MOLECULAR WEIGHT OF GASES 23 



This relation occurs most simply in the case of . the 

 elements. The ratio of their molecular weights, i. e. of 

 their gas densities, must either be the same as that of their 

 atomic weights, or, if the molecules are built up out of 

 different numbers of atoms, be in integral relation to those- 

 Exact proof of this is rendered difficult by the fact that 

 Avogadro's law is only exactly true for infinite dilution. 

 A recent extrapolation l in that direction leads to the 

 conclusion that the densities of hydrogen, nitrogen, and 

 oxygen on infinite dilution are in the ratio 



1-0074 : 14-007 : 16, 



whilst Ostwald gives for the atomic weights : 

 10032 : 14-041 : i6. 



2. Confirmation and Testing of Atomic Weights. 



The determination of atomic weights by purely chemical 

 means is attended with an uncertainty, which may be 

 noticed in the history of the atomic weights, and may 

 be illustrated in the following way. Choosing as unity 

 H = i, or rather O = 16 as better suited for analytical 

 purposes, the analysis of e.g. beryllium oxide with 36-3 

 per cent, beryllium is not conclusive as to the atomic 

 weight of beryllium, but leaves it dependent on the formula 

 of the oxide. If it is regarded as BeO we get 

 Be : O = 36-3 : 63-7 = x : 16 ; 

 x = 9-1 ; 



whilst if Be 2 3 is chosen and both formulae have had their 

 supporters it follows that 



2 Be : 36 = 36-3 : 63-7 = zx : 3 x 16; 

 x= f X9-i. 



The decision here and in corresponding cases has in the end 

 rested on determinations of molecular weight. The deter- 

 mination is not possible for the oxide on account of its 



1 D. Berthelot, Compt. Rend. 126. 954. 



