DR T. THOMSON'S Analysis of some Minerals. 471 



long ago shown by Dr WOLI.ASTON, is 106 15', instead of 105 

 5' as in calcareous spar. 



Colour white, with a shade of red. 



Translucent. 



Hardness rather exceeds that of calcareous spar. 



Specific gravity 2.815. 



Dissolves slowly in muriatic acid, unless heat be applied. 



A careful analysis of this mineral gave its composition as fol- 

 lows : 



Carbonate of lime, . . 54.256 

 Carbonate of magnesia, 47.428 

 Alumina, . . . . r ..,,. 0.680 

 Protoxide of iron, . ... 1.692 



104.056 



Excluding the alumina and oxide of iron as accidental, the 

 mineral is obviously a compound of 



1 atom carbonate of lime, . .-:.,.. 6.25 

 1 atom carbonate of magnesia, . . 5.25 



11.5 



| 



I have found several specimens of the magnesian limestone 

 from Sunderland exactly similar in composition. This is the 

 case also with several specimens of Dolomite which I have ana- 

 lyzed. 



This variety of brownspar, then, constitutes a well defined 

 species, to which the name of Magnesio-carbonate of Lime may 

 be given. It has been long distinguished as peculiar in minera- 

 logical systems. Yet nothing is more common than to find in 

 cabinets varieties of it arranged under the name of brown spar. 



