[ 397 ] 

 LIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 317.] 



February 1, 1866. — Lieut-General Sabine, President, in the 

 Chair. 



rpHE following communication was read : — 



•*- " On the Forms of Graphitoidal Silicon and Graphitoidal Boron." 

 By'W. H. Miller, M.A., For. Sec. R.S., and Professor of Mineralogy 

 in the University of Cambridge. 



Graphitoidal Silicon. 



It has been so confidently assumed that graphitoidal silicon is 

 an allotropic condition of silicon crystallized in octahedrons, that on 

 ascertaining by measurement of angles that some graphitoidal silicon 

 given me by Dr. Matthiessen was in simple and twin octahedrons, I 

 at once concluded that the substance had been wrongly named. 

 Later, however, I obtained from Dr. Percy a supply of graphitoidal 

 silicon of unquestionable authenticity. Its lustre was that of the 

 crystals I had previously examined. It occurred in small scales, 

 having for the most part the appearance of crystals of the oblique 

 system. On measurement, however, they proved to be octahe- 

 drons in which two parallel faces were much larger than any of the 

 other faces, and two other parallel faces were either too small to be ob- 

 served or were altogether wanting. One of the scales had all the 

 faces of a twin octahedron. It appears, then, that there is no reason, 

 founded on a difference of form, for separating graphitoidal from octa- 

 hedral silicon, the sole distinction being that the crystals of the latter 

 are more perfect than those of the former. 



Graphitoidal Boron. 



The forms of boron have been described by the Commendatore 

 Quintino Sella in two papers read before the Royal Academy of Turin 

 on the 4th of January and the 14th of June, 1857, and by the Baron 

 Sartorius v. Waltershausen in a paper presented to the Royal Society 

 of Gottingen on the 1st of August of the same year. They found 

 independently that the adamantine boron of Wohler and Deville, con- 

 taining a variable and not inconsiderable amount of aluminium and 

 carbon, considered by Sella as possibly a definite compound of boron 

 with aluminium and carbon with a mechanical mixture of pure boron, 

 crystallizes in forms belonging to the pyramidal system. 



Boron containing 2*4 per cent, of carbon, the boro semplice of 

 Sella, is described by him as occurring in crystals, the faces of 

 which are not so perfect as to admit of a very accurate determina- 

 tion of the angles they make with one another. The angles approx- 

 imate to some of the angles of crystals of the cubic system, but the 

 aspect of the crystals, which are usually twins, leads to the supposi- 



