248 Prof. J. P. Cooke on the Vermiculites. 



Harvard College ; and the above numbers have been abundantly 

 confirmed in repeated analyses by various students in the same 

 laboratory. The alkalies were treated by Smith's process ; and 

 the potassium was weighed as PtK 2 CI 6 . This value, compared 

 with the total weight of the alkaline chlorides and that of the 

 chlorine, also determined, showed that the alkali in the mineral 

 was almost wholly potash, although the presence of lithium and 

 sodium was plainly indicated by the spectroscope. The water 

 was determined by igniting the mineral in coarse powder, pre- 

 viously dried at 100° C. Even after ignition the finely pulverized 

 mineral is only partially decomposed by hydrochloric acid ; and 

 in the above analysis it was decomposed by fusion with sodic car- 

 bonate. The usual tests failed to indicate the presence of fluorine. 

 , Regarding the water as basic, and as forming a part of the 

 protoxides, the atomic ratio in Sterlingite between the silicon, 

 the sesquioxide radicals, and the protoxide radicals, is 



Si : S : £ = 23-40 : 18 : 6-45, or nearly 4:3:1. 



The deviation from the simple ratio will not appear so great as 

 seems at first sight, if it be noticed that a difference of one half 

 per cent, in the amount of water would make the ratio almost 

 exact. 



The corresponding ratio in the Damourite from Wermland is 



23-15 : 1 7-78 : 6-41, or, as before, nearly 4:3:1. 



These ratios point to the general formula 



S,S.0 8 .Si 2 . 



The Damourite and Sterlingite are types of a very large class 

 of hydrous micas, which in many places are widely distributed 

 through the granitic rocks. This class of minerals has been 

 especially investigated by Professor Haughton, of Dublin, whose 

 papers may be found in Phil. Mag. S. 4. vol. ix. p. 272, and 

 Quart. Journ. Geol. Soc. vol. xviii. p. 403, also vol. xx. p. 268. 

 We cite here a few of his analyses, selected from those given by 

 Professor Dana (pages 310 and 311 of the fifth edition of his 

 ' System of Mineralogy ; ) in further illustration of the subject 

 we are discussing : — 



