the Corundum Stone , and its Varieties, See. 293 
which cannot be better described, than by comparing it to that 
preparation of fish-skin which is called shagreen. This is the 
natural effect of their peculiar texture ; for, if one of these 
pieces is broken, we perceive very plainly, that it is not of a ho- 
mogeneous texture, but is mixed with small particles of the 
substance we have already described as the matrix of corundum ; 
which mixture is often in such proportion, that the quantity of 
the latter substance is equal, or nearly so, to that of the thallite 
itself.* 
The appearance the surface of these pieces exhibits, is owing 
to the destruction, at the said surface, of the forementioned small 
particles of the matrix, which, as is well known, is very easily 
decomposed. There sometimes even remains, in the little cavi- 
ties, which are very numerous, small particles of this matrix, 
generally in a state of decomposition. I11 this case, if the 
pieces are immersed in nitric acid, a slight and momentary 
effervescence takes place; and, if this immersion is continued 
for some days, the acid then acts upon those particles of the 
matrix which are inclosed in the interior part of the substance, 
as has been already mentioned in the description of this matrix ; 
* The regularity of the form in which these crystals are found, will certainly 
- appear surprising, when we consider the immense quantity of heterogeneous particles 
which are interposed within their substance, and, consequently, between their crys- 
talline molecules, the attraction of which for each other, it would appear, must be 
thereby considerably obstructed ; but the same circumstance takes place in other 
substances, for instance, in the calcareous spar known by the name of rbomboidal 
sancl-stone of Font ainb lean. The Abbe Hauy, in the article axinite, (the tbumer- 
stein of Werner,) makes the same observation, and gives a very ingenious expla- 
nation of the circumstance. This calls to our mind the remark of the celebrated 
Dolomieu, viz. that it appears, in some cases, that a foreign substance, when inter- 
posed in a crystal, instead of obstructing its crystallization, tends rather to give it' 
a greater degree of regularity. 
MDCCCII. O q 
