Law of Density -Numbers. 



25 



I have omitted from the table the data for the compounds 

 in which potassium is replaced by ammonium. The value of 

 k is here also constant, but slightly greater than that for the 

 potassium compounds. The salts here considered have ap- 

 proximately equal molecular volumes*. Thus the alums are 

 thus related : — 





Atomic 

 weight. 



9. 



Molecular 

 yolume. 



AlK (S0 4 ) 2 12H 2 



AlNa(SCg 2 12H 2 



CrK (SOJ 2 12H 2 



474 



458 

 499 



1-722 

 1-641 

 1-845 



275-3 

 2791 

 270-5 



Application of the Law to Bodies in Vapour at the 

 Boiling-point. 



If the boiling-point be s° Centigrade, and Ds be the density 

 of the vapour at that temperature, the pressure being 760 

 millim., then T = 273 + s°; and the laws of Avogadro and Gay- 

 Lussac may be expressed thus : — 



Ds a T 



)V ~~ a! X T 



On the other hand, the law of density-numbers gives 



Ds B 



DV " B'' 

 Hence it follows that 



= constant ; 



a 



or, for bodies containing only carbon, hydrogen, and oxygen, 



T xn 



= constant (see above). 



* The equality (approximate) of the molecular volumes of isomorphous 

 bodies, which was long ago recognized, may be regarded as to some extent 

 an anticipation of the law of density-numbers, and may be explained as 

 follows : — 



The constants k= — are equal according to the law. In certain cases 



^ . (a\ 



(such as the salts in the table) the relation l^j between the atomic 



weight and the density-numbers is nearly the same for different members 



of the same group ; hence it follows that the values of * (that is, the 



molecular volumes) are approximately equal. 



