322 A CrystaUographical Study of certain Double Selenates. 



" A Comparative Crystallographical Study of the Double Selenates 

 of the Series E,M(Se0 4 ) 3 ,6H 2 0— Salts in which M is Mag- 

 nesium." By A E. Ttjtton, B.Se., E.B.S. Beceived April 29, 

 —Bead May 23, 1901. 



(Abstract.) 



This memoir on the magnesium group of double selenates, in which 

 B is represented by potassium, rubidium, and caesium, is analogous to 

 that which was presented to the Society in March 1900 concerning 

 the zinc group. 



The conclusions derived from the study of the morphological and 

 physical properties of the crystals of the three salts are generally 

 similar to those arrived at from the study of the zinc group. There is 

 observed a uniform progression with regard to every property in 

 accordance with the order of progression of the atomic weights of the 

 three alkali metals present. That is to say, the constants of the 

 rubidium salt are generally intermediate between those of the 

 potassium aud caesium salts. 



The magnesium group has, however, proved particularly interesting, 

 inasmuch as the progressive diminution of double refraction, according 

 to the rule which has now been established for this series of double 

 sulphates and selenates, leads in the case of caesium magnesium 

 selenate to such close approximation of the three refractive indices 

 that the crystals of this salt exhibit exceptional optical phenomena. 

 This includes dispersion of the optic axes in crossed axial planes at the 

 ordinary temperature, the uniaxial figure being produced for wave- 

 length 466 in the blue; and the formation of the uniaxial figure for 

 every wave-length of light in turn as the temperature is raised, the 

 attainment of uniaxiality for red lithium light occurring at the 

 temperature of 91°. As the life-history of the salt terminates at 100°, 

 owing to the presence of water of crystallisation, this substance 

 exhibits the property of simulating uniaxial properties at some 

 temperature within its own life-range for every wave-length of light, 

 while still retaining the general characters of monoclinic symmetry, 

 including slight dispersion of the median lines. In this respect it 

 resembles to a truly remarkable extent the analogous sulphate, which 

 the author has shown to possess like peculiarities, but it is even more 

 striking than the sulphate, as the dispersion is much larger. It is 

 interesting to observe that these optical properties of caesium mag- 

 nesium selenate could have been predicted, given the constants of the 

 potassium salt and the rules of progression established for the double 

 sulphate and for the zinc group of double selenates. For the double 

 selenates resemble the double sulphates so closely that in general it 



