OSMOTIC PRESSURE OF PROTEIN SOLUTIONS 303 



dissolved substances, inclusive of the colloids. We have no assurance 

 that any given protein preparation is totally free from impurities which 

 may influence the direct measurement of osmotic pressure; it is, there- 

 fore, essential to employ a membrane which is permeable to such im- 

 purities and thus, if time be allowed for the system to come to equi- 

 librium, differentiates between protein and non-protein constituents 

 of the solution under investigation. For this purpose Reid employs 

 a membrane of vegetable parchment, which, as he has shown, is per- 

 meable even to nucleic acid, although it is impermeable to the proteins 

 which he employed in his investigations. By extremely prolonged 

 purification Reid has succeeded in obtaining preparations of Egg- 

 albumin which exhibit no measurable osmotic pressure when examined 

 by this method. In subsequent investigations, however, he obtained 

 osmotic pressures, due to dissolved Hemoglobin of perfectly constant 

 value and such as to indicate a molecular weight of about 48,000. 

 Barcroft and Hill have, however, demonstrated by thermodynamical 

 methods that in solutions containing hemoglobin prepared by less 

 prolonged dialysis the molecular weight of this substance is close to 

 16,669 which is the figure calculated from the content of Iron, assuming 

 each molecule of hemoglobin to contain only one atom of iron. Roaf, 

 employing the differential osmotic method just described, finds that the 

 molecular weight of hemoglobin, dissolved in distilled water, is about 

 32,000, while in sodium carbonate solutions it is 16,000. These results 

 appear to show that when protein is uncombined with acids or bases 

 it is polymerized, and so exerts a considerably smaller pressure than 

 protein salts. 



The extremely important discovery has been made by R. S. Lillie, 

 that the osmotic pressure which is exerted by proteins (determined 

 differentially as described) varies very pronouncedly with the nature 

 of the inorganic acids bases or salts which their solutions contain. The 

 following are illustrative results, obtained when dilute acids or alkalies 

 are employed as solvents: 



1.5 PER CENT. GELATIN IN DILUTE HC1 SOLUTIONS. 



Osmotic pressure of 



1.5 PER CENT. GELATIN IN DILUTE KOH SOLUTIONS. 



Osmotic pressure of 



the protein in 

 Solvent. mm. Hg 



Water 7.9 



"YsiooKOH 14.1 



m /62o " 23.7 



m /412 " 25.1 



m /no " .... ? ... r .. ? ..,. 29. Q 



