4:>4 Dr. W. -I. Soil.-,-. 



In treating of diamond, tin question of dimorphism naturally pre- 

 M-iit itself : we proceed therefore to the consideration of graphite. 



A- the recorded values for the specific gra\ ity of graphite ditlci 

 \\-idelyfroni one another, I made a fresh determiimtion of this con- 

 stant, using the graphite ol.tained as a 1 iye-product in the manufacture 

 of carliorunduni. This \vas kindly given me by Professor Miu>. 

 The graphite was introduced into a diffusion column of methylene 

 iodide and l>en/.ene: it floated at a level of specific gravity 2*286, M 

 given liy small glass indicators. The molecular volume of graphite, 

 a- deduced from this, is .Vi'.j. This value is in close accordance with 

 that found by Petersen, who gives it as 5-3.* 



The form in which graphite occurs in nature is so closely similar to 

 a hexagonal prism, that for a long time it was referred to the rhomho- 

 hedral system, lint later observations show that it is decidedly oblique 

 or monoclinic. 



Suppose that a numlier of tetrahedral groups of atoms be placed 

 each with a trigonal axis vertical, the atoms at the base forming a 

 <ingle sheet in closest contact : then suppose a similar sheet placed 

 over the first, so that the vertical trigonal axis of each of the upper 

 tetrahedra is continuous with that of each tetiahedron of the lower 

 layer. The resulting symmetry will lie that of the oblique system, and 

 will be hemimorphic. 



It is of especial interest to compare the volumetric relations which 

 exist, on the one hand, between these hypothetical modes of packing 

 for diamond and graphite, and on the other between the atomic 

 volumes of diamond and graphite themselves. It can be shown from 

 mere inspection that the volume occupied by the packed spheres in the 

 case of diamond is to that in the case of graphite as 2 : 3, for six 

 spheres in the packing of diamond occupy the same space as four in 

 that of graphite: i.<-., if we restrict attention to two sheets only, in 

 diamond both are most closely packed, every three spheres in the lower 

 layer having three corresponding spheres in the upper, while, in 

 graphite, only the lower layer is most closely packed, and in the upper 

 but one sphere occurs in correspondence to every set of three spheres 

 below. 



Next the volume found for diamond is :V4i', and that for graphite. 

 5-25, and 3'42 : 5'25 = 2 : 3-07. There is thus a correspondence 

 between the ratio of the volumes as deduced from hyjM)thesis and that 

 obtained by experiment as exact as the nature of the case permits. 



Although graphite is not truly rhombohedral, it makes a close ap- 

 proach to the symmetry of the rhombohedral system, as it might very 

 well do from the structure here assigned to it. Nordenskiold, from 

 measurements made on graphite from Pargas, is supposed to have 

 shown that the apparently hexagonal prisms are really oblique, pro- 

 * ' Zeit*. f. Fhvstiknlisrhi- CMu-mie,' vol. 8, p. 601, 1891. 



