THE 
NEW PHYTOLOGIST 
Vol. XX, No. 4 November 2, 1921 
PERMEABILITY 
By WALTER STILES 
CHAPTER IV 
DIFFUSION 
I F two gases are separated by a partition which is subsequently 
removed, the molecules of one gas pass into the space occupied 
by the other until a condition of equilibrium is reached in which the 
two gases are equally distributed throughout the whole space. A 
similar diffusion takes place in the case of two miscible liquids, except 
that owing to the internal friction or viscosity the rate of equili¬ 
bration is much slower. Similarly, if the two liquids brought into 
contact consist of solutions of different substances in the same solvent, 
then diffusion proceeds until the solutes are equally distributed 
throughout the whole of the liquid; or if the solvent contains the 
same dissolved substance but in different concentrations in “the two 
liquids, diffusion proceeds until the solute is equally distributed 
through the whole of the solvent, always supposing that no external 
force is operative. 
Phenomena involving the diffusion of gases do indeed occur in 
the plant, as in the processes of carbon-assimilation, respiration 
and transpiration. But directly any gas reaches the surface of a cell 
any further passage of the molecules of the gas through the cell 
takes place in an aqueous medium, and in problems of permea¬ 
bility it is with diffusion through a liquid that we are concerned, 
and in most cases with diffusion through an aqueous medium. 
The first systematic researched on diffusion of liquids were made 
by Thomas Graham (1851), who investigated the diffusion of a 
variety of substances dissolved in water. He showed that the quantity 
Phyt, XX. IV. 
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