566 
M. A. L. O. DES CLOIZEAUX ON THE DISPEKSION OE THE 
the modifications which heat induces in the position of the optic axes and of their 
plane *, a phenomenon less compatible with the hypothesis of a primitive rhombic form ; 
but the slight transparency of the plates on which I had operated, the wide separation 
of their optic axes, which rendered the examination of the two systems of rings almost 
impossible in air, and finally the almost complete absence of dispersion, had led me to 
believe in an anomaly as the result of the always highly complex structure of the 
crystals. 
In a memoir published towards the middle of 1867, “ Sur la deduction d’un seul 
principe de tous les systemes cristallographiques,” M. Axel Gadolix suggested the 
idea that the crystals of Harmotome might probably belong to the oblique system, and 
that then their forms would no longer present any hemihedrism. 
Desirous of verifying the truth of this suggestion, which had been communicated to 
me by M. Gadolix in the month of June 1867, I had some new plates cut normal to 
the positive acute bisectrix, from very transparent crystals of the Scotch Morvenite, for 
which I was indebted to the kindness of my friend Mr. Maskelyxe ; and I have been 
enabled to establish, both in oil and in air, a very decided twisted dispersion. Thanks 
to a separation of the optic axes less divergent than that of my former plates, I have 
been able also to satisfy myself directly that the displacement impressed by heat on the 
plane containing these axes is a rotary one, quite analogous to that which I have shown 
to exist in the cases of borax and Heulandite. 
It is therefore now beyond doubt that the crystalline type of Harmotome is the oblique 
rhombic prism, and that the figures 185, 186, and 187 (pis. 31 and 32) of the atlas of my 
• Manual of Mineralogy,’ ought to be turned so as to make the plane p, which pre- 
sents the crossed striations, into the plane of symmetry of the new form. The simplest 
plan, then, is to look on the faces of the figure as forming the lateral faces m of the 
new primitive prism, one of the former faces m as the base of this prism, and the other 
as the plane h 1 truncating its obtuse edge. 
The figures 1 to 5 (Plate XXXIV.) which represent the principal combinations of the 
forms observed have the new symbols, the relations of which with the old ones are as 
follows : — 
New symbols. Former symbols. 
.a 
Des Cloizeaux’s 
Miller’s 
Des Cloizeaux’s 
notation. 
notation. 
notation. 
m . . . 
. 110 . . 
• . H 
h 1 . . . 
. 100 . . 
. m 
P • • • 
. 001 . . 
. rn 
hi .. . 
. 410 . . 
• • 
hi .. . 
. 520 . . 
• • 55 
g 1 ■ ■ • 
. 010 . . 
. . P 
o l . . . 
. 101 . . 
• • hi 
* Nou voiles recherches sur les proprietes optiques des cristaux naturels ou artificiels, inserees dans les Memoires 
presentes par divers savants a l’Institut Imperial de France, t. xviii. 1867. 
