40 IDEAL AND ACTUAL GASES 95 



gravity remains constant, so that its expansion-coefficient 

 would have become equal to that of air, while at lower 

 temperatures it must be greater. 



41. Vapours 



With vapours developed from liquids by heat the 

 deviations from the theoretical laws are much larger than 

 with gases proper. On the whole, vapours comport them- 

 selves as gases. They obey Boyle's law approximately, 

 so that in their case, too, the pressure and volume vary very 

 nearly in inverse proportion ; and further, both pressure and 

 volume increase with rising temperature in almost equal 

 ratio, just as with gases. Dal ton's law also holds good 

 for mixtures of vapours and gases with approximate exact- 

 ness. We might, therefore, apply the kinetic theory also to 

 vapours by ascribing to their molecules, just as to those of 

 gases, a rapid rectilinear motion, and by assuming that this 

 motion increases with the heat. 



Whether and how far these assumptions suit a given case 

 can be most easily determined from the vapour-densities. As 

 vapour -density we denote the value obtained for the specific 

 gravity of a vapour when the density of air at the same 

 pressure and temperature is taken as unity. The vapour- 

 density must, therefore, be independent of pressure and tem- 

 perature if the vapour obeys Boyle's and Gay-Lussac's 

 laws as exactly as air. Observation has shown that only 

 within certain ranges of pressure and temperature can a 

 vapour-density be looked on as constant. 



In such determinations experiment has in general shown 

 that greater values of the vapour-density are found the 

 lower the temperature at which the measurement is made, 

 and that not till higher temperatures are reached do we 

 observe a constant density of the vapour, i.e. a density inde- 

 pendent of the temperature. Further, for greater values of 

 the pressure we likewise find greater values of the density 

 of a vapour, as compared with air under equal pressure ; 

 and, correspondingly, the vapour-density approaches con- 

 stancy in its value as the pressure falls off. 



