Douhle Selenates of the Series E2M(Se04)2,6H20. 



249 



intermediate character to the habits of the crystals of the potassium 

 zinc and caesium zinc salts. 



An increase of density accompanies a rise in the atomic weight of 

 the alkali metal, and it is greater for the replacement of potassium by 

 rubidium than for that of the latter by caesium in the proportion of 

 5 : 4. 



The molecular volumes show a similar progression in the order of 

 the atomic weights of the alkali metals, but the replacement of rubi- 

 dium by caesium is marked by the greater change. 



The distance ratios (topic axes) indicate an extension of the distance 

 separating the structural units in all three axial directions, the maxi- 

 mum being along the symmetry axis. 



All these rules relating to the exterior morphology of the crystals 

 are precisely analogous to those previously shown to apply to the 

 double sulphates. 



The optical elHpsoid (indicatrix) is found to rotate about the sym- 

 metry axis, when the atomic weight of the alkali metal is raised. The 

 amount of rotation is twice as great when caesium replaces rubidium as 

 when the latter replaces potassium. 



The mean refractive index (mean of all three indices) of the rubi- 

 dium zinc salt is intermediate between the mean indices of the other 

 two salts. 



The greatest change accompanies the replacement of rubidium by 

 caesium. 



The double refraction diminishes at an increasing rate as the atomic 

 weight of the alkali metal increases. 



It follows from the above that the axial ratios of the optical indi- 

 catrix for the rubidium zinc salt are intermediate between those of the 

 other two salts, the second replacement being accompanied by the 

 greater change. 



The optic axial angle of the rubidium zinc salt is almost exactly the 

 mean of the widely different optic axial angles of the potassium zinc 

 and caesium zinc salts. The optic axes have a common plane, and 

 the bisectrices are similarly situated, subject to the rotation of the 

 whole ellipsoid already specified. 



The optic axial angles show a progressive change on heating the 

 section plates — namely, a slight increase in the case of the potassium 

 zinc salt, a slight decrease in the case of the caesium zinc salt, and an 

 almost complete indifference to change on the part of the rubidium 

 salt. 



The whole of the specific and molecular optical constants of rubidium 

 zinc seleriate are intermediate between those of the potassium and 

 caesium zinc salts. The molecular refraction and dispersion increase 

 at an accelerating rate with the rise of atomic weight of the alkali 

 metal. 



