584 
M. DES CLOIZEAUX ON AMBLYGONITE AND MONTEBBASITE. 
In some specimens the interior or real angle of the axes is 90°, and in oil is equal to 
102° round each of the two bisectors. In other specimens an interchange seems to 
occur between the bisector of the acute and that of the obtuse angle, their respective 
signs remaining otherwise the same ; thus in two plates taken from the same fragment, 
108° 24' (the angle containing the negative bisector. 
r— 97° 5'{ „ „ positive „ 
These different variations in the separation of the optic axes are of course associated 
with greater or less irregularities in the form of the coloured rings, and of the hyper- 
bolas which traverse them ; hut the one invariable feature is seen in the different kinds of 
dispersion which I have already remarked on in the specimens from Hebron. Direct 
measurement indicates %<v to be the proper dispersion of the axes containing the 
negative bisector. This dispersion, strongly marked in one of the systems of rings of 
which the hyperbola examined at 45° to the plane of polarization is bordered by bright 
blue in the inside and by red on the outside, is on the contrary very weak in the other 
system, in which the borders of the hyperbola present, both on the outside and in the 
inside, blue tints that are hardly distinguishable. 
Examined in the plane of polarization, the transverse bars of the central ring of the 
two systems give evidence of the existence of horizontal dispersion by the symme- 
trical arrangement of their borders ; while these borders, on the other hand, exhibit 
colours that are distinctly more decided in the one system than in the other, but 
the difference is the inverse of that which is seen in the position 45° to the plane of 
polarization. 
The characters which have been described thus show that in Montebrasite (formerly 
Amblygonite) of Montebras, as in that from Hebron, there coexists with the horizontal a 
well-marked inclined dispersion, a combination peculiar to crystals of the triclinic system. 
About the positive bisector the inclined dispersion is united to the twisted dispersion. 
Montebrasite from Montebras, and that from Hebron, contain only lithia ; hence they 
communicate a colour of a brilliant carmine-red to the flame of the blowpipe, in which 
they melt with great readiness into a white enamel with a distinct decrepitation. In a 
closed tube they disengage a water which corrodes the glass. Their loss under the 
action of the heat is observably greater than that of Amblygonite, while on the other 
hand their density is lower. M. Pisani has found for the composition of the laminated 
specimens from Hebron and of the green variety from Montebras : — 
Hebron. Montebras. 
Fluorine 5-22 3-80 
Phosphoric acid 46-65 47-15 
Alumina 36-00 36-90 
Lithia 9-75 9-84 
Water 4-20 4-75 
101-82 102-44 
Specific gravity 3-03 (Pisani) 3-01 (Pisani) 
2-999 (Damour) 2-977 (Damour) 
