154 Scientific Intelligence. 
of the systems to which they were referred; for example, garnet, 
vesuvianite, fluorite, apophyllite, zirkon, apatite, beryl, tourma- 
ine, and so on. Mallard suggested, in explanation of cases like 
those named, the hypothesis that such crystals were to be con- 
sidered as twins or compound crystals so made up as to have a 
pseudo-symmetry corresponding to a higher grade than that be- 
longing to the individuals themselves. 
The question as to the sharpness of the line dividing the crys- 
talline systems from each other, and in many cases as to which 
stem a given species really belongs, cannot be said to be de- 
cided at the present time. It is certainly possible to exaggerate 
the “ optical anomalies” and to attribute to them a morphological 
significance when they are in fact due simply to accidental causes, 
such as the internal tension produced at the time the crystal 
example of an isometric trapezohedron, was afterward referred 
to other systems by Schrauf, Mallard, Lasaulx and others, and 
i m 
methods, Other similar examples might be given. 
M. Bertrand, working from the standpoint of M. Mallard, has 
recently published some interesting contributions to this subject. 
e shows that the apparently isometric octahedrons of ralstonite 
exhibit two optic axes with an angle of about 90°. s 
also examined a series of minerals ranging from the pure lead 
phosphate, pyromorphite, to the lead. arsenate, mimetite; the 
conclusion is that while the first is truly hexagonal and has 
each having as a base the side of the hexagon; the two optic 
axes make an angle of 64° in air. Between the two extremes 
there are various intermediate compounds containing both P,O, 
and As,O,, and it is found that as the proportion of As,O, dimin- 
ishes, the angle of the optic axes also diminishes. Similar results 
have been obtained by M. Jannettaz. ese facts recall the results 
obtained by Cooke with crystals of iodide of antimony, who 
proved the existence of a uniaxial (hexagonal) and a biaxial (ortho- 
rhombic) variety, of which the latter changes into the former on 
a slight elevation of temperature. M. Bertrand has also studied 
