556 



Mr. Horace T. Brown 



[March 22, 



Strange and paradoxical as such a conclusion may at first sight 

 appear, it will bear the test of experiment. 



I have here a thin film of celluloid — in fact, a piece of the 

 ordinary Kodak roller-film. This has been perforated with holes 

 about '4c millimetre in diameter, arranged at a little more than 2*5 

 diameters apart, so that there are just 100 of such perforations on a 

 square centimetre of area. The clear holes represent about one- 

 tenth of the area of the film, nine-tenths of the sieve being blocked 

 up with impervious celluloid. 



Here are two columns of gelatine, down which a blue solution of 

 copper-ammonium sulphate have beeu diffusing for equal times. One 

 of these columns is unobstructed in any way, being in direct contact 

 with the coloured liquid. In the other case a finely perforated 

 celluloid film has been interposed, which has the effect of blocking 

 out nine-tenths of the cross-section of the column. You see that, 

 notwithstanding this, there is no appreciable difference in the amounts 

 of coloured salt which have diffused in the two cases ; the salt has, 

 in fact, gone through the finely pierced septum as readily as if no 

 obstruction were present. 



(N.B. — The celluloid film is itself not permeable.) 



We find that exactly the same holds good with gaseous diffusion. 



If finely perforated septa of this kind are luted on to short tuhes 

 containing caustic soda and are exposed to still air, the amount of 

 carbonic acid diffusing through the holes in the diaphragm can be 

 compared with the amount which we know would diffuse down the 

 open tube under like conditions. 



Some results of this kind are given in Table IV. 



Table IV. — Diffusion of Atmospheiuc C0 2 through Multiperforate 

 Septa into Tube 4 cm. long. Diam. of Holes -380 mm. 



I must now ask you to follow me in a somewhat theoretical 

 excursion in quest of an explanation of these curious facts. 



We have seen that when steady diffusion is going on down a 

 cylindrical column which is absorbent at the bottom, there is a 



