OF VITAL PHENOMENA 31 



trolytes, but finds that they increase the permeability of the mem- 

 brane. The results of Morse do not exactly correspond to the 

 formula, and he supposes this due to the fact that the sugar 

 molecules take up so much room that the concentration of the 

 water is reduced. By dissolving the sugar in 1000 g of water 

 instead of making a liter of solution, the formula was more 

 closely followed. Such solutions are called weight normal solu- 

 tions, but the water combined with the sugar molecules is not 

 considered. According to Washburn (191 5) each sugar molecule 

 is combined with six molecules of water, which means that in a 

 liter of a mol solution, 108 g of water per liter are taken from 

 the solvent. The significance of the room occupied by the mole- 

 cules is strikingly shown in solutions containing solutes of very 

 high molecular weight. The osmotic pressure of a 30 per cent 

 gum arabic solution is 4 atmospheres, whereas that of a 60 

 per cent solution is 48 atmospheres. If blood serum whose 

 osmotic pressure is 6 atmospheres is concentrated to % volume, 

 its osmotic pressure is 35 atmospheres. 



Owing to the difficulty of determining the osmotic pressure 

 directly, it is usually calculated from the freezing point. An 

 osmotic pressure of 22.4 atmospheres corresponds to a freezing 

 point lowering (A) of 1.85 . The following conversion table 

 was calculated by Harris and Gortner (1914) from the formula 

 of Lewis (1908). 



In determining the freezing point of aqueous solutions, the 

 only advantage of the Beckmann thermometer is that it may be 

 accessible. It has the disadvantage that it must be adjusted for 

 aqueous solutions and that the zero point must be determined 

 every year. The curvature and the temperature of the two 

 mercury surfaces bordering on the vacuum are not always the 

 same and hence the vapor pressure of the mercury at these two 

 surfaces is not the same, and consequently mercury distils over 

 from one surface to the other. The Heidenhain thermometer 

 has no adjustment to cause this trouble. It is a simple ther- 

 mometer graduated in hundredths of a degree from about +0.5 

 to — 5°. The Burian-Drucker (1910) thermometer is designed 

 for fluids of which only 1.5 cc may be available. Since it has 

 a smaller bulb and consequently shorter stem, it is graduated in 

 fiftieths of a degree. The thermometer should be kept in ice 

 water several hours before the determination. 



