40 Mr Connell 07i the Chemical Constitution of Brewsterite. 



Which comes near the preceding analysis, with the exception 

 of the water. With respect to this constituent, I may observe, 

 that, on igniting a portion of the mineral containing a larger 

 proportion of crystals than the specimen analysed, I got 13-359 

 per cent. ; so that, if nothing but crystals were analyzed, the 

 proportion might be still greater, although it is not likely that 

 it would be so great as shewn by Berzelius' formula. If we 

 could suppose that the mineral analyzed by Retzius really dif- 

 fered from the Brewsterite from Strontian, only in lime and 

 soda being replaced by strontia and baryta, this mineral would 

 afford a good illustration of the doctrine of replacement. 



According to the results of my own researches, however, the 

 formula which best expresses the constitution of Brewsterite 

 from Strontian, is 



-— S^ -f- 4 A S' -f 6 A ^r ; which gives, 



Silica, 54-58 



Alumina, 17'543 



Strontia,) ,,-,- 



Baryta, | •••• 1*717 



Water, 1316 



100- 

 Or, if we suppose the proportion of the strontia and baryta 

 to be 2 atoms of the former to 1 of the latter, the constitution 

 will then be, 2 atoms bisilicate of strontia + 1 atom bisilicate of 

 baryta -f- 12 atoms tersilicate of alumina + 6 atoms of water. 



The title of this mineral to be viewed as the first instance of 

 strontia occurring as a silicate, seems to be established. 



A Series of Barometric Observations. By W. Galbraith, 

 Esq., A. M. Communicated by the Author. 



X BEG leave to communicate to you a few more barometric ob- 

 servations, to which I alluded in a former paper about two 

 years ago. In that I calculated the height of Benlomond by a 

 process depending on several tables which I had computed for 

 the purpose, involving the dew-points at the two places of ob- 



