158 PHYSIOLOGY 



tion of energy, to separate one from the other. Thus filtration, gravita- 

 tion leave the composition of the solution unchanged. It is true that, 

 by the employment of certain kinds of membrane, e.g. the semi-per- 

 meable copper ferrocyanide membrane, it is possible to separate 

 solute from solvent, but in this case the force required to effect the 

 filtration is enormous and grows with every increase in the strength 

 of the solution. The measure of the force required is the osmotic 

 pressure of the solution, and it becomes natural therefore to regard 

 the possession of an osmotic pressure as a distinguishing criterion of 

 a true solution. Is there any evidence that colloid solutions also 

 display an osmotic pressure ? 



Sabanejeff has attempted to decide this question in an indirect manner, 

 i.e. by the determination of the depression of freezing-point caused by the 

 addition to water of various colloids. The depressions observed by this author 

 were so small that they might be regarded as 'falling within the limits of experi- 

 mental error. Assuming that the depression in each case was due to the presence 

 of the dissolved colloid, Sabanejeff arrived at the following molecular weights 

 for certain colloids : 



Tannin . . . 1,322 



Egg-albumm . . . 15,000 



Starch . . . over 30,000 



Silicic acid . . 49,000 



I have shown, however, that it is possible to determine the osmotic pressure 

 of colloidal solutions directly, taking advantage of the fact that colloidal 

 membranes, while permitting the passage of water and salts, are impermeable 

 to colloids in solution. 



The method originally adopted was as follows : In order to determine the 

 osmotic pressure of the colloidal constituents of blood-serum, 150 c.c. of clear 

 filtered serum are filtered under a pressure of 30-40 atmospheres through a 

 porous cell which has been previously soaked with gelatin. The first 10-20 c.c. 

 of filtrate, which contain the water squeezed out of the meshes of the gelatin 

 and have also lost salt in consequence of absorption by the gelatin, are rejected. 

 The filtration is allowed to go on for another twenty-four hours, when about 

 75 c.c. of a clear colourless filtrate is obtained, perfectly free from all traces 

 of protein, but possessing practically the same freezing-point as the original 

 serum. (Although the colloids, if they possess an osmotic pressure, must also 

 cause a depression of the freezing-point, any such depression would be within 

 the errors of observation, since a pressure of 45 mm. Hg. would correspond 

 only to 0*005 C.) The concentrated serum left behind in the filter is then put 

 into the osmometer, the filtrate being used as the inner fluid. The construction 

 of the osmometer is shown in the diagram (Fig. 27). 



The tube BB is made of silver gauze, connected at each end to a tube of 

 solid silver. Round the gauze is wrapped a piece of peritoneal membrane, 

 as in making a cigarette. This is painted all over with a solution of gelatin 

 (10 per cent.) and then a second layer of membrane applied. Fine thread is now 

 twisted many times round the tube to prevent any disturbance of the membranes, 

 and the whole tube is soaked for half an hour in a warm solution of gelatin. 

 In this way one obtains an even layer of gelatin between two layers of peri- 

 toneal membrane and supported by the wire gauze. The tube so prepared is 

 placed within a wide tube, AA, which is provided with two tubules at the top, 



