CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 595 



Starting from Graham's observation that dissolved colloids cannot ditiuse through 

 colloidal membranes, and from the empirical fact that precipitates of colloidal substances 

 are usually themselves colloidal, Traube found that a drop of a colloid A placed in a 

 solution of a colloid B must become surrounded by a pellicle. If A is also more con- 

 centrated (or rather if its attraction for water is greater) the cell must become turgid, 

 /. e. the precipitated pellicle must become stretched by the additional water that is 

 absorbed ; and the molecules of the pellicle thus become separated to such an extent 

 that a fresh precipitate takes place between them which occasions increase in the 

 superficies of the pellicle. For a more exact study Traube chiefly employed cells the 

 pellicle of which consisted of a precipitate of gelatine tannate. For this purpose the 

 tendency of the gelatine to coagulate was destroyed by boiling for thirty-six hours. 

 A thick drop of this gelatine (called /S) of the consistency of syrup was taken up by a 

 glass rod, allowed to dry for some hours in the air, and then plunged into a flask half 

 filled with a solution of tannic acid, into the cork of which the rod was fixed. The 

 portion of gelatine which undergoes solution on the outside of the drop immediately 

 forms a completely closed pellicle with the surrounding solution of tannin ; and the 

 water which penetrates through it constantly dissolves the gelatine within. In a dilute 

 solution of tannin of o'8 to i*8 p. c. a tense pellicle which is not iridescent and is there- 

 fore thick is formed ; in a concentrated solution of from 3*5 to 6 p.c. (in which therefore 

 there is a smaller difference between the concentration of the two fluids) a thin flaccid 

 iridescent pellicle is formed ^ Traube found that the cells which are at first thick- walled 

 go through various stages of development ; they remain spherical so long as the nucleus 

 of gelatine is not completely dissolved ; a turbidity then sets in from above downwards 

 owing to the solution of a part of the pellicle in the solution of gelatine which is more 

 dilute in its upper part ; the pellicle at the same time begins to collapse and to become 

 iridescent ; and finally the contents become clear and tension again takes place. After 

 the lapse of some weeks the cell still allows gelatine to escape when torn. The greater 

 the difference in the concentration of the two fluids, the firmer and more tense is the 

 pellicle; i.e. the greater the intensity of the endosmotic attraction the greater is the 

 number of layers of atoms which coagulate so as to produce the pellicle, and therefore 

 the thicker it is. 



With reference to the properties of the pellicle, Traube shows that all pellicles 

 hitherto employed in experiments on diffusion have perforations-, while the precipitated 

 pellicles have only molecular interstices; and indeed these latter are, according to 

 him, smaller than the molecules of the precipitate of which the pellicle is composed. 

 But in spite of the greater density, the endosmose is quicker than with all other 

 membranes, because they are thinner. The pellicle becomes firmer (stiffer?) when lead 

 acetate or copper sulphate is added to the (3 gelatine. As soon as the molecules of the 

 stretched pellicle have become so far separated by the pressure of the cell-contents 

 which have increased in quantity by the action of endosmose that their interstices allow 

 the passage of the two substances from which the pellicle is formed, these substances 

 must obviously again at once mutually react upon one another at those points, and must 



* Only pellicles of gelatine behave in this way ; all others are iridescent when tense. 



2 It is easy to convince oneself of the presence of actual perforations in pig-bladder, ox-bladdcr, 

 the pericarduim, amnios, collodion-membrane, or parchment, with which experiments on diflusion 

 have hitherto usually been made, by stretching them over a wide glass tube, pouring in a column of 

 water from 20 to 40 cm. high, and repeatedly drying the free surface of the membrane with filtcinig 

 paper. Water is then almost always seen to ooze out at particular spots; a piece of membrane 

 2 or 3 cm. square is seldom watertight. The perforations are still more evident if the tube is filled 

 with a concentrated solution of common salt and the membrane dipped in water. Instead of a 

 diffusion-current equal over the whole surface of the membrane, separate threads of the solution of 

 salt are seen to sink down into the water. These experiments show how little dependence is to 

 be placed on the researches hitherto made on memljranes. 



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