24 MOLECULAR WEIGHT AND POLYMERISM 



non-volatility, but can be made with the chloride. The 

 molecular weight of the latter, according to the respective 

 assumptions, would be 



-Wi'eci, = 9-1 + 71 = 80-1, Jf Be ci 3 = I x 9-1 -4- 106-5 = 120. 



The former number was found by Nilson and Petterson l 

 and the atomic weight 9-1 so finally settled. The only 

 change possible would be a halving, &c., of this atomic 

 weight, so that the method gives a maximum value, below 

 which there is neither chemical nor physical reason for 

 going, and which has, moreover, received confirmation in 

 another way (p. 26). 



3. Nature of the Elementary Molecules, and Polymer ism. 



Poly atomicity. Hydrogen. One of the leading results 

 of molecular study of the elements is the fact that their 

 molecules usually consist of more than one atom. Thus, 

 the two facts that prove this for hydrogen are : 



The density of hydrochloric acid is 18-25 referred to 

 hydrogen, and it contains 2-74 per cent, of hydrogen. 



Since the densities, according to Avogadro's law, are in 

 the ratio of the molecular weights, we conclude from these 

 data that the ratio between the amounts of hydrogen 

 present in the hydrogen and hydrochloric acid molecules is 



i : 18-25 x-^ = a : i. 

 100 



Thus there is twice as much hydrogen in a hydrogen 

 molecule as in one of hydrochloric acid, and as the latter 

 cannot contain less than one atom of hydrogen, there must 

 be at least two in the hydrogen molecule. Such reasoning- 

 has in no case, so far, led to the necessity of assuming more 

 than two atoms in the hydrogen molecule, so it may be 

 regarded as diatomic, with the symbol H 2 . For that reason 

 hydrogen, with M= 2, is usually chosen as unit for molecular 

 weight determinations. 



1 See also Rosenheim and Woge, Zeitschr.f. anory. Chem. 15. 283. 



