58£ Mr Haidinger on the Regular Composition 



figure shows the transverse section of another regularly com- 

 pound groupe from the same country. The individual B is 

 joined to the individual A, in the plane y, according to the 

 general law; the portion M" of the face nr will therefore be 

 parallel to M 9 and coincide in one single plane with the por- 

 tion M\ which itself belongs to the individual A. In a per- 

 fectly similar manner, the individual C is attached to A, in 

 the plane z. The result is an irregular six-sided prism, having 

 three of its edges m, n, and o equal to 116° 16', since they are 

 the original edges of the prism M, which has the same angle, 

 two edges p and q ±= 127°28', and the remaining edge r equal 

 again to 116°16'. It is immaterial, whether this edge be 

 formed by the substance of the individual A, or by the meet- 

 ing of the two individuals B and C, the resultant angle will 

 be always the same. This species presents a great variety of 

 compositions of the latter kind, many of which have been de- 

 scribed and figured by Hauy, to whom we are likewise indebt- 

 ed for the discovery of its forms belonging to the prismatic sys- 

 tem, as it had been formerly considered to assume such as may 

 be derived from the regular six- sided prism. His method of 

 considering the compositions is however unnecessarily compli- 

 cated, since he assigns particular laws of decrement to the 

 planes in which two individuals like B and C in Fig. 14 will 

 meet, which jmust even vary in such species as have different 

 angles, whereas they are in fact a mere necessary consequence 

 of being attached agreeably to the general law to one indivi- 

 dual A. 



Both strontianite and witherite are usually found in regular- 

 ly compound groupes of crystals ; the former often in six-sided 

 prisms, produced by the junction of two individuals crossing 

 each other, like Fig. 5, but having the lateral angles filled up ; 

 the latter more commonly in forms like Fig. 15, the composi- 

 tion of which is shown in its projection upon a plane perpendi- 

 cular to the faces c, in Fig. 16. The angle of the prism of 

 witherite, according to Phillips, is — : 118° 30 / ; the four edges 

 at rs t and % will be each = 118° 3CK, and the other two, t>and 

 w, each = 125° 7 . Every one of the faces visible in this groupe, 

 those marked c, as well as those marked g and w, are parallel 

 to the axis of the fundamental pyramid. This yields a remark- 



