OF VITAL PHENOMENA 13 



it will evaporate until the pressure of the vapor reaches a certain 

 value, dependent on the temperature. The vapor pressure of 

 pure water at 20 is 17.4 mm, at 25 23.5 mm, at 30 31.6 mm, 

 and at 37 46.7 mm. The lowering of the vapor pressure by 

 1 mol solute at body temperature is only about .5 mm and is less 

 at room temperature since it is a relative lowering and the vapor 

 pressure of pure water is less at lower temperature. 



2 — Boiling Point. The boiling point of water is raised .54° 

 by 1 mol solute. (Vol. = 1 liter where not stated.) 



3 — Freezing Point. The freezing point is lowered 1.85 by 

 1 mol solute. 



4 — Osmotic Pressure. If a mol solution is enclosed in a 

 vessel whose walls are impermeable to the solute and permeable 

 to the water, and immersed in pure water, water will pass into 

 the vessel, causing a pressure, osmotic pressure, of 22.4 atmos- 

 pheres. 



With these colligative properties might be classed electrode 

 potentials, since they depend on osmotic pressure. The concen- 

 tration of a solute calculated from its effect on all colligative 

 properties is the same, but may differ from its concentration cal- 

 culated from other data, so the word activity is sometimes used 

 to denote concentration calculated from colligative data. 



The density of the solution is affected by the concentration of 

 the solute, but the degree of change differs with different solutes. 

 Hence, this property of a mixed solution cannot be interpreted un- 

 less the proportion of the different solutes remains approximately 

 the same (as in sea water, for instance) . Density is determined by 

 weighing a certain volume of the solution measured in a pycnome- 

 ter and dividing its weight by the weight of an equal volume of 

 water. The rule has been to have the water at 4° and the solu- 

 tion at 1 5 , but in many determinations the water is taken at the 

 same temperature as the solution, and 25 ° is rapidly becoming the 

 standard temperature for all sorts of measurements. 



The refractive index is also used to determine concentration. 

 With the Abbe direct reading refractometer only a drop of the 

 solution is necessary, and hence this instrument is used in protein 

 analysis when the protein is pure or the accompanying solutes 

 are known. The Zeiss interferometer gives a more accurate 



