ON THE MEIONITE. igj 



meet ^vitb no crystulline face in the meionite, the inclination not deducible 



of which does not occur amoas: the forms of feldspar, we ''°'" ' / P"* ^ 

 o r ' mif've form of 



will venture to request that gentleman, to endeavour to de- feldspar, 

 rive the figure of the dioctaedral meionite, represented fig. 

 5, from the primitive form of the feldspar, so that the angles 

 of incidence between all the contiguous faces shall agree 

 exactly; I say, exactly, for in such cases every thing de- 

 pends on precision; and he will soon convince himself of 

 the impossibility of succeeding. Now this consideration ^^'^ decides 

 alone is sufficient, to set aside for ever the idea of uniting the ^ '^ 1''f*«i»<^"' 

 meionite with the feldspar, and decides the question beyond 

 dispute. 



The author of the memoir, after having asserted, that all Difference as- 



the faces of the meionite may exist in the feldspar with the "^^^^ by Mr. 



,• ,• n y 11 1 f, . .\ Mohs to er- 



same incUnations, nnding that the angles of incidence men- roursin mea- 



tioned by Mr. Hauy differ evidently from each other, as- ®"™e- 

 cribes this difference to erroursin the goniometer, and a want 

 of agreement in the data; and he leaves it to the skilful 

 oryctometer, to remove the difficulty that this want of har- 

 mony presents. But the crystallographer finds nothing here But the form 

 to reconcile, since every thing is regular in each of the two of each is re- 



—,1 . . 1 c ^ r gular, distinct, 



crystalhne forms. The incidences of the faces have that re- and agreeable 

 lation to the primitive forms proper to each species, which to the theory, 

 calculation, agreeing with observation, indicates in a precise 

 manner by virtue of certain laws of decrement. If the angle 

 of incidence between T and P approach that between / and 

 M ; if those between O and M and O and P differ from the 

 latter, as well as from each other ; it is because the form of 

 the integrant molecules and the laws of decrement require 

 it. These laws have been determined with the more cer. 

 tainty, as there has been no difficulty in procuring well de- 

 fined crystals of feldspar and meionite. They who are fully 

 acquainted with the theory of Haiiy, and at the same time 

 know the precision, with which he applies it, see no difficulty 

 in the case. They know, that the angles are rigorously de- 

 termined by calculations founded on certain laws of decre- 

 ment, the truth of which is in turn confirmed by the agree- 

 ment of observation with calculation ; and they require no 

 jnore. 

 One example will be sufficient, to give an idea of the ac- Insanccofthe 



curacy 



