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The Meyer Molecular Weight Calculation. 



By Percy N. Evans. 



In the Victor Meyer method of determining molecular weights of 

 vaporizable substances, as usually carried out, the material is converted 

 iuto vapor at the bottom of the inner tube, the latter 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 clue 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 



