214 



ELEMENTARY CHEMICAL MICROSCOPY 



be in contact with an atmosphere, the vapor pressure of which is 

 greater than their own. Condensation will take place on these 

 drops and they will increase. The others, alternating with 

 them, will have a vapor pressure greater than that of the adjoin- 

 ing air spaces; these drops will evaporate and thus decrease. 

 Hence, there is a distillation from the drops of one series to 

 those of the other series. By measurement we can tell which 

 drops increase and hence ascertain which solution has the 

 smaller vapor pressure. If the solvent is identical in both cases 

 and if the solutes are non-volatile, the solution with the smaller 

 vapor pressure will have the greater concentration of molecules 

 and vice versa." 



A series of tubes must be made in which the strength of the 

 standard solution has been systematically varied in small frac- 

 tions of a gram-molecule per liter. A tube is thus obtained in 

 the series where there is little variation in the lengths of the 

 drops of known and unknown or where there is change in the 

 character of the variation, say from an increase in length to a 

 decrease in length. It is evident that the molecular concentra- 

 tion of the unknown must correspond to that of the known solu- 

 tion at this point. 



Molecular weight = 



Weight of unknown in grams per liter 

 Concentration in gram-molecules found 



This may be made clear by quoting one experiment: Standard 

 used, cane sugar. Unknown, glucose. Solvent, water. 



