^If/^ Messrs, W, and R, Phillips on [April, 



Fig. 2 represents the primary octohedron having the edges of 

 its pyramids, though not of their common base, replaced ; and it 

 so occurs in Derbyshire. 



Fig. 3 represents the ordinary form of its crystals ; namely, a 

 tetrahedron having its angles replaced, and in the direction in 

 which it will most obviously appear to be constituted of the 

 planes m m\ m'' iv!^\ of fig. 2. Hence the tetrahedron is a 

 secondary crystal, arising from the complete replacement of all 

 the primary planes by those which truncate the edges of the 

 primary crystal. It exhibits a remarkable deviation from that 

 symmetry of form generally so apparent in substances of which 

 the primary crystal is a perfectly regular solid ; for here, although 

 the crystal is so far symmetrical that the planes are alternately 

 large and small, it deviates by so much from that more 

 perfect equality in the proportions of the secondary planes 

 observable in those belonging to regularly geometrical primary 

 crystals. 



Fig. 4 represents a crystal in my possession from Cornwall, in 

 which all the planes, except two or three, are sufficiently bril- 

 liant for the use of the reflective goniometer, and this crystal, 

 which exhibits planes that are not very common, or rather that 

 are very uncommon, might alone be assumed as affording suffi- 

 cient evidence that the primary form is not the regular tetrahe- 

 dron. Annexed are the measurements ; but it seems requisite 

 to premise, that angles taken on natural planes, however bril- 

 liant, are rarely perfectly accurate, and in proof that the following 

 are not so, it will be observed that P on P' taken on the natural 

 planes is 102° 15', but on planes of cleavage 101° 52', as before 

 stated ; but taking the latter as the basis of his calculation, these 

 measurements will assist the mathematician ; for experience 

 leads me to the conclusion that the difference between truth and 

 error lies generally within the narrow compass of 30'. 



PonP', orP^onP'" 102 15 



P on P", or P'on P'" 126 30 



Ponr 151 25 



Ponw.. 169 32 



P on w, or P' on m 141 15 



m" on w 141 15 



m! on m, or m^' on m"' 108 35 



wi" on e, or m' on e' 144 20 



m on m over o 71 35 



o on m 126 30 



/ on / 149 2 



wonr 160 54 



r on r' 133 50 



r on m 143 24 



w on c 144 10 



«onw' Ill 50 



