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 
'Qxvd 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 iii 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. 
Weight of unknown in grams per liter 
Molecular weight - in gram-molecules found 
This may be made clear by quoting one experiment: Standard 
used, cane sugar. Unknown, glucose. Solvent, water. 
Concentration of 
Standard 
in gram-molecules. 
Nature of change 
in length of drop 
of unknown. 
Tube I. 
1 ^ 
1 o 
0 
Increase 
Tube 2 . 
O. lO 
Tube 3 . 
0.12 
Tube 4. 
0.13 
U 
Tube 5 . 
0. 14 
Decrease 
Tube 6. 
O-15 
Tube 7. 
0.20 
Tube 8. 
0.25 
