96 Dr. Thomas Thomson on the [Feb. 



excess of silica, which I believe to be only apparent. The 

 constitution of thulite may be stated as follows : — 



3 atoms bisilicate of cerium. 

 3 atoms bisilicate of lime, 

 lj atom bisilicate of potash. 

 1 atom bisilicate of iron. 



Whether the water be an essential constituent or not is 

 not easily determined. Probably it is only mechanically 

 lodged in the pores of the mineral. The constituents of 

 thulite may be represented by the following formula:— 



3 Cer. SH3CSH 1J K S 2 + /S 2 

 Whether all these bisilicates be essential to the mineral 

 can only be determined by the analysis of purer specimens 

 than I possessed. Were the bisilicate of cerium the only 

 essential ingredient, thulite would differ from cerite by 

 containing twice as much silica. Cerite is a simple silicate 

 of cerium, while the cerium in thulite is in the state of 

 bisilicate. 



Mr. Richardson analyzed in my laboratory cyprine which 

 accompanies thulite. Its specific gravity is 3*2278, and its 

 constituents, 



atoms. 



Silica 38-80 „ 19-4 



Alumina 20-40 „ 9*07 



Lime 32-00 „ 9-14 



Protoxide of iron - - - 8*35 „ 1*8 



99-55 

 It is obviously a garnet, and analogous to grossularite in 

 its composition. No trace of copper could be detected in 

 cyprine. 



Article III. 



Analysis of Arden Limestone. By Thomas Thomson, M. LT. 

 F. R. S., L. and E., &c. Regius Professor of Chemistry in 

 the University of Glasgow. 



Arden limestone is found about six miles south-west from 

 Glasgow, and is well known and highly valued, because it 

 furnishes a lime which sets readily under water. 



