MOLECULAR WEIGHT OF GASES 21 



interest at the present time in its special application to 

 solutions. 



The method is based on the application of the second 

 law, which, as applied to the process of evaporation, was 

 given (Part I, p. 19) as 



where A is the thermal equivalent of work = 1 ^ B , V the 

 increase in volume on evaporation, in cub. metres, of 

 a quantity (say i kilogram) that absorbs q calories, P the 

 pressure of the saturated vapour in kilograms per sq. metre. 

 A vapour density and therefore a molecular weight 

 determination may be obtained from this equation, since 

 the volume of the vapour itself may be taken as V when 

 the dilution of the saturated vapour is sufficiently great, 

 so that the measurement of volume is based upon that of 

 the change of vapour pressure with temperature, and the 

 latent heat of evaporation. Taking water as an example, 

 with the data : 



P 10 = 9-14 mm., P 20 = 17-363, <?io = 5 8 4cal., 

 we get, without integration, using the formula 



_ 584x10 



ATAP" 388 x 8-233 x 13-6x4 



where 13-6 is the density of mercury. Now 77 cub. metres 

 of hydrogen at 13-2515 mm. and 15 weigh 



77 x 0-08956 x ^p- 5 x 2 || = 0-114 kg. 



700 2oo 



So that the molecular weight of water vapour according to 

 Avogadro is 



M : 2 = i : 0-114. 



M= 17-6, 



which is close to the known value H 2 O = 18. 



In the same way the observations for acetie acid show 

 an abnormal molecular weight. We may perform the 

 calculation in a simpler way by using, instead of the 



