Carbon Assimilation. 
127 
such shells of equal density is essentially the same. It is:— 
Q = 2k P D, 
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. 
Fi S-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. 
