METHOD OF DETERMINING MOLECULAR WEIGHTS. gg 



one liand, to tlic ](nveriiig of the freezing point and, on the otlier, to 

 the rise of the ])oilini>' point. The former relation, which was 

 established by lîaoult in a purely empiric:d way (Compt. rend., 87 

 [1878]; 103 [1886]; 104 [1887]; 105 [1887]; 107 [1888]; 

 Ann. Clihn. Plnjs. [6], 15 [1888]), liad already been deduced by 

 Guldberg- {Compt. rend., 70 [1870]) from tlie mechanical theory of 

 heat. Moreover, van't Hoff (Zeitsch. plujsik. Chcm., 1[1887]) theore- 

 tically deduced the laws of the lowering of the freezing points of 

 solutions from the relation which exists ljetv;een vapour pressure and 

 osmotic pressure. 



The determination of, the other relation, the rise of the boiling 

 point also affords us a means of ascertaining the molecular weight of 

 the dissolved substance, inasmuch as it is possible to determine the 

 temperatures at which the solvent and tlie solution exhibit equal 

 vapour pressures, instead of ascertaining their vapour pressures at an 

 equal temperature. This method was lirst practically carried out by 

 Beckmann. 



The calculation of the molecular weight is made by taking into 

 consideration the projiortionality which exists between the difference 

 of pressure and the difference of tempeniture. î^ow, since the solutions 

 subjected to examination are all very dilute, the equation 



p—p' _ n 

 p ~ N+n 



may assume the following form witliout causing any material dif- 

 ference in the results : 



p—p' _ n 

 ^} ~ A' ■ 



The difference of tem])orature prodnred in tlie l)oiling point by the 

 dissolution of one gram-molecule of the substance in one gram mole- 

 cule of the solvent and which corresponds to the above pressure- 



