MEASUREMENT AND CALCULATION 39 



by Beckmann's improved apparatus for this purpose, 1 a de- 

 scription of which will be found along with that for the 

 freezing-point determinations. 



The correction for temperature may be made, as in the 

 last case, by the application of the principle of Gay-Lussac 

 directly, or by interpolation between P b and P f , the latter 

 having been determined by the previous method. 



The expression for the Gay-Lussac principle is of course 

 the same, mutatis mutandis, as that given above: 



P --P I- 



*-T * 6 rr 5 

 -*& 



in which P T is again the pressure in millimeters at the de- 

 sired temperature T, in the absolute scale, and T b is the 

 absolute boiling-point of the solution. For this calculation 

 the boiling-point of a weak aqueous solution may be con- 

 sidered the same as that of pure water. Thus T b 373, 

 the boiling-point of water, and T = 273 -j- t, where / is the 

 desired temperature in the Centigrade scale. Making these 

 changes in the above equation, 



273 -f- 1 



Pr=P> 



373 



The method of interpolation is expressed by the follow- 

 ing equation: 



where P t is the pressure at the desired temperature, t (Centi- 

 grade), and the other symbols are the same as above. 



3. Method by observed vapor tension: As has already 

 been stated, the vapor tension of the solvent is decreased by 

 the presence of a solute. It is found that, for dilute solu- 

 tions, this decrease in vapor tension is proportional to the 



IE. BECKMANN, "Zur Praxis der Bestimmung von Molekulargewichten nach 

 der Siedemethode," Zeitschr.f. physik. Chem., Vol. VIII (1891), pp. 223-8. 



