^ the Ores of Manganese. 137 



/ 



manganese of commerce has been for this reason facetiously called 

 by the French le savon des verriers, or le savon du verre. 



There can be no doubt that pyrolusite should form a species 

 of its own, if we only attend to the marked differences in its 

 hardness, strength, &c. from all the rest. As yet, however, its regu- 

 lar forms are unknown. For some time past I have endeavoured 

 to collect specimens either of crystals or cleavable masses of this 

 substance, but have not succeeded in getting any fit for measure- 

 ment. Mr VON LEONHARD kindly communicated to me some crys- 

 tals from Tiefe Kohlenbach, near^Eiserfeld, in the province of Sie- 

 gen, possessing the form Fig. 24., with uneven surfaces, and yield- 

 ing a black streak. They form a coating on the reniform shapes of 

 the uncleavable manganese-ore. Professor GUSTAVUS ROSE had 

 obtained a similar specimen from the same source ; and by some 

 approximate measurements, but which were far from decisive, we 

 found the inclination of a on , over the small face b, to be 

 = 86 20'. The faces of the horizontal prism d, did not admit 

 of measurement at all. There exists cleavage parallel to a and b, 

 but not very perfect. Among the forms of manganite, there is no 

 prism, parallel to the axis, which even comes near the one here 

 mentioned, though the approximation at the angles be ever so 

 rude ; and the crystals may be therefore considered as the actual 

 type of the species of pyrolusite, which is likewise the^opinion of 

 Mr ROSE. I have observed crystals of the form of manganite, 

 yielding the characteristic brown streak only in the interior por- 

 tions of the crystals, while that of the exterior strata is black. 

 This may be the result of one of those changes of substance, the 

 form remaining the same, which are recorded in a preceding part 

 of this volume ; it may, however, be also one of those curious in- 

 stances, where two' species, of different forms, enter, as it were, 

 into a regular composition with each other, as in felspar and al- 

 bite, disthene and staurolite, and others ; many of which 1 have 



VOL. XI. PART I. S 



