424 Prof. Young on the Boiling-points, Molecular 



ratio of the specific or molecular volumes of two substances 

 at " corresponding " pressures if we know their critical tem- 

 peratures and pressures and their boiling-points under low 

 pressures, provided, as is usually the case, the saturated 

 vapours at these low pressures behave as normal gases. 



For the molecular volume of the saturated vapour of any 

 substance under such conditions will be given by the equation 



»=22-32x^x™°, 

 273 p 



where T is the boiling-point on the absolute scale of tempera- 

 ture, and p is the pressure. So also the molecular volume of 

 the saturated vapour of any other substance at the " corre- 

 sponding " pressure p'= 



T_ 760 

 273 X / " 



The ratio of the molecular volumes of the saturated vapours 

 will therefore be 



v < - T / x y 



The ratio of the molecular critical volumes will be the same ; 

 and as the critical volumes apply equally to the gaseous and 

 liquid states, the ratio of the molecular volumes of the liquids 

 at corresponding pressures will also be given by the equation 



In the special case of the haloid derivatives of benzene, the 

 critical pressures of which are equal, the equation becomes 



y _ V ; 



and this relation Prof. Masson has shown to apply not only 

 to this group of compounds, but also to other groups of haloid 

 compounds of hydrocarbon radicals. 



So far the experimental results extend only to the liquid 

 state, but I hope to be able to determine the densities of the 

 saturated vapours of the benzene derivatives within sufficiently 

 wide limits to test the accuracy of the relation for the gaseous 

 state also. 



When the critical pressures of two substances are not the 



v V T p r 



same it would seem improbable that the relation —. = —- = — x — 



r v' V T p 



would really hold good ; for it is quite certain that for such 



