1880.] NATURAL SCIENCES OF PHILADELPfflA. 319 



altered micaceous gneiss, in which the muscovite has become 

 margarodite, and the orthoclase become white and crumbling. 

 Such mica exfoliates slightly when heated, is uniaxial, fusible 

 with difficult}^, and might be called Hijdr-ohiotite for convenience. 

 It frequentl}' occurs enclosed in cr^'stals of margarodite, or in 

 muscovite passing into margarodite. 



Hygroscopic Properties. — In the determination of water in its 

 different states in PhiladeliDhite, the principal difiicult}^ was on 

 account of the strong hygroscopic properties possessed by the 

 mineral. After the water has been expelled by heat or desiccation, 

 it is rapidly absorbed again from the air, if exposed. Upon the 

 balance, the dried mineral gains so rapidly that it was found 

 necessary while weighing to enclose it in corked tubes. It appears 

 to absorb water with the avidit}^ of chloride of calcium. Even 

 when enclosed in watch-glasses clasped together and standing in 

 the closed balance-case with dry CaCL, it gains decidedly in weight. 

 The following experiments by Mr. Haines illustrate its h3^gro- 

 scopic properties : 



G-rammes. 



(1) Weight of undried mica, .9935 

 Heated at 100° C. for li hours, .9616 

 Weight after standing; in balance-case with CaCL 



for 3 days, .9915 



Reheated for 3 hours at 100° C, .9580 



Left on balance 20 minutes. Gain in weight, .0070 



Left on balance 2 hours. Total gain in weight, .0085 



(2) Weight of undried mica, 1.1280 

 Heated at 100° C. for 3 hours, 1.0965 

 Left in balance-case with CaCl, for 1 hour, 1.1175 

 Left in balance-case with CaCI.^ for \\ hours, 1.1230 

 Left in balance-case with CaClj for 2^ hours, 1.1250 

 Left in balance-case with CaCl,^ for 2 days, 1.1260 



(3) Undried mica heated at 100° C. for 6^ hours. 



Loss, 2.49 p. c. 



On standing in balance-case with CaCL for 2^ 

 daj'S, regained nearly the whole of its original 

 weight (all but 2 milligrammes). Again 

 heated at 100° for 3 hours, loss of weight, 3.09 p. c. 



These experiments, showing that nearly the total amount of 

 hygroscopic water is regained even in the presence of such an 



