288 Mr Haidinger on the Parasitic Formation of Minerals, 



namely, gradually formed in the crystal of the phosphate, as- 

 sume such positions, that two of their faces are parallel to the 

 sides, and two to the terminations of the six-sided prism ; the 

 two remaining ones will be perpendicular to the lateral and 

 the terminal faces. The direction of them appears distinctly 

 in the annexed sketch of the transverse section of a crystal, as 

 indicated by the lines parallel and perpendicular to the sides 

 of the hexagon. On breaking the prisms, we obtain fractures 

 situated like the line abed, Plate IV. Fig. 4. which I have 

 sometimes observed giving a clear demonstration of the actual 

 composition of the crystal in the manner described. General- 

 ly the portion adjoining the centre, as it were the axis of the 

 prism, consists of perfectly compact lead-glance, provided the 

 original species has entirely disappeared ; then comes a more 

 or less considerable stratum of thecleavable mass, which, how- 

 ever, is often wanting ; and then a coating of a coarser tex- 

 ture. From the mere arrangement of the particles, it is placed 

 beyond a doubt, that the crystals of the sulphuret have not 

 been formed in moulds from the phosphate. They are pro- 

 bably the product of the gradual decomposition of the latter 

 by sulphuretted hydrogen, an explanation which was first pro- 

 posed by Rome de ITsle, even though the real chemical com- 

 position of the rhombohedral lead-baryte was then unknown, 

 to account for the appearances which he so well describes. 

 Cristallog. vol. iii. p. 400. Such a decomposition easily takes 

 place even at the common temperature of the atmosphere, if 

 a stream of sulphuretted hydrogen is allowed to pass over the 

 brown variety from Huelgoet, reduced to powder. Both the 

 phosphate and the chloride of lead are decomposed, sulphuret 

 of lead is formed, while the oxygen, phosphorus and chlorine 

 are carried off, forming hydrophosphoric and hydrochloric 

 acid and water. 



VI. — Changes in Minerals containing Manganese. 

 The ores of manganese have not yet been sufficiently exa- 

 mined, in regard to their chemical composition, to allow us 

 clearly to establish the changes that take place in what may 

 be rightly supposed the decomposition of the prismatoidal man- 

 ganese-ore. I have shown on another occasion, Edin. Journ. 



