SIZE OF PARTICLES 47 



We shall only indicate here, that when the colloid electrolytes 

 (thus BILTZ designates salts in which one ion is a colloid) and the 

 electrolyte in the outer water have originally the same osmotic 

 pressure, there is a gradual penetration of the outer electrolytes but 

 the colloid electrolytes cannot escape. Consequently the osmotic 

 pressure in the cell is the resultant of osmotic pressure of the colloid 

 electrolytes plus that of the electrolytes which have entered. If the 

 latter have an ion in common with the colloid electrolytes, we do not 

 find, as might have been expected, that there is an equal division of 

 the true electrolytes (e.g., NaCl), but on the contrary, outside the 

 osmometer there is proportionately more NaCl the more dilute the 

 NaCl solution is (see page 62) . The osmotic pressure of the colloid 

 electrolytes is consequently depressed. 



From this it follows that the values determined by the direct 

 osmotic methods require revision. E. H. STARLING obtained 4 mm. 

 Hg osmotic pressure for serum colloid in 1 per cent serum, thus ex- 

 pressing an apparent molecular weight of about 50,000. E. W. REID 

 obtained a pressure of 369 mm. Hg for a 1 per cent hemoglobin solu- 

 tion from which is deduced an apparent molecular weight of about 

 65,000, a figure which approaches the values obtained by the diffusion 

 method by R. 0. HERZOG and Sv. ARRHENIUS. 



These figures are 4 to 10 times greater than those determined for 

 the molecular weight by chemical means. 



As a matter of fact the theory of the direct measurement of osmotic 

 pressure is so difficult that I do not know any results which are not sus- 

 ceptible of adverse criticism (see F. G. DONNAN'S theory). As the re- 

 sult of direct measurement, we know that colloid solutions actually have 

 an osmotic pressure and that it increases with the amount of dispersion. 

 Theoretical considerations, however, show us that this osmotic pressure 

 must be low. Theoretically, all solutions which contain the same 

 number of particles of the dissolved substance exert the same osmotic 

 pressure. Thus all normal solutions (leaving out of consideration 

 dissociations, associations and other changes) exert the same osmotic 

 pressure, namely, 22.4 atmospheres. Normal solutions are such as 

 contain the same number of molecules, namely, one gram molecule 

 per liter. A normal salt solution is one containing 58.5 gm. NaCl 

 per liter and a normal hydrogen solution is one containing 2 gm. of 

 hydrogen per liter (theoretically). By various means which yield 

 rather concordant results it has been determined that 2 gm. H contain 

 6.1 X 10 23 molecules 1 or fragments of molecules or molecular com- 

 plexes, i.e., particles in solution exerting 22.4 atmospheres of osmotic 



1 This figure (6.1 X 10 23 ) is called Avogadro's figure, and the various methods 

 for deriving it give quite uniform values. 



