358 WILLIAM IIAMSAY. 



Hélium. Néon. Argon. Krypton. Xénon. 



0,1238 0,2345 0,968 1,419 2,364. 



Thèse figures are proportional to tlie quantity (,v. — 1); and on divi- 

 ding by the densities of tlie gases, or wliat cornes to the same tliing, 

 their atomic weiglits, a séries of figures is obtained whicli expresses tlie 

 retardation of liglit by equal quantifies of matter. 



ïliese quantifies are given in the following short table. 



0,1238 

 4 



0,2345 

 20"" 



. 0,968 

 40 



1,449 

 81,76 



2,364 

 ^28 



= 0,0309 



= 0,0172 



= 0,0242 



= 0,0177 



= 0,0184 



It will be noticed that thèse figures may be divided info two classes; 

 hélium and argon are both higher than néon, krypton, and xenoii, and 

 the last three yield almost the same nuraber. Whatever be the reason 

 of tins, it appears that the molecular structure of the last three offers 

 less opposition to the passage of light than that of the first two; it is 

 curions to notice such a variation of behaviour of gases which on ail 

 other grounds must be pronounced to possess a similar character. 



Soniewhat similar relations are found with neighbouring séries of 

 éléments of tlieir refractivities, ascertained, for the raost part by mea- 

 surements of that of their compounds, are reduced to the scale of air 

 as unity, The numbers are far less certain, and must be accepted with 

 caution; but they are: 



