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In Table III. are given the values of the dielectric con" 

 stant e for several gases at normal temperature and pressure, 

 together with the values of y/ e which on the electromagnetic 

 theory of light should, for insulating media with very small 

 magnetic susceptibility, when corrections for dispersion are 

 taken into account, be equal to the refractive indices of the 

 said gases. The measurements of the dielectric constants 

 have all been executed with frequencies exceeding 10 6 , so 

 that they may be regarded as referring directly to waves 

 of infinite length. Although strictly speaking a small 

 correction for the deviations from the gas laws should be 

 applied to the values of the dielectric constants, which 

 refer to normal temperature and pressure, its magnitude 

 does not, in Ve, exceed the limits of experimental errors. 



For the purpose of comparison the values of the refractive 

 indices should strictly speaking be extrapolated to infinite 

 wave-length. Since in all cases we are dealing with normal 

 dispersion, this procedure would lead to smaller values of 

 the refractive indices. As a matter of fact, however, the 

 higher values of the refractive indices, corresponding to the 

 shorter wave-lengths, are in better agreement with the ob- 

 served values of v e than are the extrapolated values. 

 Assuming the correctness of the usual dispersion formulae, 

 and the theoretical relation between the refractive index and 

 the dielectric constant, this result would seem to indicate that 

 the determinations of the latter are affected by some constant 

 error. It would seem more probable, in view of the better 

 agreement among the values of the refractive indices given 

 by different observers, than among the values of the dielectric 

 constants, that the origin of the discrepancy is to be sought 

 in the latter rather than in the former. 



Table III. 



Gas. 



6. 



*/e. 



,, 





1-000264 

 1-000590 

 1 000547 

 1-000606 

 1-000693 

 1-000987 

 1-000993 

 1-000940 

 1-000610 

 1-000949 

 1-001384 



1-000132 

 1-000295 

 1-000274 

 1000303 

 1000347 

 1-000494 

 1-000497 

 1-000470 

 1-000305 

 1-000475 

 1-000692 



1-000139 (\ = fofr 

 1-000292 (\= 656) 

 1000270 (X=656) 

 1-000298 (X=656) 

 1-000335 (X= 589) 

 1000444 (X =671) 

 1-000648 (X =671) 

 1-000512 (X= 589) 

 1000375 (X=589) 

 1-000439 (X=656) 

 1-000711 (X=671) 



Air 



Oxygen 





Carbon monoxide 



Carbon di oxide 





Nitrous oxide 



Methane 



Ethylene 





Phil Maq. S. 6. Vol. 29. No. 160. Jan. 1915. 



D 



