f6$ Theory of Cryflallifation. 



decrement, by pointing out the angle which ferves it as a 

 point of departure. 



Other Examples of Decrements on the Angles. 

 Acute calcareous Spar (fig. 43). 



Spath calcaire rhomboidal aigii. Daubenton, Tab. Miner, 

 edit. 1792, p. 15. n. 3. Spath calcaire muriatique. De 1'Ifle 

 Chryfl alio graphic, t. i. p. 520. var. 12. 



Geomet. CharacJ. Inclination of p z ry to pu oy 78° 27' 

 47'' and to irzs ioi r 32' 13". Angles of the rhombus 

 f z ry, p qr r — 75 31' 20"; z or y - 104 28' 40 - v . In- 

 clination of the oblique diagonal drawn from p to r with the 

 edge/>«, 71° 33' 54". 



Geopiet. Propert. The angles of the rhombus are equal to 

 the refpective inclinations of the faces of the nucleus, and 

 reciprocals. 



The angles of the principal quadrilateral, or that which 

 paries through two oppofite oblique diagonals pr, ui, and 

 through the intermediate edges pu, ir, are the fame as on 

 the nucleus. 



To conceive the ftructure of this rhomboid, let us fuppofe 

 that a h df (fig. 44) reprefents the face of the nucleus 

 marked by the fame letters (fig. 4), fubdivided into a multi- 

 tude of partial rhombufes, which are the exterior faces of fo 

 ' many moleculae. Let us fuppofe farther, that the laminae of 

 fuperpofition, applied on this face, decreafe by one range 

 towards the lateral angles ah d, afd in fuch a manner that, 

 on the firft the two rhombufes h h k 1, f m i n are uncover- 

 ed, that on the fecond the uncovered rhombufes are thofe 

 traverfed by the diagonals c 0, uy, on the third thofe traverfed 

 by the diagonals s t, qz, 8cc. in which cafe the decreafing 

 edges will fucceiTlvely correfpond with thefe diagonals. 

 This law of decrement will produce two faces, which pro- 

 ceeding from the angles h, y'will rife in the form of a roof 

 above the rhombus a b df, and will meet on a common edge 

 iituated immediately above the diagonal a d, and which will 



b e 



