THE PROPERTIES OF COLLOIDS 



159 



One of these, (), is for filling the outer tube ; the other is fitted with a mercurial 

 manometer, M. Two small reservoirs, CO, 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, CO, 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 

 removed, 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 corre- 

 spond to a molecular weight of about 30,000. 



A more convenient form of osmometer has been devised by B. 

 Moore, using parchment paper as the membrane. With this osmo- 



meter, the existence of an osmotic pressure in colloidal solutions 

 has been definitely established both by Moore in the case of haemo- 

 globin, proteins, and soaps, and by Bayliss in the case of colloidal 

 dyes, such as Congo red. The osmotic pressure of haemoglobin was 

 found by Hiifner to correspond to a molecular weight of about 

 16,000, i.e. a molecular weight already deduced from its composition 

 and its combining powers with oxygen. Often, however, the osmotic 

 pressure shows a molecular weight which is very much smaller than 

 would be expected from the molecular weight of the substance, owing 

 to the fact that colloids in solution may be in many different condi- 

 tions of aggregation. Thus the molecule of colloidal silica must be 

 many, probably thousands of, times larger than the molecule as 

 represented by H 2 Si0 3 . The osmotic pressure being proportional 

 to the number of molecules in a given volume of solution, the larger 

 the aggregate the smaller would be the total number of molecules, 

 and the smaller therefore the osmotic pressure of the solution. 



It is in consequence of the huge size of the molecular aggregates 



