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Walter Stiles 
is obvious, and the solution theory of permeability has found many 
supporters, e.g. Nernst (1890), Kahlenberg (1906), and Overton 
(1897), to whose work more detailed reference will be made later. 
From his work on the influence of pressure and temperature on 
permeability, Bigelow (1907) concludes that the passage of water 
through membranes of collodion obeys Poiseuille’s formula for the 
movement of water through capillary tubes, and consequently that the 
passage of substances through such membranes is capillary in nature. 
The inconsistent results first obtained with porcelain were shown by 
Bartell (1911) to be due to mechanical clogging of the pores, a 
phenomenon which M. Traube thought he had been able to bring 
about in the case of precipitation membranes and which he adduced 
as evidence in support of the sieve theory. By clogging the pores 
of a membrane, the latter may become impermeable to substances 
to which it was formerly permeable. It has been pointed out by 
Bayliss (1915) that such clogging can also take place as a result of 
adsorption. The views of Bigelow seem at first sight to approach 
the sieve theory, but Bigelow shows that the capillary theory 
reconciles the opposed sieve and solution theories. In the case of 
porcelain the passage of water undoubtedly takes place through 
capillary pores, while in the case of water diffusing through collodion 
there is good evidence that the water passes through intermolecular 
spaces, i.e. dissolves in the membrane. But the phenomenon is 
essentially the same in the case of these and other membranes. 
Consequently Bigelow concludes that the rate of passage of liquids 
through molecular interstices is expressible by the same laws which 
formulate the rate of passage of liquids through capillary tubes. 
This view, that capillary phenomena only differ in degree but not 
in kind, from chemical phenomena, can be traced back to L’Hermite. 
Apart from the two definite theories of permeability here dis¬ 
cussed, there has been suggested a third theory, which may be spoken 
of as the chemical theory, according to which the membrane is 
supposed to combine chemically with the substance to which it is 
permeable. This reaction is supposed to be reversible so that on the 
far side of the membrane the compound of membrane and diffusing 
substance breaks down with the result that the diffusing substance 
is set free on the far side of the membrane. This theory has been 
applied chiefly to cell problems, but has not found the support 
accorded to the sieve and solution theories. 
(To be continued) 
