106 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



removal of the substances produced, a feeble chemical reaction may finally be 

 brought to completion, but an account of this phenomenon and of osmotic pressure 

 will be given later (Sects. 22 and 93). 



Precipitation membranes. For the study of osmotic processes, the precipitation 

 membranes which Traube ' first employed are of great importance. These demon- 

 strate how a limiting precipitation-membrane may be formed and maintained where 

 two fluids, or a fluid and a solid, come into contact. The precipitation-membranes 

 formed by tannic acid and gelatine or mucilage form especially favourable experi- 

 mental material. Thus, if the end of a glass rod, on which a little mucilage has been 

 allowed to dry, be immersed in a two per cent, solution of tannin, after a short time 

 a transparent continuous membranous film is formed around the dissolving mucilage. 

 This membrane is kept tense by the osmotic nature of the mucilaginous contents, 

 and is forced to increase in surface area by the stretching to which it is subjected. 

 To obtain marked growth, a gelatinous gum should be employed containing from 

 ten to fifteen per cent, of sugar. If in addition the gum 

 contains a little indigo-carmine, or aniline-blue, it may be 

 observed that these substances do not diosmose through the 

 separating membrane. The solid American mouth-glue 8 may 

 be conveniently employed for this purpose, but sometimes the 

 addition of a little gelatine is necessary. The experiment 

 represented in Fig. 5 is of even more general utility and 

 simplicity. The cylinder contains a two to five per cent, 

 solution of potassium ferrocyanide, and the capillary end of 

 the glass tube contains a drop of a solution of cupric sulphate. 

 The other end of the glass tube is closed by means of the 

 FIU. 5- finger, and the capillary end is immersed in the fluid con- 



tained in the cylinder, so that the column of fluid inside the 

 tube is slightly higher than that outside. If a crystal of sulphate of copper is 

 thrown into a solution of ferrocyanide of potassium, a similar precipitation-mem- 

 brane is formed. (On the production of membranes for diosmotic researches, see 

 Sect. 22.) 



Moreover, as Traube has shown, an osmotic membrane may be formed by the 

 mere contact of a solution with pure water, provided a certain amount of precipita- 

 tion is thereby induced. This is actually the case when a concentrated solution of 

 tannin containing dissolved tannic-acid-gelatine is employed, for when diluted the 

 latter is precipitated, and hence when the solution mentioned is in contact with 

 water, a precipitation-membrane is formed between the two. The works already 

 quoted 3 give evidence to show that the specific permeability varies in different 

 membranes. According to Traube, infiltrations of the membrane may modify its 

 permeability, and Pringsheim *, from results obtained with gelatinous membranes, 



1 Traube, Archiv f. Anat. u. Physiol., 1867, p. 87 ; Bot. Zeitung, 1875, p. 56. See Pfefier, 

 I.e., p. u. 



a [A soluble tasteless gum used upon the adherent flaps of envelopes, &c.] 



3 See p. 103, note i. 



* Pringsheim, Jahrb. f. \viss. Bot., 1895, Bd. xxvin, p. i. 



