6o 



Carbon Assimilation. 



such shells of equal density is essentially the same. It is : — 



Q = 2kpD, 

 where Q is the quantity absorbed in any time, 



k the coefficient of diffusion of carbon dioxide in air, 

 p the density of atmospheric carbon dioxide, 

 D the diameter of the disc. 

 Brown and Escombe explain their results in regard to the rate 

 of diffusion through perforate septa as due to the same cause, 

 namely, that when a gas is diffusing through such a perforate 

 septum, shells of equal density are formed outside the perforation 

 just as in the case of the absorbent disc, and the same ' diameter 

 law ' will hold. 



The accompanying diagrams show the various systems of 

 shells. Fig. 1 is the case of the shells over a perfectly absorbent 

 disc. The density of the diffusing gas varies from p at a remote 

 distance from the surface to zero at the surface itself. In Fig. 2 

 are represented the shells produced on the inner side of a perforated 

 diaphragm opening into a large space in which the gas is rapidly 

 absorbed and where the density of the gas at the perforation is 

 kept at a maximum by a constant current of air. The density of 

 the gas here varies from p at the diaphragm to zero at the surface. 

 In Fig. 3 is represented the case of a perforated septum like the 



Fig. 4. 



Ff«.3, 



Figs. 1—4. 



Figs. 1 and 2. Diffusion " shells " formed outside and Inside a perforation 

 in a septum. 



Fig. 3. Diffusion " shells " outside and inside a perforation in a septum 

 in perfectly still air. 



Fig. 4. Lines of flow through a multiperforate septum. 



