MH. A. PI TUTTOX OX A COMPARATIVE CRYSTALLOGRAPHICAL STUDY 
with that ol this salt, shows the brushes triiicred accordino; to the foliowiujy colour 
scheme : — 
blue I red 1 st M.L. red i blue. 
The obtuse morphological axial angle ac is to the left in this scheme. 
It is thus evident that the optic axial angle is greater for red than for blue, and 
determinations for C-light and F-light gave the angles for these respective wave¬ 
lengths 47° 34' and 4G° 3', values so close to those derived from the determinations of 
2Ha and 2Ho as seen in bromnaphthalene {aide the table) as to confirm the accuracy 
of the amount of dispersion of the axes shown in the table. 
Dispersion of the Median Lines. —The actual circle readings obtained during the 
benzene immersion observations indicated that the first median line is so dispersed 
that it lies nearer to the morphological axis a for blue than for red by about 7-8' 
(between C- and F-light). 
Effect of Rise of Temperature on the Optic Axial Angle. —Determinations of the 
apparent angle in air at 75° indicated that the 2 E increases 25' for 60° rise of 
temperature. 
C.ESiuM Magnesium Selenate, Cs^Mg(Se0^).,,6H;0. 
A determination of magnesium in 1‘2312 gramme of crystals gave 0’2077 gramme of 
magnesium pyrophosphate, which corresponds to 3'64 per cent, of magnesium. The 
theoretical percentage of this metal is 3’51. 
Goniometry. 
Ten excellent crystals of this salt, selected from three of the best crops, were used 
in the goniometrical measurements. 
Habit; flattened 23rismatic. 
Axial angle : /3 = 73° 43'. 
Ratio of axes : a : h : c = 07314 ; 1 : 0‘4960. 
Forms observed : b = { 010 } oo R oo ; c = ) 001 }oP; p = {HO} go P; 
q = {011} Pod; o' = {Til} + P; / = {201} + 2P oo . 
The accomjoanying table re^iresents the results of the measurements. 
The cresium salt of the magnesium grouj) exhil)its precisely the habitual configura¬ 
tion which has been shown to be characteristic of all the Ccosium salts of the sulphate 
grouf) of this series and of cmsium zinc selenate. It invariably 2 )resents large q (clino- 
dome) faces, giving the jorismatic ap 2 )earance to tlie crystals, narrower (often very 
narrow) c basal plane faces, and for the end faces those of the jjiTinary 2 )rism qj and of 
the hemi-pyramid o', sometimes one and sometimes the other j^i’t'dominating. The 
clinopinacoid h is generally present as a strij), but the ortho 2 )inacoid a has not been 
observed. The faces of the orthodome r' were usually small, but in one of the best 
crops were frecpiently found much more j)romlnently develo})ed. 
