418 



Mr. Miller on Crystals found in Slags. 



The planes M, are striated parallel to their common inter- 

 section, q, t, X, have not hitherto been observed in the natural 

 minerals, s, t are too imperfect to be measured accurately. 



Some of the crystals from Merthyr, exhibiting the secondary 

 planes represented in Fig. 2, are transparent, of a dark olive 

 green colour, and possess a high refractive power, the minimum 

 deviations of yellow rays through MM" being 49'',32' and 52'',48'. 

 Opaque crystals from the same place having a semi-metallic 

 lusture, and apparently containing a mechanical mixture of foreign 

 matter, invariably take the form of Fig. 1. The crystals from 

 Birmingham, which also have the form of Fig. 1, are black, and 

 cleave readily in a direction perpendicular to the planes M, M. 



In chemical composition, peridot is found to be a silicate 

 of magnesia and of protoxide of iron, that is, a compound of 

 the magnesia and the protoxide of iron with silica, so that the 

 oxygen of the two first is equal to that of the latter. It is 

 worthy of notice, that Professor Mitscherlich has already found 



