

 but by a little arrangement we can make the apparatus cut its own 

 sections as the diffusion goes on. 



This is done by making the aperture in the diaphragm semi- 

 circular instead of circular, and bringing its straight edge close up 

 to the side of the glass vessel. 



I will now throw on the screen some photographs of vertical 

 sections of spheroids of diffusion of this kind. (See Figs. 3 and 4.) 



On comparing the lines of equal density around the aperture 

 with the diagrams on the wall, you will at once see that their shape 

 is exactly that required by theory — they describe a series of ellipses 

 having their common foci in the edges of the aperture through which 

 the diffusion is taking place. 



The actual stream lines of the diffusing subtance are not visible 

 but as these must necessarily be normal to the curves of equal 

 density, they can only be represented by a series of hyperbolas, also 

 having their foci in the edges ot the aperture. 



