CHAP, i.] OF NUTRITION. 81 



ebullition of thirty-six hours has lost its coagulability. He lets 

 it dry in the air for several hours, and by the help of a rod fixed 

 in the cork of a vessel half filled with a solution of tannin, he 

 plunges it into this liquid. Then the small quantity of gelatine 

 which dissolves on the surface of the drop combines with the 

 tannin, and the result is a closed cellular membrane. But this 

 membrane is homogeneous, unperf orated, as the organic membranes 

 are. Also the diffusion which is established between its contents 

 and the exterior liquid must be effected osmotically across the 

 molecular interstices. The osmosis is produced very energetically. 

 The membrane distends more and more ; as a consequence the 

 constituent molecules sever from each other : at a given moment 

 when the molecules of the two liquids brought face to face can 

 easily be introduced into the molecular interstices and blend, they 

 form anew molecules of tannate of gelatine. Consequently, the 

 membrane grows by intussusception. In effect, it suffices to arrest 

 all increase, to substitute water for the solution of tannin. 



M. Traube forms also, in the same manner, endosmotic mem- 

 branes, very curious, impermeable by certain substances, very 

 permeable by others ; in a word, exercising on the substances 

 in contact with them an elective action, such as the living 

 membranes exercise. 



According to M. Traube, every precipitate whose molecular 

 interstices are smaller than the molecules of its components 

 must take the form of a membrane. 



Lastly, the endosmosis across the membranes depends solely 

 on the attraction of the body which is dissolved for its solvent. 



These experiments are infinitely interesting; nevertheless, 

 they imitate very imperfectly what takes place in the living 

 cells. It is something to have obtained by simple chemical 

 processes a membrane which grows by intussusception, foras- 

 much as, from time immemorial, this mode of growth has 

 been considered peculiar to living bodies. But in the living 

 cell there is something more ; the contents are as little inert as 



G 



