474 DR T. THOMSON'S Analysis of some Minerals. 



This mineral was from Traversella in Piedmont. I got it 

 under the name of Convex Rhomboidal Pearl- Spar. Unfortu- 

 nately the rhomboids are not susceptible of measurement. This 

 puts it out of our power to determine whether its shape be pre- 

 cisely the same with that of the first variety or not. 



The constituents of this variety were found to be, 



Carbonic acid, . , . 47. 



Lime, 29.072 



Magnesia, . . . . . 14.140 



Protoxide of iron, . . 13.392 



Alumina, 0.720 



104.324 

 This is equivalent to 



8 atoms carbonate of lime, 



5 atoms carbonate of magnesia, 



3 atoms carbonate of iron. 



We may consider it as composed of two different minerals, 

 united together. The first mineral is common carbonate of 

 lime ; the second a compound of 



5 atoms carbonate of magnesia, 

 3 atoms carbonate of iron. 



This compound we may represent thus, 



1 atom carbonate of lime, 



1 atom carbonate of (f magnesia + f iron). 



This specimen constitutes a peculiar species, not hitherto no- 

 ticed by mineralogists, which may be termed Calcareo-carbonate 

 of Magnesia-and-Iron. 



