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XXI. Upon the Quartz from Euba, and on the Biaxial character 

 of Pyramidal and Rhombohedral Crystals. By Professor A. 

 Ereithaupt*. 



IN volume cxx. of PoggendorfPs Annalen Prince Salm- 

 Horstmar alludes to a quartz from Euba, near Chemnitz in 

 Saxonv, and makes mention of its inferior hardness and low spe- 

 cific gravity, and observes also that it is optically biaxial. I 

 conceive that it is I who may claim to have been the first to 

 discover these peculiarities, for there was assuredly no one who, 

 prior to my doing so, had called attention to them. 



What first of all interested me was the felspar that is associ- 

 ated with this quartz, of which, under the name of Paradoxite, 

 I shall shortly publish a description. I accidentally found, on 

 separating the two, that this quartz is of less hardness than is 

 usually attributed to this mineral, and I was thence led to address 

 myself to M. Steeg to prepare me some slices thereof for optical 

 purposes. He on that wrote me to say that he too, when cutting 

 slices at right angles to the principal axis, found this quartz to 

 be only of the hardness of adularia. On scratching it I find, 

 however, that it is somewhat, but very little, harder than adu- 

 laria. According to my scale, its hardness is from 8 to 8 \ \ at 

 the free summits of the crystals it even attains a hardness of 8|-, 

 taking that of the Zinnewald smoky quartz, or the transparent 

 quartz of St. Gotthard, or from Graubiindten, as equal to 9. 



Not knowing whether anyone besides myself has determined 

 its specific gravity, I may state that it fluctuates between 2*578 

 and 2*632, and that it so far goes hand in hand with the hard- 

 ness, that fragments of the crystals on the ends where they 

 were seated are of the lowest specific gravity, while fragments 

 of the free ends were of the highest, yet never attaining that of 

 other varieties of quartz that I have examined. 



In the optical sections prepared for me by M. Steeg, I dis- 

 covered at once the very distinct biaxial character of the mineral. 

 The hyperbolas are not, however, black, but appear bluish. M. 

 Jenzsch, the Councillor of Mines, has met with both left-handed 

 and right-handed individuals. 



Although it is stated in the notice referred to that this quartz 

 is cloudy, this is only partially the case, for some of the crystals 

 met with are quite transparent. 



This quartz possesses also another peculiarity, being more 

 liable to be weathered than any other with which I am acquainted. 

 I cannot say how many thousand times I have noticed quartz 

 lying on the surface of the ground, but I have never met with 



* Communicated by W. G. Lettsom, Esq. From Poggendorff 's Annalen 

 der Physik, vol. cxxi. p. 326. 



