OP ARTS AND SCIENCES. 35 



III. 



THE VERMICULITES : 



THEIR CRYSTALLOGEAPHIC AND CHEMICAL RELATIONS TO 



THE MICAS; 



TOGETHER WITH A DISCUSSION OP THE CAUSE OF THE VARIATION 

 OF THE OPTICAL ANGLE IN THESE MINERALS. 



Bt Josiah p. Cooke, Jr., 



Erving Professor of Chemistry and Mineralogy in Harvard College. 



Read, Dec. 9, 1873. 



Introduction. — In the American Journal of Science (VII. 55, 1824), 

 T. H. Webb described a mineral from Millbury, near Worcester, Mass., 

 which has since been a mineralogical curiosity on account of its singular 

 reaction when heated. The mineral consists of " small foliated scales dis- 

 tributed through a steatitic base." " When heated, it exfoliates prodig- 

 iously, the scales opening out into long worm-like threads made up of the 

 separate folise. Exfoliation commences at 500° to 600° Fahr., and takes 

 place with so much force as often to break the test-tube in which the 

 mineral may be confined. Before the blowpipe it fuses at 3.5 to a gray- 

 ish-black glass." It was named by Webb, as he says, " from the Latin 

 vermiculor, ' I breed worms.' " The hardness of the mineral is 1-2, 

 the specific gravity 2.756, the lustre talcose, and the color grayish, 

 somewhat brownish. It was analyzed by Crossley, who " separated 

 with great care from the base the scaly mineral, which is the true 

 vermicuUte," and his results were as follows : — 



99.92 101.03 



