260 Dr T. THOMSON'S Account of the 



Opaque. 



Dull. 



About the hardness of calcareous spar. 



Specific gravity, 2.2055. 



Does not effervesce in acids. In this respect my specimen dif- 

 fers from that analysed by HISINGER, which effervesces weakly 

 in acids, when in masses ; but strongly when in powder. 



Before the blowpipe it fuses with difficulty on the edges (like 

 table-spar), into a colourless, semitransparent glass. With bo- 

 rax it fuses easily, and forms an amethyst-coloured glass. 



Its constituents, as determined by Mr R. MITCHELL, are as 

 follows : 



Silica, 55.200 



Lime, 34.284 



Alumina, 4.160 



Protoxide of iron, . . 2.896 



Moisture, 3.400 



99.940 

 The constituents, as determined by Mr HISINGER, are as fol- 



lows: 



Silica, 43.368 



Lime 38.433 



Protoxide of Manganese, 4.962 



Protoxide of Iron, . . 1.434 



Carbonic acid, . , . 11.368 



99.565 



If the carbonic acid was combined with lime, as it must have 

 been, it would require 14.46 grains, reducing the lime in combi- 



