Mr. Miller on Crystals found in Slags. 419 



minerals, having the form of peridot in the slags of the furnaces 

 of Sweden and Germany, (Brewster's Journal, Vol. ii. p. 130, 

 and Annales de Chemie el de Physique, Tom. xxiv.) Some 

 of the varieties, of which he gives the analysis, consist almost 

 entirely of protoxide of iron and silica, in the proportion of 

 s6 of the former, to 16 of the latter; in others he found lime or 

 magnesia, replacing part of the iron, so as always to make up 

 a quantity of oxygen equal to that of the silica. These chemical 

 constitutions may be expressed by the formulae 



Fe Fe 



Fe + S, f, +S, jyj + S, 



and all these cases are included in the chemical formula 



Fe 



M+ S where Fe, M, C, &c. 



C 



are isomorphous or plesiomorphous ; that is, may replace each other 

 with no change of form and angle, or with a slight change of 

 angle, and a preservation of the system of crystallization. 

 The mineral called Lievrite or Yenite has for its formula 



4(Fe + S) + (C+S). 



This is manifestly a particular case of the formula already given. 

 Also Be 4- S, the formula expressing the composition of silicate 

 of zinc, is analogous to it, Zc being isomorphous with Fe. We 

 might expect therefore the angles in these two cases to ap- 

 proximate to those of peridot. It will however be more con- 

 venient to compare them with the angles of the slag, than with 

 those of peridot, the former having a greater number of second- 

 ary planes. 



