1866.] Prof. W. H. Miller 07i Graphitoidal Silicon and Boron. 11 



And hence the specific gravity of mercury at 62° F., as compared with 

 water at the same temperature, will be 13*569 nearly. 



Again, if we assume the correctness of Regnault's Table of the absolute 

 dilatation of mercury, and also that of Despretz's Table of the absolute dila- 

 tation of water, we shall find that the weight at 32° F. of a volume of mer- 

 cury weighing 13590-86 grs. at 62° F. will be 



13590-86xl-00298=13631-361 grs. 



Also the volume at 4° C, or 39°"2 F., of a volume of water weighing at 

 62° F. 1001-62 grs., will be 



i001-62xl-0011437=1002-766 grs. 



Hence the specific gravity of mercury, according to the French method of 

 determining it, will be 



13631-361 

 1002766 



A determination by Regnault gives 13*596. 



These two results agree very nearly with one another ; and this agree- 

 ment tends not only to verify the correctness of Regnault's determination, 

 but to show that Regnault's Table of the dilatation of mercury, and Des- 

 pretz's Table of the dilatation of water, agree together ; a remark that had 

 been previously made by Dr. Matthiessen in a paper which he recently 

 presented to the Society. 



II. " On the Forms of Graphitoidal Silicon and Graphitoidal Boron.^^ 

 By W. H. Miller, M.A., For. Sec. R.S., and Professor of Mi- 

 neralogy in the University of Cambridge, Received February 1, 

 1866. 



Graphitoidal Silicon. 



It has been so confidently assumed that fgraphitoidal silicon is an allo- 

 tropic 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 authen- 

 ticity. 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 

 octahedrons 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 observed 

 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 differ- 

 ence of form, for separating graphitoidal from octahedral silicon, the sole 



c2 



