94 J. van't Hoff on the Function of Osmotic Pressure 







Molecular 







diminution 



Solvent. 



Molecular 



of vapour- 





weight. 



pressure. 





(M'). 



(K). 



Water 



18 



0-185 



Phosphorous chloride . . 



137-5 



1-49 



Carbon disulphide . . 



76 



0-80 



Carbon tetrachloride . 



154 



1-62 



Chloroform .... 



119-5 



1-30 



Amylene 



. 70 



0-74 



Benzene 



78 



0-83 



Methyl iodide . . . 



. 142 



1-49 



Methyl bromide . . 



. 109 



1-18 



Ether 



. 74 



0-71 



Acetone 



. 58 



0-59 



Methyl alcohol . . . 



. 32 



0-33 



VIII. Third Confirmation o/Avogadro's Law in its Applica- 

 tion to Solutions. — Molecular Depression of Freezing-point 

 of Solvent. 



Here, again, a general law may be stated, connecting 

 osmotic pressure with the freezing-point of a solution: — 

 Solutions in the same solvent, and of the same freezing-point, 

 are isotonic at that temperature. This statement admits, like 

 the former, of proof by means of a reversible cycle ; but the 

 solvent when returned is here in the condition of ice, not of 

 vapour ; the inverse change is also brought about by means 

 of a semipermeable diaphragm, and, as it cannot be accom- 

 panied by gain or loss of energy, isotonism must exist. 



We shall apply the above statement to dilute solutions; and 

 applying the relations previously described, the simple con- 

 clusion follows that solutions containing an equal number of 

 molecules in equal volume, and which therefore, from Avo- 

 gadro's law, are isotonic, also have the same freezing-point. 

 This law has been actually discovered by Raoult, and is ex- 

 pressed by him as "normal molecular depression of freezing- 

 point." It holds for the great majority of dissolved substances 

 examined, and consists in the statement that the depression of 

 freezing-point of a one-per-cent. solution, multiplied by the 

 molecular weight of the dissolved substance, gives a constant 

 product ; it is stated of solutions containing equal numbers of 

 molecules in unit volume, assuming a close proportionality 

 between concentration and lowering of the freezing-point. 

 For an aqueous solution of nearly all organic bodies the 

 constant product is about 18*5. 



