334 Dr. Thomas Thomsons 



The atomic constituents deduced from these analyses are 

 Silica, . . . . 31-16 2-34 

 Alumina, . . . 13-28 1 

 Protoxide of iron, 0-88 0-066 



There is an excess of silica, because the grains of nacrite 

 occcur in quartz, which it is impossible to exclude com- 

 pletely. From this I am disposed to consider nacrite as a 

 bi-silicate of alumina. 



Talcite. — The specimens of talcite from Ireland are from 

 the county of Wicklow, where it occurs crystallized in 

 granite. I have received it also in scales interspersed 

 through a soft matter having somewhat the aspect of chlo- 

 rite. The locality of this last specimen is Strathbane in 

 the county of Tyrone. 



The scales are silvery white without any of the green 

 shade which characterizes nacrite. 



The crystals are in large four-sided prisms with rough 

 surfaces, the faces of which meet at an angle of about 91°. 



Lustre of crystals dull ; of scales splendent and pearly. 



Scales translucent, crystals opaque. 



Hardness 2-25. Specific gravity 2-6918. 



When the crystals are pounded they appear to be composed 

 of white silvery scales. 



The constituents of talcite are as follow : 



Atoms. 



Silica, .... 46-000 44-55 22-27 1-48 



Alumina, . . . 35-200 33-80 15.02 1 



Protoxide of iron, 2-880 7-70 1-54 



Lime, .... 9-608 1-30 0-37 



Magnesia, . . . 3-30 1-32 



Protox. of mangan., 3-944 2-25 0-45 



Water, .... 2-000 6-65 5-91 £ 



99-632* 99-55f 

 Calculating from the scales, which are obviously purer 

 than the crystals, the mineral is a sesqui-silicate of alumina. 

 The quantity of oxide of iron, lime, magnesia, oxide of 

 manganese and water vary so much in the two specimens 

 analyzed, that the} cannot be considered as essential to the 

 constitution of the mineral. 



* The crystals analyzed by Dr. Short, 

 t The scales analyzed by Mr. J. Tonnant . 



