139 
Tue Mever Monecutar WEIGHT CALCULATION. 
By Percy N. Evans. 
In the Victor Meyer method of determining molecular weights of 
raporizable substances, as usually carried out, the material is converted 
into vapor at the bottom of the inner tube, the iatter being kept at a con- 
stant temperature at least twenty degrees above the boiling point of the 
substance by keeping a suitable liquid in the outer jacket steadily boiling. 
When the vaporizing occurs, a quantity of air equal to the increase in vol- 
ume is forced out from the upper part of the inner tube, through the lateral 
capillary, and collected over water in a eudiometer. It is assumed that 
this increase in total volume is the volume of the vapor; it would be more 
correct.to deduct from this volume that of the original liquid, but failure 
to do so introduces an error of usually only one part in two hundred or 
more, and this may be considered negligible in view of unavoidable ex- 
perimental inaccuracies. 
In passing from the heated tube to the eudiometer the temperature 
of the air changes to that of the room, with a corresponding volume 
change; it is assumed that the vapor would undergo the same change in 
volume if reduced to the same temperature without condensation, since 
all gases and vapors show a nearly identical behavior with changes in 
temperature. 
After passing into the eudiometer the air is saturated with water 
vapor. If the air in the inner tube at the beginning of the experiment 
is already saturated with moisture at room temperature no change in 
the degree of moistness results, and hence no change in volume due to 
this cause. It would therefore be incorrect in calculating the volume of 
air under standard conditions to deduct from the observed barometer read- 
ing the tension of aqueous vapor. 
If, on the other hand, the air in the apparatus had been perfectly 
dry its volume is increased by its becoming saturated with moisture, and- 
this should be allowed for by deducting the tension of aqueous vapor from 
the barometer reading. 
If, lastly, the air in the apparatus at the beginning of the experiment 
