xv THE MOLECULAR THEORY 359 



whilst the presence of ^ volume of hydrogen and \ volume of 

 chlorine in i volume of hydrogen chloride proved the divisi- 

 bility of both hydrogen and chlorine. 



2H 2 + 2 -> 2H 2 



I Vol. 2 Vols. 



2NH 3 -> N 2 + 3 H 2 



2 vols. i vol. 3 vols. 



H 2 + C1 2 -> 2HC1 



I VOL I 0/. 2 ZX7/.T. 



In each of the cases studied by Avogadro, H 2 , O 2 , N 2 , C1 2 , 

 the molecule proved to be divisible into two parts : his sugges- 

 tion that subdivision into four or eight parts was possible has 

 been realised in the molecules, P 4 , As 4 , Sg ; cases are also known 

 in which the molecule contains only a single atom and cannot 

 be subdivided, e.g. Hg, Zn, Cd, Ar ; in the case of ozone, O 3 , 

 the molecule contains three atoms. 



D. CANNIZZARO'S EXPOSITION. 



Avogadrds Hypothesis states that 



" If the temperature and pressure are the same, equal volumes 



of different gases contain equal numbers of molecules." 



From this hypothesis, which is strictly true only at zero 

 pressure, it follows that the molecular weights of different gases 

 are in the same ratios as their densities. Caimizzaro, in 1858, 

 selected the half-molecule or atom of hydrogen as the standard 

 for all molecular and atomic weights. The molecular weight of 

 a gas is then equal to its density relative to hydrogen = 2, or, in 

 the modern system, relative to oxygen = 32. The atomic weight 

 of an element is the smallest weight which is found in the mole- 

 cular weights of its volatile compounds. 



The molecular and atomic weights deduced from vapour- 

 densities are not exact. But the ratio of atomic weight to the 

 equivalent (the valency of the element) is a whole number, and 

 it is therefore easy to deduce the exact atomic weight from the 

 equation 



Atomic weight = valency x equivalent. 



After the atomic weights have been determined, the formula 

 of any volatile compound can be deduced from its vapour density 

 and percentage composition by using the method illustrated 

 on p. 353. 



