106 



COLLOIDS IN BIOLOGY AND MEDICINE 



In order to remove the gelatin cylinder easily, H. BECHHOLD and 

 J. ZIEGLER coated the interior of the test tube with a lining capsule 

 of parchment, paraffined paper, pergamyn or the like, so that the 

 paper is closed below; on the side, it is closely adherent to the glass, 

 while above it projects about 1 cm. above the rim. This paper lining 

 is filled with gelatin, allowed to cool quickly and removed with the 

 gelatin at the end of the experiment. The 

 gelatin cylinder is sliced after unwrapping the 

 paper. 



One might imagine that instead of determin- 

 ing the quantity of substance which had diffused 

 into the jelly, i.e., the diffusion path, the percen- 

 tage of substance that has been lost by the re- 

 maining fluid could be determined. For inves- 

 tigations of colloids this method is not to be 

 recommended, because with the slight diffus- 

 ibility of colloids, the loss of substance and the 

 limits of error approach each other closely. 



The experiments of VOIGTLANDER, placing 

 scales of glue in solutions and determining the 

 amount of the dissolved substance that they 

 took up, are not suitable for use with colloid 

 material. 



R. LiESEGANG* 4 has developed a special 

 method. He covers a plate with a jelly and 

 puts on it drops of a solution which diffuse in 

 rings. The method is especially suitable for 

 qualitative studies. If the jelly is impregnated 

 with a substance which forms precipitates 

 with the diffusing solution, structures appear 

 whose form and growth may be beautifully 



studied. Instead of aqueous solution, sheets of 

 may be placed on the ge l atni l ayer . 



FIG. 22. Osmometer of 

 Biltz and VonVegesack. 



Diffusion and capillary ascension in filter paper (which must be ab- 

 solutely clean) may, under certain circumstances, give useful quali- 

 tative information. 



Osmotic Pressure. 



While in the case of crystalloids, indirect methods of determining 

 the osmotic pressure are used (lowering the freezing point or raising 

 the boiling point), in the case of substances lying at the border 

 line of colloids, the direct osmotic method is most useful. W. 

 BILTZ and A. VON VEGESACK* constructed an osmometer whose main 



