50 M. Rose on Felspar, [Jan. 



analysis of Klaproth, M. Berzelius has found that the formula 

 was N S 3 + 3 C S 3 + 12 A S. 



The fourth species is the scarcest of all ; I have only met with 

 it in small groups of crystals in blocks of carbonate of lime, which 

 are found near Vesuvius. I have found that their chemical 

 formula was MS + 2CS + 8AS, and I have given them 

 the name of anorthite. 



I shall now describe the principal properties of these four 

 species. In the description of the crystals I have only given 

 the primitive form, the signs of the secondary planes, and the 

 principal angles. I have thought it useless to describe more 

 minutely the secondary crystals.* The figures I have given, 

 especially when compared with their signs, are perfectly suffi- 

 cient to form an exact idea of the relative situation of these 

 planes, and of the parallelism of the edges. The signs of the 

 secondary planes are given according to the method of Haiiy, 

 and I have calculated them from the angles of the primitive 

 form, which I have measured with as much exactness as possi- 

 ble, by means of spherical trigonometry, and by the parallelisms 

 of the edges. But the primitive forms of these species beina; 

 doubly oblique prisms, the theory of which is not yet perfectly 

 known, their determination depends on five measurements, while 

 the determination of oblique rhombic prisms depends only on 

 two : it is for that reason that I can only consider the angles I 

 have given as approximations not very far from the truth. 



The specific gravity has been determined with care. When I 

 had only small crystals to examine, I weighed some of them in 

 a small flask of glass, the weight of which both in water and in 

 air was subtracted from the weight of the flask containing the 

 small crystals, and weighed under the same circumstances. I 

 have given the temperature of the water I used in my experiments. 

 I have not reduced my results to the same temperature, because 

 they would be but very little altered by that reduction. 



The hardness of all the species described is less than that of 

 quartz, and differs but little from that of felspar. Albite has in 

 general appeared to me to be the hardest, and labrador the 

 softest. 



First Species. — Felspar. 



The system of crystallization is, according to M. Weiss, bino- 

 singulaire. The primitive form is an oblique rhombic prism, in 

 which the ratio of the three dimensions which are perpendicular 

 to each other, and equal to the diagonals of the section perpen- 

 dicular to the lateral edges, and to the length of one of these 



edges is — */ 13 : \f 3-13 : V 3. 



The chemical formula is, according to Berzelius, KS J + 



• The figures of crystals are not all given in the translation.— Edit, 



