ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 255 



cation into semipermeable and permeable membranes is prob- 

 ably best suited for our purposes. 



A true semipermeable membrane is one which allows only 

 the solvent and none of the substances dissolved in the solvent 

 to pass through. True semipermeable membranes are really 

 very rare. The existence of such membranes was discovered 

 by TRAUBE, but we are indebted to PFEFFER for the idea of 

 supporting these in unglazed vessels of earthenware, so that 

 accurate studies of them could be made. The nature of true 

 semipermeable membranes may be made somewhat clearer 

 if the method of making a so-called precipitation membrane 

 is described. 



An ordinary PASTEUR-CHAMBERLAND filter is sawed in half 

 transversely (see Fig. 28, p. 263). The resulting small clay 

 cylinder after proper washing is filled with a solution of 

 copper sulphate, and the whole is then dipped into a second 

 vessel containing a potassium ferrocyanide solution. . The two 

 solutions penetrate the unglazed clay wall from opposite 

 sides, meeting in the middle, where a precipitate of copper 

 ferrocyanide is produced. 



This precipitate of copper ferrocyanide constitutes a semi- 

 permeable membrane, that is, one which is permeable to 

 water but not to substances dissolved in the water. We 

 must point out at once, however, that this statement is not 

 strictly true. It is true for the salts which have been used to 

 produce the precipitation membrane, but a certain amount 

 of nearly all other substances can pass through such a mem- 

 brane. Strictly speaking, therefore, even the semiperme- 

 able membranes are permeable to some extent, though, as 

 we shall see, not at all to the same degree as the truly perme- 

 able membranes. Other precipitates have also been used as 

 semipermeable membranes, such as zinc ferrocyanide and 

 calcium phosphate, but copper ferrocyanide is perhaps the 

 best. 



Almost any one of the animal membranes, such as the 

 bladder of various animals, portions of intestine, or ordinary 



