42 Biological Chemistry. 



air chamber. This again is contained in a thick glass 

 vessel C, which is provided with a stout stirrer (generally 

 of aluminium) and a metal top. A freezing mixture (ice 

 and salt) is introduced into this outer vessel, which is well 

 stirred. The liquid in A can then be supercooled without 

 freezing, and its temperature will sink. If, now, it is 

 inoculated with a crystal (in the case of water, a crystal 

 of ice), the supercooled liquid will freeze, and the ther- 

 mometer will rise to a given point and remain constant. 

 The point on the scale at which this happens is then noted. 

 The tube A is then removed from the apparatus and the 

 liquid is then melted, and a weighed amount of the sub- 

 stance of which the molecular weight is required is then 

 introduced into A'. The tube is then replaced in the 

 apparatus, and the melting point of the solution as 

 indicated on the arbitrary thermometric scale is then 

 determined in the same way as that of the pure solvent. 

 The difference between the two measured points gives the 

 depression of the freezing point due to a given weight of 

 the solvent. To get accurate results with this method, 

 various precautions must be taken, and a certain amount 

 of practice is necessary. The principle only of the method 

 is indicated in the above description. 



It has been found now that the depression of the 

 freezing point of any given solvent produced by dissolving 

 another substance in it is proportional to the number of 

 molecules of the dissolved substance (solute). Equimolec- 

 ular proportions of different substances will, therefore, 

 always produce the same amount of depression of freezing 

 point for a given solvent. This amount will vary with 

 different solvents. It is possible now to calculate what 

 this depression should (theoretically) be, when one gramme 

 molecule of a substance (that is to say, the same number 

 of grammes as the molecular weight of the substance) is 



