ii 4 PRINCIPLES OF GENERAL PHYSIOLOGY 



passage is very slow ; and about 70, it ceases altogether. Of course, the actual space occupied 

 bv a molecule does not depend only on the number of atoms it contains. The chemical 

 arrani;i'iin nt must al>"> l>e taken into account ; accordingly, chemical structure was found to 

 have some effect on the results. The "sieve theory," then, appears to hold in the case i.f 

 < "lloids and, as we cannot draw a lino of demarcation between them and iT\>tallnids, the 

 general application of the theory receives support. 



Abel ( 1914) finds in his " vividiffusion " method, that the rate of diffusion through collodion 

 membranes is independent of their thickness, a fact which suggests pores rather than solution 

 in the substance of the membrane. 



When we recollect that the copper ferrocyanide membrane is freely permeable 



to water, in fact, contains water in its constitution, it seems not so easy to 



understand how a substance such as sugar, which is easily soluble in the water 



-contained in the membrane, fails to pass through, unless something like a sieve 



is present, opposing a mechanical constraint on molecules above a certain size. 



Traube (p. 280 of the "Collected Papers") points out that his precipitation 

 membranes always contain considerable quantities of water, and that, if dried, 

 they become completely impermeable, both to water and to solutes. 



Bartell (1911) showed that, when water is forced by pressure through a 

 membrane of copper ferrocyanide, the rate at which it flowed through obeyed 

 Poiseuille's formula for the case of capillary tubes. 



But, before the question at issue can be finally decided, it will be necessary 

 to understand more completely the nature of the process of solution, and it may 

 very probably be found that there is no real contradiction between the two 

 opposing views. 



With regard to the structure of colloid membranes in general, it will be 

 clear that the remarks on page 14 above are of importance. If a membrane of 

 gelatine has a honeycomb structure, any substance passing through it must 

 traverse a structure consisting of much finer pores than if the membrane were 

 of a sponge-like nature, where it could pass, by a tortuous channel, between 

 the actual trabeculse of the solid phase. 



Another point to be remembered is that the surfaces of the elements of the 

 / membrane adsorb dissolved substances. In the filtration of salts through a 

 gelatine filter, the first portions of the filtrate contain less salt than the original 

 solution ; this continues until the adsorption capacity of the membrane is 

 saturated. A colloid, when adsorbed, may diminish considerably the dimensions 

 of the pores, so that the filter becomes impermeable for substances to which 

 it was at first permeable. 



It is frequently found that a solute, to which a membrane appears to be 

 impermeable, will pass through in very small amount, if allowed a long time. 

 Theie are two possible causes for this fact. The pores in an artificial membrane 

 are not all of exactly the same size, as was noticed by Bechhold in his measure- 

 ments of various membranes. Suppose that there are a few of them which 

 will allow a certain solute to pass, while the great majority are impermeable 

 to it ; it will take a long time for an appreciable amount of the solute to find 

 the small number of channels available for it, owing to the slowness of diffusion. 

 A similar state of affairs would be found if the particles of the solute varied 

 in dimensions, even if the membrane were of a uniform structure. 



These facts lead to reference to the rate of passage through ;i membrane. 

 In addition to the factors mentioned in the previous paragraph, a little con- 

 sideration will show that a membrane may be freely permeable to a solute, 

 but, if the rate of diffusion is very slow, comparatively little will pass in unit 

 time, owing to the supply at the surface of the membrane not being kept 

 up sufficiently. This state of affairs plays a part in certain osmotic phenomena 

 to be discussed in the next chapter. 



THE SURFACE MEMBRANE OF THE CELL 



The present chapter was commenced by pointing out the necessity for some 

 arrangement by which, in such organisms as the amoeba, sugar and other soluble 

 food-stuffs are prevented from diffusing out and being lost to the protoplasm. 





