340 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



double that of each of them. Thus in all these cases there 

 must be a division of the molecule into two ; but it is 

 possible that in other cases the division might be into four, 

 eight, etc." (A.C.R. IV. 32). 



Molecules which can be divided into four atoms have 

 since been found in the vapours of phosphorus and arsenic, 

 whilst the vapour of boiling sulphur contains molecules 

 which are divisible into eight atoms, exactly as suggested 

 by Avogadro in the preceding paragraph. 



Molecular weights determined by means of Avogadro's 

 hypothesis. Avogadro pointed out that, on the theory of 

 equal spacing, the density of a gas is proportional to the 

 weight of its molecules. 



. 



" Setting out from this hypothesis, it is apparent that we 

 have the means of determining very easily the relative 

 masses of the .molecules of substances obtainable in the 

 gaseous state ... for the ratios of the masses of the 

 molecules are then the same as those of the densities of the 

 different gases at equal temperature and pressure" ( A.C.R. 

 IV. 30). 



This argument may be illustrated by the following 

 example. Suppose that the weights of i litre of oxygen 

 and hydrogen are i -42900 and 0*089873 gram respectively, at 

 oC. and 760 mm. ; and that there are n molecules in i litre 

 of either gas. Then : 



i '42900 grams Weight of i litre of oxygen, 



0*089873 gram Weight of i litre of hydrogen. 



Weight of n molecules of oxygen, 

 Weight of n molecules of hydrogen. 



Weight of i molecule of oxygen, 

 Weight of i molecule of hydrogen 



