234 THE PLANT. X. 



(1) The area of the aperture say A. 



(2) The partial pressure of the carbon dioxide in the atmo- 



sphere outside say P. 



The number of molecules moving outwards similarly depends 

 upon 



(1) The area of the aperture A. 



(2) The partial pressure of the carbon dioxide in the 



chamber say P'. 



Let x = number of molecules entering in one second and 

 y= number leaving in one second. 

 Then x = &AP 

 and y = &AP'. 



Of these quantities A is common, P is constant, about '0003 

 of an atmosphere, but P' depends upon the ratio of x to the 

 rapidity with which the carbon dioxide is absorbed. This last 

 will be proportional to the area of the absorbing surface say S. 



Now the rate of diffusion is 



x-y = fcAP-fcAP 



Dividing by k~P it is seen that the rate of diffusion is 

 proportional to 



A - C | A* 

 or, per unit area, to 



Hence the smaller the value of the area of the aperture the 

 greater is the amount of diffusion per unit area. The essential 

 point in connection with this phenomenon is that by means 

 of small apertures it is possible to have on the one side air 

 containing practically its full amount of carbon dioxide, while 

 on the other, the inside, the air is kept practically devoid of 

 that gas ; consequently very little diffuses outwards, provided 



