616 



Silica, G7.96 



Alumina, 23.20 



Lime, 3.23 



Magnesia, 0.63 



Oxide of Iron, 1.19 



Water, 2.80 



99.01 



As no washing completely removes the presence of sand from 

 this clay, which always feels gi'itty to a glass rod, and as it 

 contains comminuted mica, it could not be expected that its 

 analysis should present a precisely mineralogical result. 



From the close analogy, however, which the above figures 

 present to the composition of various felspathic rocks, as 

 analysed by Beudant, Berthier, &c., there can be little doubt 

 but that the geothetic origin of this clay is the decomposition 

 of felspar, or other allied granitic minerals. In fact the re- 

 sults approximate to the formula (taking the iron and mag- 

 nesia together). 



(AU + Si,5 -f Ca -f Mg + FeO) -f HO, 

 or, 



3(A1 -f Sia) + (Ca + Sia) + ((Mg + FeO)-f Sia) -i- HO. 



This clay is of very great economic value, and capable of 

 being used for the manufacture of the finer descriptions of 

 pottery or even of porcelain ; it has, however, hitherto only 

 been brought into use for the manufacture of crucibles, by 

 Mr. Mallet. 



The President read a paper on an improvement in the 

 double achromatic object glass. 



DONATIONS. 



Life of IV. V. Morrison, Esq., M.R.I. A. By John 

 Morrison, Esq. Presented by the Author. 



