226 DIFFUSION AND OSMOSIS. 



■within flowed out onlj' to meet the exterior liquid, and there 

 formed instantlv a precipitate cohering with the edges of the 

 rupture. If a fragment of chloride of copper is placed in a test- 

 tube containing a strong solution of potassic ferroe3anide, the 

 action is more rapid than with copper-sulphate. The fragment 

 dissolves at once, and forms a green globule at the bottom of 

 the tube. If it now be carefully watched, it will be seen that a 

 delicate transparent film (the precipitate of cupric ferroC3-anide, 

 ■which in a flocculent state is brown) is produced over the glob- 

 ule, and the sphere begins at once to grow into a cylindrical 

 body. The liquid in the upper jjart of the closed cylinder is 

 almost colorless ; that at the bottom is deep greeu. The inter- 

 mittent growth in height appears to admit of only one explana- 

 tion ; namely, that the membrane is torn by the great pressure 

 ■within, and the solution of copper chloride which flows through 

 is immediatel}- covered bj' a newly formed film. By careful 

 management, such growths of C3-Iindrical form can be produced 

 several inches long. 



Traube also discovered that v/hen a drop of yS gelatin (gelatin 

 which has been boiled contiuuousl3- for about three days, there- 

 by losing its power of coagulation) is placed in a solution of 

 tannin, a film forms at once, which begins to grow into a spheri- 

 cal cell, but without the appearance of irregular and intermit- 

 tent rupture. Such an artiflcial cell is best prepared b}- placing 

 a glass tube having a drop of yS gelatin on the tip into a solution 

 of tannin. Its growth is even and uninterrupted, and unless 

 the apparatus is disturbed, no appearance of rupture is observed. 

 A further discovery was made by Traube ; nanieh', that a co- 

 herent film ma}- be formed even by the contact of pure water. 

 The coagulum produced when gelatin is acted on by tannin (the 

 so-called tannate of gelatin) is soluble in a concentrated solution 

 of tannin, but is insoluble in a dilute solution. If a drop of a 

 solution of tannate of gelatin thus prepared is placed in pure 

 water, a coherent film forms at the surface which can increase 

 up to a certain size.^ 



615. Pfeffer has employed the precipitation-membranes dis- 

 covered by Traube, in an ingenious apparatus bj' which the 

 pressure developed in the so-called artificial cell can be accu- 

 rately measured. The apparatus consists of a porous porcelain 



' At this point should be mentioned the observation of Nageli, tliat when- 

 ever cell-contents rich in protein matters come in contact with watery media, a 

 membranous film is fovmed over the surface (Pflanzenphysiologische Unter- 

 suchungen, 1855, pp. 9, 10). 



