1942 Journal of Applied Microscopy 



GENERAL PHYSIOLOGY. 



RAYMOND PEARL, University of Michigan. 



Books and Papers for Review should be Sent to Raymond Pearl, Zoological Laboratory, 

 University of Michigan, Ann Arbor, Mich. 



Moore, B., and Parker. W, H. The Osmotic The problem which the authors investi- 

 Properties of Colloidal Solutions. Amer. g^ted was whether, in a colloidal solu- 

 Jour. Physiol. 7: 261-293, 1902. ° 



tion, the large molecule (or " solution 

 aggregate " ), consisting of many chemical molecules in physical combination, 

 behaves like a molecule of crystalloid solution, and exerts osmotic pressure, or 

 is perfectly inert with none of the properties of matter in solution. It has been 

 found by previous workers that such indirect methods of observing osmotic 

 pressure as lowering of freezing point and raising of boiling point in case of 

 collodial solutions, give results so low as to fall within the limits of experimental 

 error. As a consequence the authors decided to use the method of measuring 

 directly the osmotic pressure by means of an osmometer. To guard against the 

 source of error introduced by the presence of crystalloids in the solution, the 

 following general method was used. The size of the solution aggregate was 

 altered by the action of reagents, which left unaffected the amount of indiffusible 

 or feebly diffusible salts in solution. Since osmotic pressure depends upon the 

 I rmolecular concentration rather than upon the molecular weight, a higher pressure 

 should be obtained when the solution aggregate is broken down and consequently 

 made smaller in size, since there will then be a larger number of such aggregates 

 per unit volume. If the amount of crystalloid is kept the same in both experi- 

 ments, any observed difference in osmotic pressure must be due to the colloidal 

 substance. 



Great difficulty was experienced in obtaining an osmometer in which there 

 was no leakage around the edges of the membrane. 

 An instrument was finally devised by the authors 

 which entirely overcame this difficulty. Their ap- 

 paratus is shown in section in Fig. 1. It consists 

 essentially of a brass receiver, hemispherical in 

 form, thickly plated with silver, and finally on the 

 inner surface with gold. The purpose of this gold 

 plating is to exclude chemical action of the solution 

 ^^§' '• on the metal. The receiver has at its upper pole a 



tubular continuation for connection with the manometer, and a broad flange 

 around its circumference, which has a narrow circular groove, let into its under 

 surface. In this groove lies a somewhat thicker rubber band, against the under 

 surface of which the membrane is tightly pressed. The pressure is equally 

 applied by means of four screws (two of which are shown in the section) passing 

 through four equidistant holes in the outer part of the flange of the receiver, and 

 engaging in four similar but threaded lioles in a heavy ring of metal of similar 

 dimensions to the flange. In fitting up the instrument for an experiment, the 



