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XXII. New Researches upon the Dispersion of the Optic Axes in Earmotome and 
Wohlerite, proving these Minerals to belong to the Clinorhombic \Oblique\ System. 
By M. A. L. O. Des Cloizeaux. Communicated by Professor W. H. Miller, For. 
Sec. B.S. 
Received March 12, — Read April 23, 1868. 
We are already acquainted with a considerable number of crystals, natural as well as 
artificial, the forms of which have only been determined with precision by the examina- 
tion of their optical properties as doubly refracting bodies. Harmotome and Wohlerite 
furnish two fresh examples of this ; and they afford all the more important proof of the 
necessity of appealing to these properties, inasmuch as the crystals of these substances 
would appear certainly to be derived from a right rhombic prism, so long as we consider 
only the apparent symmetry of their external forms, or the orientation of the plane 
containing their optic axes. The different sorts of dispersion which these axes might 
be capable of presenting* are so feeble, and so difficult of appreciation on account of 
the slight transparency of Wohlerite, and the complex structure of the crystals of Har- 
motome, that the determination of these dispersions has hitherto been too incomplete 
to allow of any conclusion being drawn as to the crystalline type they might otherwise 
serve to characterize. 
It was a remark of M. Axel Gadolin, to which I shall make further allusion, that 
induced me to resume the attentive study of the phenomena of dispersion, first in Har- 
motome, and then in Wohlerite, and as a consequence to modify the crystallographic 
type to which these minerals have been in general referred. 
Harmotome. 
It is now some yearsf since I showed that simple crystals of Harmotome did not 
exist, and that those of Strontian in Scotland (Morvenite), considered as such, presented, 
in fact, a twinning formed by the interpenetration of two principal individuals. The 
particular orientation of the plane of the optic axes in each of the crystals of which the 
least complicated of such groups are composed had led me to refer their crystalline form 
to a right rhombic prism of 124° 47' ; and I had been induced to look on this prism as 
presenting a peculiar sort of hemihedrism, or rather hemimorphism, such that only the 
half of the fundamental rhombic octahedron existed, that namely formed of four faces 
parallel two and two, and lying in the same zone. More recently I had established, in 
* See my memoir “ Sur l’emploi du microscope polarisant,” Annales des Mines, 6 me serie, t. vi. 1864. 
t “ Sur l’emploi des propriety optiques birefringentes pour la determination des especes cristallisees,” 2nd 
memoir, Annales des Mines, t. xiv. 1858. 
