62 



Carbon Assimilation. 



Table IX. 



Diameter of each hole 0-380 mm. 



Length of tube 1-0 cm. 



It will be observed from these numbers that the obstruction 

 offered to the diffusion of gases by a multiperforate septum is 

 considerably less than the actual obstruction of area. Thus when 

 the area of perforation was less than 3% of the whole area of the 

 septum, the actual diffusion through the perforations was 51-7% of 

 the diffusion taking place through an open tube of the same area of 

 cross section. That is, the diffusion through the septum is nearly 

 15 times as great as it would be if it were simply proportional to 

 the area of the cross section. As the distance between the holes is 

 increased, the efficiency of the area of the perforations increases 

 until the holes are about 10 diameters apart. In this case and in 

 cases where the distance apart of the holes is increased, the diffusion 

 through the perforations is about 40 times as much as it would be 

 if it were proportional to the area of cross section of the tube. 



The accompanying figure (Pig. 4) illustrates what Brown and 

 Escombe imagine to be the lines of equal density and the lines of flow 

 of gas through such a multiperforate septum. The lines of flow of gas 

 diffusing towards the septum will be approximately parallel at some 

 distance from the septum, but as they pass through the perforations 

 they converge, the velocity of flow increasing at the same time 

 owing to the production of ellipsoidal density shells round the 

 opening. After passing through the opening, the lines of flow 

 diverge and as lines of flow from adjacent perforations cannot 

 cross each other (for otherwise there would be shells of different 

 density crossing each other, which is impossible) they must bend 

 round and become once more parallel, the velocity of flow at the 

 same time diminishing. From a consideration of Fig. 4, it is easy 

 to understand how it is that the perforations under favourable 

 conditions of distribution are so efficient for diffusion through them.' 



Brown and Escombe's results lead them to the general 



' Brown and Escombe's work deals only with the simpler cases of diffusion 

 through perforate septa. A fuller treatment of the subject in regard to the 

 diffusion of water vapour has more recently been the subject of investigation 

 by Renner (1910). 



