THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



APRIL 1874. 



XXXI. The Vermiculites ; their Crystallographic and Chemical 

 Relations to the Micas ; together with a discussion of the Cause 

 of the Variation of the Optical Angle in these Minerals, By 

 Josiah P. Cooke, Jun., Erving Professor of Chemistry and 

 Mineralogy in Harvard College*. 



[With a Plate.] 

 To Professor Tyndall, F.R.S. 



Chemical Laboratory of Harvard College, 

 My dear Processor, Cambridge, Feb. 15, 1874. 



I SHALL send you by this mail the advance sheets of a paper 

 of mine to be published in our Academy Proceedings, giving 

 an account of some new observations, which I am sure will 

 greatly interest you. The mineralogical details of the paper 

 will not probably appeal to you ; but they also have importance 

 as bearing on the optical question. You will find that I have 

 succeeded in constructing uniaxial crystals of the hexagonal type 

 from rhombic crystals of 60° and 120° angle, and have also been 

 able to show that, in some cases at least, hexagonal crystals are 

 so constructed in nature. You will find, further, that I have 

 been able to reproduce the optical conditions of a quartz crystal, 

 and in such a way as to be able to point out the mode of mole- 

 cular structure by which the result is produced. It would be 

 unnecessary to recapitulate these points, which you will find 

 fully stated in my paper, were it not that you might otherwise 

 be repelled by the mineralogical crust with which the optical 

 results are necessarily enclosed. 



* From the Proceedings of the American Academy of Arts and Sciences, 

 read December 9, 18/3. Communicated by the Author. 



PhiL Mag. S. 4. Vol. 47. No. 312. April 1874. R 



