116 



COLLOIDS IN BIOLOGY AND MEDICINE 



mined before and after the stay in the solution gives the amount of 

 fluid taken up or lost. Before weighing, the jelly is to be wiped 

 with filter paper or a cloth, in order to free it from the adhering 

 fluid. Fluid is pressed out in this method also, especially in the case 

 of very much swollen material, by the weight of the jelly itself as 

 well as by its contraction in the air; this fluid is dried off and cannot 



be taken account of in the 

 weighing. 



Swelling Pressure. The deter- 

 mination of this factor offers the 

 greatest prospect for an exact 

 method, especially as the appa- 

 ratus of J. REINKE* may be 

 adapted for other swelling sub- 

 stances. J. REINKE used his 

 apparatus (Fig. 25) to measure 

 the swelling pressure of laminaria 

 (a sea weed). The dry algae are 

 placed in the bore F of the metal 

 cylinder M . On top of the algae 

 rests the piston A, perforated with 

 fine holes. From E water may 

 penetrate to the algae through the 

 holes. As the mass of alga3 swells 

 it raises the piston and the rod 

 ABD, which may carry various 

 weights. The lifting power is 

 indicated on the dial. The 

 theory of the apparatus, however, 

 requires careful investigation to 

 establish the relationship be- 

 tween swelling pressure, water 

 absorbed and the lift. 



The apparatus of E. POSNJAK 

 (see Fig. 26) offers less theoretical difficulties but may be employed 

 only for pressures up to 6 atmospheres. The principle employed 

 follows: the substance to be investigated, Q, is placed at the bottom 

 of a tube G which is closed by a porous clay cell T. The swelling 

 tube dips into a vessel of water. To overcome the swelling pressure 

 the whole vessel is filled with mercury which is connected with a 

 manometer M. The swelling substances may be placed under a 

 given pressure by permitting compressed gas to flow from a steel cylin- 

 der g. The details of the experiments appear in the original paper. 



FIG. 25. J. Reinkes' apparatus for 

 measuring swelling pressure. 



