M. DES CLOIZEAUX OX AMBLYGONITE AND MONTEBEASITE. 
581 
doubly oblique form of this mineral can, in fact, only be established in an incontestable 
manner by the obliquity of the cleavages p and m on the plane perpendicular to that in 
which the optic axes lie, because this obliquity is absolutely incompatible with a 
clinorhombic form, in which the plane of symmetry would be perpendicular to the 
plane of the optic axes and to the bisector of their acute angle, in which, furthermore, 
the twisted dispersion would be very strong. 
Heat changes the angle of separation of the optic axes by diminishing it in a 
sensible degree. A plate only slightly oblique to their plane gave in air for the 
red rays 
2E=86° 26' at 14°C ; 82° 16' at 120° C. 
Between these two temperatures one of the axes moved about 2^- times faster than the 
other, the first having advanced towards the bisector by 2° 56', while the second had 
only moved 1° 14' ; the bisector, therefore, had moved from its initial position, and had 
been displaced by 0° 51' towards the same side as the axis which had made the least 
progress*. 
When I published my first notice, “ Sur les proprietes optiques birefringentes et sur 
la forme crystalline de l’amblygonite,” Comptes Eendus des Seances de 1’Academie cles 
Sciences, tom. lvii. p. 357, I had only been able to place under the polarizing micro- 
scope a very small plate parallel to the more difficult cleavage m of the Penig variety ; 
consequently I was only able to examine the optical phenomena in this case in a very 
incomplete manner. Having lately again examined a plate nearly perpendicular to the 
edge of the intersection of the two unequal cleavages p and m, I could perceive that 
around the negative acute bisector the plane of the optic axes is in the acute angle 
p 971=74° 16', that the dispersion proper to the axes indicates § > v, and that there is 
also a very observable twisted dispersion combined with a slight inclined dispersion. 
In oil the separation of the optic axes is about 2 H=56° 30' (in white light). These 
characters belong to Amblygonite (formerly Montebrasite) of Montebras. The only 
difference that the specimens from Penig present, as compared with those from 
Montebras, is that their physical constitution is more homogeneous, and that they are 
not traversed, as the latter are, by twin plates. 
Amblygonite melts easily in a simple alcoholic flame, without decrepitation and with 
a slight bubbling, into an opaline white blebby glass. Before the blowpipe it commu- 
nicates to the flame a reddish-yellow coloration, owing to the soda and lithia which it 
contains. This coloration enables one immediately to distinguish it from Monte- 
biasite, which contains only lithia, and consequently only yields a flame of a rich 
carmine tint. 
The composition of Amblygonite is established by the following analyses : — 
* A plate of Montebrasite from Hebron heated to about 100° exhibits no change in the appearance of its 
coloured rings. 
