66 On the elementary Particles of certain Crystals. 
former, and will be more or less obtuse according to the 
degree of oblateness of the primitive spheroid. 
It is at least possible that carbonate of lime and other 
substances, of which the forms are derived from regular 
thomboids as their primitive form, may, in fact, consist of 
oblate spheroids as elementary particles. 
It deserves to be remarked, that the conjecture to whick 
we are thus led by a natural transition, from consideration 
of the most simple form of crystals, was long since enter- 
tained by Huyghens*, when treating of the oblique refrac- 
tion of Iceland spar, which he so skilfully analysed. The 
peculiar law observable in the refraction of light by that 
crystal, he found might be explained on the supposition of 
splieroidical undulations propagated through the substance 
of the spar, and these he thought might perhaps be owing: 
to a spheroidical form of its particles, to which the dis- 
position to split into the rhomboidal form might also be 
ascribed. 
By some oversight, however, the proportion of the axes. 
of such an elementary spheroid is erroneously stated to be 
1 to 8; but this is probably an error of the press, instead 
of 1 to 2°8, for I find the proportion to be nearly 1 to 2°87. 
In fig. 15, F is the apex of a tetrahedron cut from an acute 
rhomboid similar to fluor spar, and the sections of two 
spheres are represented round the centres F and C. I is: 
the apex of a corresponding portion cut from the summit 
of arhomboid of Iceland spar, as composed of spheroids 
having the same diameter as the spheres. In the former,. 
the inclination FCT of the edge of the tetrahedron to its 
base is 54° 44’; in the Jatter, the inclination ICT is 26° 15’; 
and the altitudes FT, IT are as the tangents of these angles 
1414 to 493:: 2°87: 1, which also expresses the ratio of 
the axis of the sphere to that of the spheroid, or the pro- 
portional diameters of the generating ellipse. 
Hexagonal Prisms. 
If our elementary spheroid be on the contrary oblong, 
instead of oblate, it is- evident that by mutual attraction 
their centres will approach nearest to each other when their 
axes are parallel, and their shortest diameters in the same 
plane (fig. 13.) The manifest consequence of this struc- 
ture would be, that a solid so formed would be liable to 
split into plates at right angles to the axes, and the plates 
would divide into prisms of three or six sides with all their 
angles equal, as occurs in phosphate of lime, beryl, &c. 
* Huyghenii Op. Relig. tom, is Tract, de Lumine, p.70. 
It 
