354 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



therefore be monatomic, a conclusion that is in agreement 

 with the rather scanty data for the vapour density of 

 mercury and its compounds (pp. 349 and 351). The same 

 method of investigation has shown that the inert gases 

 HELIUM, NEON, ARGON, XENON, and KRYPTON, discovered by 

 Sir William Ramsay in atmospheric "azote," are mon- 

 atomic gases; their atomic weights are therefore identical 

 with the molecular weights calculated from their densities. 



Accurate determination of atomic weights. Valency. 

 The vapour-density of an element or compound can as a 

 rule be determined only approximately. Moreover, even an 

 accurate measurement must be corrected for compressibility 

 before it can be used to determine the true molecular weight 

 of a volatile substance. It is therefore evident that the 

 method described above does not lend itself readily to the 

 accurate determination of atomic or molecular weights. 



On the other hand the equivalent of an element or 

 compound can often be determined with extraordinary 

 accuracy by gravimetric analysis : thus, the figures given by 

 Stas for the weight of silver chloride produced from 100 

 parts of silver show an average deviation from the mean of 

 only i part in 40,000. 



I32'84i 132-840 132-846 132-842 

 132-843 132-849 132-848 13^845 = Mean. 



It is therefore important to consider how these accurate 

 analyses may be utilised in drawing up a table of atomic 

 weights based upon Avogadro's hypothesis. 



Dalton's Atomic Theory finds expression in tables of 

 combining weights or equivalents, based solely upon the 

 chemical data supplied by analysis, e.g. : 



H=roo8, C = 3'oo, O = 8 , 8=16-03, = 35-46, etc. 

 Avogadro's hypothesis, on the other hand, finds its 



