£o Count de Bournon’s Description of a 
interposition of these particles, in the substances into which 
they are admitted, is made in such a regular manner, that the 
homogeneity of those substances is in some measure preserved, 
if not with respect to single molecules with each other, at least 
with respect to collections of molecules with similar collections. 
Hence it follows, that the substance, although it may not possess 
that complete transparency which belongs to it when in its 
highest state of chemical purity, still retains that property in a 
very considerable degree. This is exemplified in many crystals of 
magnesian carbonate of lime, of martial carbonate of lime, of 
garnet, &c. But, for the most part, this kind of attraction, which, 
on account of its being more weak than any of the others, is 
more easily disturbed, does not admit of the forementioned regu- 
larity ; in that case, the substance which was under its influence, 
possesses a greater or less degree of opacity. 
In short, it appears, that the molecules of foreign substances, 
introduced into mineral bodies, in the above-mentioned manner, 
by the heterogeneous attraction of aggregation, do not prove 
any obstacle to the action of the crystalline attraction. The 
only effect the former mode of attraction seems to produce 
upon the latter, is to cause the form of the substance submitted 
to its influence, to approach as nearly as possible to the most 
simple forms, or even to the primitive one, belonging to it ; and, 
at the same time, to render those forms more constant. Thus, 
the magnesian carbonate of lime, and also the martial carbonate 
of lime, generally assume either the form of the primitive rhom- 
boid', or that of the lenticular rhomboid, of pure carbonate of 
lime. Thus also, the quartzose carbonate of lime, commonly 
known by the name of sandstone of Fontainebleau, constantly 
assumes the form of the muriatic rhomboid (named by Hauy 
