the Liuminifevous 'Ether 



237 



indirect manner. For instance, the greater the atomic weight 

 of the substance the greater ought to be the refractive power 

 or the less the velocity in the body. Unfortunately, how- 

 ever, this rule cannot be generally applied, because of other 

 conditions which prevent the free motion of the molecule. 

 But we know that in the gaseous condition the molecules are 

 less hampered than in the liquid and solid states ; the refrac- 

 tive indices of gases ought therefore to exhibit some increase 

 with the density. The following are the indices of refraction 

 and densities of the five simple gases : — 



Gas. 



Eefractive Power. 



Density. 



Index. 



Compared, 

 with Air. 



Hydrogen 



Nitrogen 



Air 



1-000138 

 1-000300 

 1-000294 

 1-000272 

 1000772 



0-470 

 1020 

 1-000 

 0-924 

 2-623 



0069 

 0-971 

 1-000 

 1-106 

 2-470 



Oxygen 



Chlorine 



And it will be seen that, with the exception of oxygen, this 

 fulfils the condition mentioned above. But the position of 

 air is most instructive : it is seen that, like its density, its re- 

 fractive power is intermediate between that of its constituents. 

 What stronger indirect evidence than this can we have that 

 the velocity in a medium is due to the density of the molecules 

 of that medium ? 



A further examination of this list is of value. We notice 

 that oxygen is an exception. Now the other three gases are 

 what chemists call monads; oxygen, on the contrary, is a 

 dyad. The molecule of oxygen, let us assume, consists of one 

 atom *, while the molecules of H, N, and CI consist of two. 

 Altering, then, the density of oxygen to one half that given, 

 we find it occupies its proper place in the list. The following 

 table exhibits this and other relations : — 



Gas. 



Density of 

 Molecule. 



Sq. Et. of 



Density. 



Sq. Et. of 

 Density. 



Eefractive Power. Air=l. 



Calculated. 



Observed. 



Hydrogen . . . 



Oxygen 



Nitrogen ... 

 Chlorine . . . 



2 

 16 

 28 

 71 



1-414 

 4-000 

 5-2915 



8-426 



1 

 2-8 



3-7 

 5-9 



•470 

 1-3160 

 1-7390 

 2-7730 



•470 



•924 



1-020 



2623 



* Other considerations lead us to consider that the molecule of O con- 

 sists of two atoms \ if this be so, we must consider O an exception. 



