THE PHYSICAL PROPERTIES OF THE PROTEINS 25 



proteins. The determinations were made with a view to the estima- 

 tion of molecular weights. The data arrived at by this method are, 

 however, of little value. In the first place it is, as has been re- 

 peatedly stated, extremely difficult to free proteins from the last 

 traces of inorganic substances ; very small contaminations of such 

 substances with low molecular weight would cause relatively very 

 large errors in the determination of the molecular weight of such 

 complex substances as the proteins. In the second place, from the 

 direct determinations of osmotic pressures, discussed in greater detail 

 below, the depression in the freezing point of a salt-free protein 

 should be so small as to be well within the limits of experimental 

 errors. Moore has calculated that 'OOi C. should correspond to an 

 osmotic pressure of 9 mm. of mercury. The smallest depression 

 that can be measured by a Beckmann thermometer is '005 C., 

 which corresponds to an osmotic pressure of 45 mm. As, in prac- 

 tice, pressures of this order have to be measured in the investigation 

 of proteins, when estimating osmotic pressures directly, it is obvious 

 that but little value can be attached to determinations of freezing- 

 point depressions. Nevertheless, some of those determinations 

 made with peptones and other degradation products are of interest, 

 as substances of much lower molecular weight than the proteins from 

 which they have been derived are here being dealt with. It must be 

 remembered, however, that these peptones are not pure bodies. A 

 few data from the literature, with these reservations, are given below : 



Substance. 

 Egg-albumin 



Protalbumose 



Deutero-albumose 



Albumose 



Peptone (commercial) 



Propeptone 



Antipropeptone (albumose) 



Gelatin 



Peptone (Merck) 



(Grubler) 

 Glutin-peptone hydrochloride 



Molecular 

 Weight. 



14,270 



6400 



2467-2 



32CO 



2400 



1504-1754 



I20I-I2I5 



776-823 



878-960 



52Q-555 



278-704 Paal. 



Observer. 



Sabanejevv and Alexandrow. 

 Bugarsky and Liebermann. 

 Sabanejew. 



> 



Bugarsky and Liebermann. 

 Sabanejew. 

 Paal. 



Ciamician and Zanetti. 

 ii 



(B) The Direct Determination of Osmotic Pressure. 



Considerable controversy has arisen as to whether colloids exert 

 any osmotic pressure in solution. Several investigations on this sub- 

 ject are, however, recorded in the literature. Starling, in the course 

 of some researches on the function of the glomeruli of the kidney, 

 measured the osmotic pressure exerted by a serum solution of 

 known protein content. A vessel was used the walls of which were 

 permeable to the salts but not to the proteins contained in the 

 serum ; for this purpose they were constructed of peritoneal mem- 

 brane of calf previously soaked in gelatin. On the other side of 

 the membrane was placed a fluid which possessed approximately 

 the same salt contents as the serum ; this was generally prepared by 

 freeing the serum experimented with from its protein by pressing it 

 through gelatin filters under a pressure of 30-40 atmospheres. By 

 having on one side of the membrane natural serum, and on the 

 other serum deprived of proteins by filtration, and by connecting 

 the protein containing liquid with a manometer, the osmotic pressure 



3* 



