50 MOLECULAR WEIGHT AND POLYMERISM 



increase of pressure of p mm. of mercury applied on the 

 right-hand side, and let h be the difference in level of 

 the aniline produced, a the ratio of the cross section of the 



reservoir to that of the tube, so that - is the difference in 



a 



level produced in the reservoirs ; then 



13-56 > = k (1-022 0-998) H 0^998, 



a 



so that if a is very large 



or 



^ = 5 6 5P> 



i. e. in the most favourable case i mm. of mercury will 

 make 565 mm. difference in level. 



With such apparatus the following result was obtained 

 for cane sugar (C 12 H 22 O n ) : 



7-31198 grams in loco grams of water gave at o 6 , for 

 which p = 4-62 mm,, a value of A_p = 0-00178 mm. 



If we determine the molecular weight of sugar from 

 these data we get : 



= 342 



in exact agreement with the formula C 12 H 22 O n . 



Molecular weight determination by lowering of solu- 

 bility. As previously remarked, every means of separation 

 between solvent and dissolved body implies a possible 

 measurement of molecular weight. If the separation is 

 effected by evaporation, then if the solvent alone is volatile 

 we arrive at the law already given ; if on the other hand 

 only the dissolved body is volatile i. e. a gas then 



