xx.] VAPOUR DENSITY. 281 



which absorbs the red is more complex than the one which 

 absorbs the blue light. 



Have we the like information in the case of the passage from 

 the blue molecule to the fluted one ? I think we have in the 

 case of sulphur ; and here we approach a question of great 

 interest to us in our inquiries that of vapour densities, on 

 which something must be said before the question of sulphur is 

 discussed. 



Chemists, by their vapour-density determinations, endeavour, 

 so far as the elements are concerned, to ascertain the weight 

 of say a cubic inch or a litre of any permanent gaseous 

 element or of the vapour of any liquid or solid element; and 

 in order that relative statements may be made they take the 

 weight of one of the permanent gases, hydrogen, as the unit, 

 because they have found that this is the lightest. Thus, in the 

 case of the permanent gases hydrogen, oxygen, and nitrogen, 

 we may construct the following table, showing also the atomic 

 weights, hydrogen being taken as the unit in that case also 



Vapour density. Atomic weight. 



Hydrogen 1 1 



Nitrogen 14 14 



Oxygen 16 16 



In addition to the permanent gases, there are three other 

 bodies which readily lend themselves to such determinations ; 

 these are chlorine, bromine, and iodine. Tabulating out the 

 results, we get 



Vapour density. Atomic weight. 

 Chlorine 35 "5 35 -5 



Bromine 80 80 



Iodine 127 127 



As before, we find the vapour density on all fours with the 

 atomic weight, and represented by the same values. Hence with 

 chemists the " normal " state is that the vapour density and 



