DIFFUSION OF GASES 225 



Work of Brown and Escombe. — Brown and Escombe (1899- 

 1900) then interested themselves in the question of the absorp- 

 tion of carbon dioxide through the stomata, and studied especially 

 the question of rate. They worked on the Catalpa leaf, which 

 has stomata only on the under side, and found that, under favor- 

 able conditions, one square meter of leaf surface absorbed 700 c. c. 

 of carbon dioxide in an hour. They then computed the rate of 

 absorption of each molecule of the gas and learned that the ab- 

 sorption in the leaf was only half that exhibited by a free surface 

 of sodium hydroxide (NaOH) of the same area. But since the 

 carbon dioxide goes in only through the stomata, one must com- 

 pare not the entire leaf surface with the free surface but only the 

 stomatal area. This is about one-hundredth the area of the leaf, 

 so that the absorption through the stomatal areas is then not half 

 that of a free surface but fifty times as great ! 



This led to the following experiment: Solutions of NaOH were 

 placed in test tubes covered with thin perforated plates. Differ- 

 ent plates had openings of different diameters, and the rate of 

 diffusion was compared with the areas of the openings and their 

 diameters. The following table gives some of the results : 



Diameter of opening Ratio of opening Ratio of Ratio of amounts 



in mm. areas diameters of C0 2 entering 



22.70 1.000 1.000 1.00 



6.03 0.070 0.260 0.26 



3.23 0.023 0.140 0.16 



2.12 0.008 0.093 0.10 



This shows that the rate of diffusion through the small pores into 

 the NaOH is a function not of the area of the openings but of their 

 diameter. Thus the largest opening has an area more than a 

 hundred times that of the smallest and a diameter only slightly 

 more than ten times as great. The gas diffuses through the smaller 

 opening, however, not one-hundredth as rapidly but one-tenth. 

 It follows, therefore, that if a vessel of sodium hydroxide were 

 covered with a perforated plate with very small openings properly 

 spaced, the amount of carbon dioxide passing in would be as great 

 as though the surface were entirely free and there were no plate 

 present at all, even though the total area of the openings is con- 

 siderably less than the total area of the surface. 



Further experimentation showed that diffusion was most rapid 

 when the distances between the openings were ten times their 



