1901.] 



on Some Recent Work on Diffusion. 



555 



I have here a short burette-like tube with a vvido rim of metal 

 round the top. When this tube is completely filled by lettiug in a 

 solution of caustic soda, we have a circular surface of the solution 

 lying in the same plane as the rim. When this has been exposed to 

 the air for a given time, the carbonic acid absorbed by the disc of 

 liquid can be determined by drawing off and titrating. 



If such absorptive discs of different dimensions are exposed to 

 air which is in slight movement, we shall find that the carbonic acid 

 absorbed is proportional to the area of the surface. The smaller, 

 however, we make the discs, and the greater precautions we take to 

 keep the air over them perfectly still, the nearer do the absorptions 

 become proportional to the diameters. (See Table III.) 



Table III. — Absorption of Atmospheric CO ; by Circular Surfaces 

 of Solutions of Caustic Alkali. 



Diameter 



of 

 Surface. 



mm. 

 10-25 



20-25 



29-25 



40-00 



5-00 

 10-25 



Ratio 



of 

 Areas. 



10 



3-9 



8-1 



15-2 



1-0 

 4-2 



Ratio 



of 



Diameters 



1-0 

 1-9 

 2-8 

 39 



1-0 

 2-05 



Mean Ratio 



of Areas 



and 



Diameters. 



1-0 

 2-9 

 54 

 9-5 



Ratio 



ofC0 2 



Absorbed. 



1-0 

 3-0 

 5-3 

 9-2 



1-0 

 2-47 



There is always, however, more difficulty in obtaining these 

 results witli plane absorbing surfaces, than by diffusion through a 

 perforated diaphragm. The reason for this will be apparent later. 



Before entering on an explanation of these facts, I wish you to 

 note a very important conclusion to be drawn from them, and one 

 which readily admits of experimental verification. 



We have seen that when we partially obstruct the diffusive flow 

 of a gas or liquid by a thin septum with a single circular perforation, 

 the velocity of the flow through each unit area of aperture increases 

 as the diameter of the aperture decreases. One might therefore ex- 

 pect that if a number of fine holes were suitably arranged in such a 

 septum, the acceleration of flow through the individual holes might 

 assume such proportions that a perforated septum of this kind would 

 exercise little or no obstruction on the diffusive flow, although in 

 such a case the aggregate area of the holes might only represent a 

 small fraction of the total area of the obstructing septum. 



