536 Dr. T. Thomson on Sillimanite, 



rightjy understood the latest anatomical labours on the struc- 

 ture of muscles, on their relations with the tendons and the 

 sarcolema, 1 cannot hesitate to regard the proper current, or 

 that from the tendon to the surface of the muscle, as the most 

 simple case of the muscular current. The tendinous fibres 

 are in connexion witii the muscular fibres, whilst the sarco- 

 lema envelopes the muscular fibres only. This result is ren- 

 dered still more probable when we recollect, that the same 

 laws regulate the proper current and the muscular current. 



LXXVII. On Sillimanite. By Thomas Thomson, M.D., 

 F.R.S., Sfc.j Professor of Chemistry^ Gla'sgoixi^. 



THE name Sillimanite (from Prof. Silliman) was imposed 

 by Mr. Bowen on a mineral discovered in 1817 at Say- 

 brook in Connecticut. An account of it, together with an 

 analysis by Mr. Bowen, was published in the 8th volume of 

 Silliman's Journal (p. 114), in 1824. A few years afterwards 

 I received a specimen of this mineral from Mr. Nutall, and 

 gave it for analysis to Dr. Thomas Muir, who was at that 

 time a practical student of chemistry in my laboratory. He 

 had devoted several years of his life to mineral analysis, and 

 had acquiied much skill and was remarkable for his accu- 

 racy. He found sillimanite composed of 



Silica 38-670 



Alumina 35*106 



Zirconia 18*510 



Peroxide of iron 7*216 



99*502 



In consequence of this analysis I placed sillimanite among 

 the zirconia minerals in my Outlines of Mineralogy. The 

 specimen of sillimanite from which Dr. Muir made his ana- 

 lyses was small; so much so, that only S'Qi- grains could be 

 obtained for taking its specific gravity. This put it out of my 

 power to draw up a correct description of the mineral, or to 

 compare it with others to which it seemed to bear an affinity. 

 But five or six years ago I got from a friend in America a 

 very fine collection of specimens, partly from Saybrook and 

 partly fiom Petty Pang, both in Connecticut. The mineral 

 was in crystals, several inches long, running through granular 

 quartz in a gneiss rock. I was immediately struck with their 

 resemblance to bucholzite, a specimen of which, from Ches- 

 ter on the Delaware, I had also got from Mr. Nutall. This 

 induced me to subject it to analysis. I was surprised to find 



* Communicated by the Author. 



