62 On the elementary Particles of certain Crystals. 
which is a rhomb of 70° 39% and 109° 29’ nearly; and if 
we again split these rods transversely, we shall obtain a 
number of regular acute rtiomboids, all similar to each 
other, having their superficial angles 60? and 120°, and 
presenting an appearance of primitive molecule, from which 
‘all the other modifications of such crystals might very sim- 
plv be derived. And we find, moreover, that the whole 
mass of fluor might be divided into, and conceived to con- 
sist of, these acute rhomboids alone, which may be put to 
gether so as to fit each other without any intervening vae 
cuity. 
But, since the solid thus obtained (as represented fiz. 2.) 
may be again split by natural fractures’ at right angles to its 
axis (fig. 3.), so that a regular tetrahedron may be detached 
from each extremity, while the remaining portion assimes 
the form of a regular octohedron ; and since every thom- 
boid, that can be obtained, must adinit of the same division 
into one octohedron and two tetrahedrons, the rhomboid 
can no longer be regarded as the primitive form; and since 
the parts into which it is divisible are dissimilar, we are left 
in doubt which of them is to have precedence as primitive. 
In the examination of this question, whether we adopt 
the octohedron or the tetrabedron as the primitive form, 
since neither of them can fill space without leaving va- 
cuities, there is a dificulty in conceiving any arrangement 
in which the particles will remain at rest: for, whether we 
suppose, with the Abbé Hatiy, that the particles are tetra- 
hedral with octohedral cavities, or, on the contrary, octo- 
hedral particles regularly arranged with tetrahedral cavities, 
in each case the mutual contact of adjacent particles is only 
at their edges; and although in such an arrangement it 
must be admitted that there may be an equilibrium, it is 
evidently unstable, and ill adapted to form, the basis of any 
permanent crystal. 
More than three years have now elapsed since a very 
simple explanation of this difficulty occurred to me. As in 
the course of that time I had not discovered it to be lable 
to any crystallographical objection, and as it had appeared 
satisfactory to various mathematical and philosophical friends 
to whom I proposed it, I had engaged to make this the 
subject of the Bakerian Lecture of the present year, hoping 
that some further speculations, connected with the same 
theory, might lead to more correct notions than are at prer 
sent entertained of crystallization in general. 
At the time when I made this engagement, I flattered 
myself that the conception might be deserving of aie we 
rom 
° A 
