ON ARC40N AND ITS COMPANIONS. 
85 
of the inactive gases, the only elements of which the refractivities have Ijeen 
measured directly ; the other refraction-equivalents are deduced from measurements 
Avith compounds of the elements. 
These figures have been plotted on the accompanying curve; the ordinates are 
atomic weights, and the abscissae, the refraction-equivalents. It is noticeable that those 
of helium, neon, argon, krypton, and xenon form the lower apices of each series. It 
must of course be understood that many of the numljers for the other elements are 
far from accurate ; still the diagram is interesting inasmuch as it shows clearly tlie 
Fig. 8. 
periodicity of the property, and carries with it the conviction that tlie inactive 
gases find a fitting place among the other elements. 
The comparative retardation of light caused Ijy equal numbers of molecules of 
the inactive gases compared Avith that produced Avlth air is given by the figures on 
p. 84 ; on diAuding by tlie atomic Aveiglits the retardation for equal quantities of 
matter is arrived at :— 
Helium. Nenii. Argon. Krypton. Xenon. 
0-0309 0-0118 0-0242 0-0177 0-0184 
It is remarkalile that if, for example, krypton be submitted to such a pressure 
that its density is equal to that of xenon at atmospheric pressure, it offers nearly 
equal opposition to the passage of light, a fact Avhlch is highly suggestive, Avhen 
the ultimate nature of these gases is considered. 
SomeAvhat similar relations are found with the neighbouring series of elements if 
their values are reduced to the scale of air as unity. The refractivities are far less 
certain, and the results must be accepted Avith caution; but the figures are :—■ 
