THE PROPERTIES OF COLLOIDS 



141 



branes, 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 are obtained, per- 

 fectly 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 osmo- 

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

 is shown in the diagram (Fig. 2^). 



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 cigirette. 

 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 



, 



A* 

 FIG. 27. 





revent 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 peritoneal 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. One of these, O, is for filling the outer tube ; the other is fitted with a 

 mercurial manometer, M. Two small reservoirs, CC, are connected with the outer 

 ends of BB, by means of rubber tubes. The whole apparatus is placed in a wooden 

 cradle, DD, pivoted at X, and provided with a cover so that it may be filled with fluids 

 at different temperatures if necessary. The colloid solution is placed in AA, and the 

 reservoirs, CC, and inner tube, BB, are filled with the filtrate, i.e. with a salt solution 

 approximately or absolutely isotonic with the colloid solution. The apparatus is then 

 made to rock continuously for days or weeks by means of a motor. In this way the 

 fluid on the two sides of the membrane is continually renewed, and the attainment 

 of an osmotic equilibrium facilitated. With this apparatus I found that the colloids 

 in blood-serum, containing from 7 to 8 per cent, proteins, had an osmotic pressure of 

 25 to 30 mm. Hg, which would correspond to a molecular weight of about 30,000. 



