20 MOLECULAR WEIGHT AND POLYMERISM 



grams, litres, atmospheres, and in absolute centigrade 

 measure respectively. Since under the specified circum- 

 stances there is, according to Avogadro, proportionality 



between total weight and molecular 



weight, we get : 



PV 



where M is the desired molecular weight, 

 and 2 that of hydrogen (see p. 24). 



In Victor Meyer's method a cylindrical 

 vessel b (Fig. 2) of about 200 c.c. capacity, 

 ending in a long neck with a side tube af, 

 is filled with air or nitrogen, and brought 

 to a convenient constant temperature. If 

 now the weighed quantity of substance 

 be introduced into the cylinder by means 

 of the dropping arrangement d, the vapour- 

 produced expels an equal volume of air 

 which passes through the side tube of 

 and is collected in the graduated tube g 

 over water. 



The great advantage of this process is 

 that it allows of the evaporation being 

 performed at a very high temperature, 

 which heed not be exactly determined. 

 Ignorance of this temperature has no in- 

 fluence on the result, as it is the tempera- 

 ture of the graduated tube, not that of 

 the cylinder, that is needed to calculate 

 Fig. a. the molecular weight. 



2. Determination of Vapour Density by means of the 

 Second Laiv of Thermo-dynamics. 



We will mention here a second method which, though, 

 so far, of no consequence in the practical measurement 

 of molecular weights of gases and vapours, may be of 



