1824.] Albite, Labrador, and Anorthite. 51 



3 A S s . If we calculate from this formula the proportion of the 

 constituent parts, we find that 100 parts of felspar contain 



Silex 65-94 



Alumina ] 7-75 



Potash 16-31 



Observations. — Although felspar is common, yet it is rarely- 

 met with in such perfectly brilliant crystals as are necessary 

 for measurement by the reflecting goniometer The collection 

 of minerals in the University of Berlin, which is extremely rich 

 in crystals of felspar, does not contain a specimen the crystals of 

 which could have been measured by that instrument. The best 

 for that purpose with which I am acquainted are the cry stals of 

 glassy felspar from Vesuvius, and I have measured the angles of 

 some which differ a little from those given by M. Weiss. I have 

 found for instance the obtuse incidence of the lateral planes of 

 the primitive to be 119° 18', and that of the base of the primitive 

 upon one of the lateral planes 1 12° 14^*. These measurements, 

 however, I did not consider as sufficiently exact to ground my 

 calculations upon. 



I was rather surprised by what I found to be the specific gravity 

 of the felspar of Baveno. I had weighed it several times, and 

 1 had chosen not only the hemitrope crystals which are so fre- 

 quently met there, but also simple crystals which are perfectly 

 pure, and did not appear to contain any foreign substance. The 

 results I obtained were always the same, and I was induced to 

 think that the composition of the crystals of Baveno differed from 

 that of felspar, and that since the crystallisation was perfectly 

 the same in both, some isomorphous principle was replaced by 

 another. I, therefore, analyzed a crystal from Baveno. In 

 fusing it with carbonate of potash, and in treating it in the 

 usual manner, I found the proportion between the silex and 

 the alumina exactly the same as that which exists in common 

 felspar; so that though I had not separated the potash, I 

 thought I had no reason to suppose the composition different 

 from other felspars. 



Second Species.— 'Albite. 



The primitive form of albite is a doubly oblique prism (Plate 

 XXV), figs. 1,2. The planes M and T of which are inclined 



_- _ 1 /»_■_—»—_—_--_ . 1 - — _ _• --_ - - ______ 



. perpendicular to the planes M and T is an oblique- 

 angled parallelogram, fig. 2, the obtuse angle of which is divided 

 by a plane / produced by a decrement of two rows along the 



• Mr. W. Phillips gives fur the same angles 119° 20' and 11 2° 5'. 



E2 



