558 Mr. Horace T. Brown [March 22, 



Fig. 2 represents what we might expect to be the state of tilings 

 when diffusion takes place through a circular aperture in a diaphragm. 

 Here the stream lines of the substance, which are convergent as they 

 approach the aperture, diverge again when the opening is past, and 

 we should expect to get a double system of the ellipsoidal zones of 

 equal density on either side of the aperture. 



Did time admit I could show you that this hypothesis is not only 

 capable of giving reasonable and consistent explanations of all the 

 phenomena of diffusion into and through apertures, but completely 

 explains the " diameter law " and also enables us to predict the amount 

 of gas, vapour, or solute which will pass under given conditions, and 

 the results can be verified by experiment. 



I have only time to glance at one or two readily verifiable 

 deductions from this hypothesis. In the first place it fully accounts 

 for what I have called the " diameter law," that is to say that dif- 

 fusion through circular apertures in a diaphragm is proportional to 

 their diameters, not to their areas. 



In two diagrams on the wall we have represented the arrange- 

 ment of the equi-density curves and stream lines over two absorbent 

 discs, one double the diameter of the other. We may take these 

 discs to represent an alkaline solution absorbing carbonic acid from 

 the air. 



The two systems are on the same relative scale, one in fact being 

 the image of the other magnified by two diameters. 



It will be seen that a curved line corresponding to any given 

 actual density of the diffusing substance must be twice as far from 

 the surface of the larger disc as it is from the surface of the smaller ; 

 that is to say, the gradient of density on which the flow depends, is 

 twice as steep over the small disc as it is over the large one. From 

 this it follows that for equal areas the flow into the smaller disc is 

 twice that into the larger, and that, the total flow must be proportional 

 to the diameters, which is just what is found to be the case. 



Wherever we get conditions favourable for the formation of a 

 system of equi-density zones on one or both sides of a perforated 

 diaphragm, diffusion will go on in accordance with this " diameter 

 law." But one system of zones is quite sufficient for the purpose, so 

 that in a case like that of Fig. 2, which represents the course of 

 diffusion of atmospheric C0 2 in perfectly still air into an absorbent 

 chamber, we might allow the outer system of equi-density shells over 

 the aperture to be completely swept away by air currents and still 

 the " diameter law " would hold good on account of the inner series 

 of zones, which, from their position, are protected from the air currents. 

 This explains in a very satisfactory manner why it is much more 

 easy to demonstrate the diameter law with apertures in a diaphragm, 

 than simply with absorbing discs, where only one external system of 

 equi-density shells can exist, which is of course extremely liable to be 

 influenced by disturl ing currents. 



Satisfactory, however, as this hypothesis is in explaining every- 



