igQ ON THE MEIONITE. 



nucleus has here. Mr. Mohs has chosen this form among 

 all those of the feldspar, as being the simplest, and best 

 calculated to lead to the object he had in view, the reduction 



; of the forms of the mei'onite to those of the feldspar. On 



the other hand fig. 4 represents the dioctaedral meionite. It 

 now remains, to compare these two forms together ; and this, 

 I must apprise the reader, is the essential point of the dis- 

 cussion. 



These do not Mr. Mohs, having measured the angle of incidence be- 



*S'^*^' tween T and P in the crystal of feldspar fig. 3, found, that 



it agreed manifestly with that between / and M in the dioc- 

 taedral meionite, fig. 5. In fact we find by calculation a 

 difference of 21' only between these angles; the first being 

 111° 28', the second 111° 49'. But on proceeding with the 

 comparison, instead of evident resemblances, we have no- 

 thing but striking differences. For instance, the angle of 

 incidence between I and each of the two sides M, M, is the 

 same ; while that between T and M, fig. 3, differs 8° 32' from 

 that between T and P, since it is of 120°. On the other 

 hand, the angle between O and M is of ll6° 21', and that 

 between O and the face opposite to P is of 124° 15; yet 

 each ought to be of 11 1° 49' for the form of the meionite to 

 agree with that of the feldspar. It is the same with all the 

 other faces, that can arise on the edges or angles of the face 

 T. There are none similarly situate but the faces analo- 

 gous to M and S, fig. 5, the angles of incidence of which 

 are 90° and 135°. But this is only an accidental resemblance 

 owing to the symmetrical position of the lateral faces in the 

 two nuclei; otherwise we might say, that feldspar is an ore 

 of oxide of tin, since the same angles of incidence are found 

 on the prism of the latter. As to the essential difference 

 between the" summits of the crystals of feldspar and those 

 of the/prystals of meionite, this is owing, as has already 

 been Observed, to a want of symmetry in the positions of 

 the bases of the nucleus with respect to the lateral faces, 

 which does not allow the faces produced, in consequence of 

 the laws of decrement, to preserve that regularity with re- 

 spect to each other, which appears in the terminal faces of 

 the dioctaedral meionite. 



The mesonit'; So far then from acknowledging with Mr. Molis, that we 



meet 



