Notices of some Minerals. 341 



are A1S 2 , all the others being accidental impurities. If 

 that supposition be admitted, Weissite will be merely an 

 impure variety of the Nacrite described at the beginning of 

 these notices. 



VII. — MAGNESITE. 



In page 178, vol. i., of my Mineralogy, I have given an 

 imperfect description of this mineral, which occurs in the 

 marl limestone round Paris, and which was first noticed by 

 M. Alex. Brogniart, and analyzed by Berthier. As I had 

 never seen a specimen of the mineral, my description was 

 necessarily imperfect. But, in a box of minerals, which I 

 received last October from Dr. Holmes, there was a small 

 specimen, labelled anhydrous deweylite? This specimen 

 agreed exactly with magnesite, as far as I could judge from 

 Berthier's description. I may add (as Berthier has for- 

 gotten to mention it) that the specific gravity is 2-0964, and 

 that the mineral is opaque. It was analyzed by Mr. J. 

 Tennant. I shall place Berthier's analyses of magnesite 

 beside it, that the reader may judge how far they agree. 



99-5 100-2 

 The French specimen is the purest ; but the formula for 



both, leaving out what a comparison of the two analyses 



shows to be impurities, is Mg S 3 + 2Aq. 



The mineral is, of course, a hydrous tersilicate of 



magnesia. 



Article III. 



On the Causes of the Motion of the Blood in the Capillary 

 Vessels. By Dr. Poiseuille.* 



When the globules of the blood in the capillaries of the 

 mammiferse are examined, they are found to possess diffe- 



* Bibliotbeque Universelle, Nov. 1835.. 



