Notes . 
538 
dition has been established, there is a steady flux of the carbon dioxide 
down the air column which may be quantitatively investigated by the 
same simple mathematical treatment as the ‘ flow ’ of heat in a bar 
when the permanent state has been reached, or the ‘ flow * of electri¬ 
city between any two regions of a conductor maintained at a constant 
difference of potential. 
By a long series of experiments of this nature it was found that the 
diffusivity constant, k, for very dilute C 0. 2 does not materially depart 
from the value assigned to it by Loschmidt and others, when experi¬ 
menting with much higher ratios of mixture, and that the difference 
is certainly not of sufficient magnitude to be taken into serious account 
in the study of the natural processes of gaseous exchange in the 
assimilating organs of plants. 
In the static diffusion of a gas, vapour, or solute, as the case may 
be, the amount of substance diffusing in a given time, all other con¬ 
ditions being the same, is directly proportional to the sectional area 
of the column. It is found, however, that if the flow is partially 
obstructed by interposing at any point in the line of flow a thin 
septum pierced with a circular aperture, the rate of flow across unit 
area of the aperture is greater than it would be across an equal area 
of the unobstructed cross-section of the column at this point. If 
the margin around the aperture has a width of at least three or 
four times its diameter, the rate of flow is now found to be directly 
proportional to the linear dimensions of the aperture and not to its 
area, so that the velocity of flow through unit area varies inversely as 
the diameter. 
A large number of experiments on the diffusion of carbon dioxide, 
water-vapour, and sodium chloride in solution, are given in support 
of this proposition. All these show that the rate of diffusion across 
such a septum, all other conditions being the same, is directly pro¬ 
portional to the diameter of the aperture, and not, as might have 
been expected, to its area. 
Exactly the same result is obtained when small circular disks of 
an absorbent, such as a solution of caustic alkali, are surrounded 
by a wide rim and exposed to perfectly still air, the amount of carbon 
dioxide absorbed under these conditions being proportional to the 
diameters of the disks. 
If, however, there are any sensible air currents the absorption 
becomes proportional to the areas. 
