60 Mr Haidinger on the Regular Composition 



the one be changed by a revolution of 180°, no change in the 

 appearance will be produced, unless the combinations possess 

 a particular hemi-pyramidal character, in which there is not 

 an absolute symmetry round the pyramidal axis, owing to the 

 enlargement of the alternating faces of isosceles, or half the 

 number of faces of scalene eight-sided pyramids. 



Pyramidal forms, in general, are the rarest among those 

 which the products of inorganic nature affect, and few species 

 among those which present them, have been observed in regu- 

 lar compositions ; but, wherever these occur, they are very 

 frequent, and in some instances exceedingly varied ; so much 

 so, that their existence has not a little contributed for a long 

 time to conceal the true form from the inquiries of minera- 

 logists. 



The only law of regular composition hitherto observed in 

 pyramidal Tin-ore is, that two individuals joined will meet in a 

 face of the pyramid P, the angles on the terminal edges of 

 which are = 133° 2&, on the lateral edges = 67° 59'. It oc- 

 curs, however, in a great many varieties of forms, and vari- 

 ously repeated on the similarly situated parts of the indivi- 

 duals. This law was first ascertained by Lermina, and de- 

 scribed in the second edition of the Crystallography of Rome 

 de l'lsle. Hairy has given a good drawing of the most sim- 

 ple case of it as found in nature ; but the most exact and 

 ample description of the various appearances which it pro- 

 duces in different crystals of the substance, has been given in 

 Mr W. Phillips's excellent paper on the Oxyd of Tin.* No- 

 thing new can here be mentioned on the subject ; but it will 

 be interesting to compare these compositions with what is 

 found in other species, particularly by means of preserving 

 for the figures of all of them a parallel position. 



The fundamental pyramid itself, when composed parallel 

 to one of its faces, would produce a form like Fig. 1, Plate III. 

 analogous to the twins of the regular octahedron described 

 among the regular compositions of the tessular system. "f 



• Trans. Geol. Soc. vol. ii. p. 336. 



t Edin. Journ. of Science, vol. i. p. 56- PI- III. Fig. 5.- 



