20 THE MECHANISM 0E LIFE 



osmotic pressure are said to be iso-osmotic or isotonic. 

 When comparing two solutions of different concentration, the 

 solution with the higher osmotic pressure is said to be hyper- 

 tonic, and that with the lower osmotic pressure hypotonic. 



Lowering of the Freezing Point. — Pure water freezes at 

 0° C. Raoult showed that the introduction of a non-ionizable 

 substance, such as sugar or alcohol, lowers the freezing point 

 of a solution in proportion to the molecular concentration of 

 the solute. One gramme-molecule of the solute introduced 

 into one litre of the solution lowers ' its temperature of 

 congelation by 1S.VC. Thus a normal solution of any non- 

 ionizable substance in water freezes at — l - 85 c C. The measure- 

 ment of this lowering of the freezing point is called Cryoscopy, 

 a method which is becoming of great utility in medicine. 



Cryoscopy of Blood. — In order to determine the osmotic 

 pressure of the blood at 37° C, i.e. 98*6° F., the normal 

 temperature, we proceed as follows. On freezing the blood, 

 we find that it congeals at — -56°. Its molecular concentration 



is therefore — — = "30, or about one-third of a gramme- 

 T85 



molecule per litre. Its osmotic pressure at 0° C. is therefore 



"3 x2^"35 = 6"7 atmospheres. The increase of pressure with 



temperature is the same as for a gas, viz. ^} ... or "00367 of its 



pressure at 0° for every degree rise of temperature. The 



increase of pressure at '■>!' is therefore "00367 X 37 X G'7= '9 



atmospheres. The total osmotic pressure at 37° is therefore 



(i'7 + '9 = 7 "6 atmospheres. 



Rise of Boiling Point. — Water under atmospheric pressure 

 boils at a temperature of 100 < '. The addition of a solute 

 whose solution does not conduct electricity, such as sugar, 

 causes a rise in the boiling point proportional to the molecular 

 concent ration of that solute. 



Lowering of tin- Vapour Tension. — The vapour tension of 

 a liquid is lowered by the addition of a solute. A liquid boils 

 at the. temperature at which it- vapour tension equals that 

 of the atmosphere. Since an aqueous solution of sugar at 

 atmospheric pressure does not begin to boil at 100 ('..it is 

 manifest that its vapour tension is then less than that of the 



