179 



the upright strias on the prism, and the apex exhibits the 

 diagonal strias, and on some crystals a slight beginning of 

 the two faces which sometimes meet on the centre. The 

 left hand figure shows these striae meeting in four directions 

 to the centre, with the cross diagonals, giving signs of 4 or 

 -more faces. The prism is sometimes truncated so as to 

 form eight sides. Hauy knew of no other than these eight- 

 sided ones, with 2 or 4 summits at the apex. 



We first read of short tetraedral prisms of oxide of man- 

 ganese in Catal. de Raab. v. 2. 130, from Naila, in the mar- 

 gravate of Bareith, in Germany, and soon after of rhom- 

 boidal tetraedral prisms, neatly truncated at their extremi- 

 ties, from Ilmenau in Saxony. These of course are in Mr. 

 Greville's matchless collection; we find the latter mentioned, 

 as from Ilfield, inDr.Babington's catalogue of the collection, 

 now .belonging to sir John St. Aubin, p. 255. We are 

 glad to be possessed of British specimens from the works 

 near Aberdeen, which I have been given to understand 

 were first discovered by the Rev. Mr. Smith. My friend, 

 Mr. James Reid, among other similar favours, procured 

 me the specimen figured, some time in the year 1803. It 

 agrees exactly with the two last specimens mentioned in 

 Catal. de Raab., in which the word truncated is certainly 

 superfluous, rhomboidal prisms simply, being assuredly 

 meant. This is considered as the primitive form by Hauy. 



Oxide of manganese is used in glass-houses in small 

 quantities, to clear and discolour glass by giving up some of 



