depending on their internal Changes. 289 



of Science, vol. iv. p. 41, 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 character 

 of a black streak, a degree of hardness lower than that of cal- 

 careous 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 

 outward layers. The form remains the same, and even cleav- 

 age continues in those parts whose streak is black; nay, it 

 seems to be more easily obtained, particularly the faces parallel 

 to the short diagonal of the prism of 99° 40'. From chemical 

 considerations, Prof. Gmelin had formed nearly the same opi- 

 nion in regard to a change of composition within the crystals 

 or crystalline masses of one of the species. One of them is a 

 hydrate of the oxide of manganese, and that is the prismatoi- 

 dal manganese-ore, giving a brown streak : the other is the 

 hyperoxide, formed by loss of water and absorption of oxygen, 

 and it gives a black streak. Hitherto no crystals of the latter 

 substance have been described, that did not depend upon the 

 previous existence of the prismatoidal manganese-ore. Prof. 

 Rose of Berlin showed me small crystals, having the form of 

 right rhombic prisms, with their acute lateral edges replaced, 

 and measuring 86° 20' and 93° 40', a prism not to be found 

 in any of the known varieties of the former species. But the 

 faces not being very bright, and the measurements therefore 

 not quite decisive, inferences drawn from the observed differ- 

 ence in the angles might prove erroneous. 



The Pyramidal Manganese-ore, too, sometimes appears to 

 be a product of the decomposition of the prismatoidal species. 

 In a specimen in Mr Allan's cabinet, the pyramidal species 

 forms very distinctly the substance of elongated crystals, re- 

 sembling those of the latter ; but unfortunately the decompo- 

 sition has proceeded so far, that the surface of the original 

 crystals no longer exists, in a manner similar to what occurs 

 in several instances of malachite in the shape of blue copper. 

 We cannot guess at the chemical change taking place here, 



