98 Mr HAIDINGER on the Parasitic Formation 



by sulphuretted hydrogen, an explanation which was first pro- 

 posed by ROME' DE L'!SLE, 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 *. Such a 

 decomposition easily takes place even at the common temperature 

 of the atmosphere, if a stream of sulphuretted hydrogen is allow- 

 ed to pass over the brown variety from Huelgoet, reduced to 

 powder. Both the phosphate and the chloride of lead are de- 

 composed, sulphuret of lead is formed, while the oxygen, phos- 

 phorus 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 manga- 

 nese-ore. I have shewn on another occasion f, that the regular 

 forms belonging to that species, are properly found in specimens 

 having a brown streak, a degree of hardness equal or superior to 

 that of fluor, and a specific gravity contained between the limits 

 of 4.3 and 4.4, but that the same form is often united to the cha- 

 racter of a black streak, a degree of hardness lower than that of 

 calcareous spar, and a specific gravity often approaching to 4.7. 

 These latter varieties frequently form a coat round the former ; 

 and a crystal whose internal particles afford a brown streak, may 

 give a black streak when the experiment is tried with the out- 

 ward layers. The form remains the same, and even cleavage con- 



* CristaUographie, vol. iii. p. 400, 



( Edinburgh Journal of Science, vol. iv. p. 41. 



