OSMOTIC PRESSURE . 185 



the latter diffuses rapidly into the gelatin solution thereby 

 lowering the concentration of the gelatin solution. To avoid 

 this error so much gelatin phosphate solution was poured into 

 each bag and glass tube that at the beginning of the experiment 

 the liquid reached already to about that level which from pre- 

 ceding experiments we knew the gelatin solution would reach 

 in the manometer at the point of osmotic equilibrium. All 

 experiments were made in duplicate. In addition to the osmotic 

 pressure we measured the pH inside and outside after equilibrium 

 was reached. From these latter data the osmotic pressure due 

 to the H and H 2 PO 4 ions could be calculated, being equal to 



(2y + z - 2x) X 2.5 mm. H 2 O 



By deducting this value from the observed osmotic pressure in 

 each case it was hoped to obtain a rational value for the share 

 of the protein particles in the observed osmotic pressure. Table 

 XXXVI gives the results. 



The reader's attention is called to the last two rows of figures 

 (Table XXXVI) giving the difference between the observed and 

 the calculated osmotic pressures, since if this difference actually 

 represents the osmotic pressure due to the gelatin particles, the 

 figures should be in direct proportion to the concentration of the 

 gelatin. The experiments were all made in duplicate to give 

 some idea of the magnitude of error, and it is obvious that the 

 error may be considerable, 25 per cent or more, because the 

 errors in the observed and the calculated values are additive. 

 Thus the " difference" is for 0.75 per cent solution in one case 92, 

 in the other 61, a variation of 50 per cent! If we take this into 

 consideration we may conclude that the differences between 

 the observed and the calculated osmotic pressures are compatible 

 with the idea that the difference is the value for the osmotic 

 pressure due to the gelatin particles in solution. 



This would lead us to the conclusion that the osmotic pressure 

 due to the gelatin particles in a 1 per cent solution (of originally 

 isoelectric gelatin) of gelatin phosphate of pH 3.60 is about 100 

 mm. H 2 O. Since the osmotic pressure of one grammolecule is 

 about 250,000 mm. H 2 O and since 1 liter of a 1 per cent solution 

 of gelatin contains 10 gm. of gelatin, the molecular weight of 

 gelatin should be expected to be in the neighborhood of 25,000. 



