424 
CRYSTALLOGRAPHY. 
cated variety hitherto obferved among cryftals of this 
kind. To form an idea of its ftrudture, let us fuppofe 
that bpyr, fig. 55, reprefents a fedtion of the nucleus 
AR, fig 56, made by a plane perpendicular to the pa¬ 
rallelograms GOAD, B O A II, and fubdivided into a 
multitude of fmall parallelograms, which are the analo¬ 
gous ledtions of fo many molecules. Here the fide yr, 
fig- 55> which is the fame fedtion of the cutting plane as 
GOAD, is greater than it ought to be in regard to the 
fide cr, fig. 55, which is the fame fedtion as BO AH, 
fig. 56; but thefe dimenfions are fuited to thofe of the 
fecondary cryftal, and here occafion no difficulty, becaufe 
we may fuppofe that the primitive form has been ex¬ 
tended mere in one direction than in another; for this 
form is only a convenient datum for the explanation of 
the (trudture, and the cryflal confifts merely in an affern- 
blage of fimilar molecules ; fo that it is tire dimenfions 
of thefe moleculce which remain invariable. 
This being premifed, we fiiall find, by comparing the 
figures 54 and 55 ; ill, that the plan c fabnklih, fig. 54, 
and its oppofite, which correfpond to mn, dg, fig. 55, 
are parallel to two of the planes of the nucleus, viz. 
G O A D, B R NH, fig. 56, and. eonfequently do not re- 
fult from any law of decrement; 2d, that the plane 
Poff/N, and its oppofite, fig. 54, which correfpond to ao, 
eg, fig. 55, are alfo parallel to two of the planes of the 
nucleus, viz. BO AH, RGDN, fig. 56; 3d, that the 
plane on km and its oppofite, fig. 54, which correfpond to 
on, eg, fig. 55, refult from a decrement by two ranges 
parallel to the ridges AO, NR, fig. 56 ; 4th, that the 
plane cfgk and its oppofite, fig. 54, which correfpond to 
my, dc, fig. 55, refult from a decrement by four ranges 
parallel to the ridges GDBH, fig. 56; 5th, that the 
plane ctYg and its oppofite, fig. 54, which correfpond to 
fy, ca, fig. 55, refult from a decrement by two ranges 
parallel to the fame ridges G D, B H, fig. 56, which de¬ 
crement takes place on the, other fide of thefe ridges. 
Tt may be feen by what has been already faid, that de¬ 
crements different in their meafure give rife to planes 
fiinilarly fituated, fuch as onkm and cfgh, fig. 54, which 
is a confequen.ee of the particular figure of the moleculae. 
With regard to the faces of the fummit, the heptagon 
pG tedez, fig. 54, is fituated parallel to the bafe BRGO, 
fig. 56. The enneagon srPonbez, fig. 54, is produced 
jn confequence of a decrement by one range on the 
angle OB R, fig. 56, or parallel to the diagonal OR; 
which decrement does not attain to its full extent, and 
leaves fubfifting the neighbouring heptagon parallel to 
the bafe BRGO. It may be readily conceived, after 
what has been faid on the pofition of the diagonal OR, 
why the line ez, fig. 54, which feparates the two large 
faces of the fummit, is fituated horizontally, fuppofing 
that the planes have a vertical pofition. 
The trapeziums dafe, hpGG, refult from a decre¬ 
ment by one range 011 the ridges GO, BR, fig. 56. The 
facet deba, fig. 54, arifes from a decrement by two 
ranges, parallel to the ridge BO, fig. 56. With regard 
to the other facet ABz£, which has the fame pofition 
as the preceding, in regard to the oppofite part of the 
crvftal, it refults-from an intermediary law by a range of 
double moleculae on the angle OBR, fig. 56. The 
rhombufes bclh, klsu, fig. 57, reprefent the horizontal 
fedtions of two of thefe double moleculte, taken in the 
fame range, and whofe relation to the reft of the affort- 
liient will become fenlible by comparing the rhombufes 
in queftion with thofe marked by the fame letters in fig 55. 
The cryftals of this variety are fubjedt to a change of 
dimenfions, which is, that the faces pG tedez, fabnklih, 
and their oppofites, which are at right angles to each 
other, are ftretched out, in the direction of their breadth, 
in fuch a manner that they exhibit the appearance of a 
quadrilateral rectangular prifm, the fummits of which 
would be formed by the faces fituated towards the ridges 
PN, FA This variety is found in opake cryftals, and 
of a whitifh, yeliowill:, and fo me times reddilli, colour. 
in the granites of Auvergne, and of different countries. 
There are fome of them in groups and fome fingie, but 
the latter are uncommon. 
The ingenious author of the above theory of cryftal- 
lization, having fince revifed it attentively with a view 
of publilhing a treatife on mineralogy, found that he had 
fallen into an error with regard to the cryftalline forms 
ot iron ore. He had before announced, that the cryftals 
of volcanic iron were not fegments of the regular octahe¬ 
dron, as believed, and that the difference between their 
angles and thofe of thefe fegments was more than twelve 
degrees. He has fince found that they have for their 
primitive'form a rhomboid fomewhat acute, in which 
the angle of the fummit is about eigluy-feven degrees. 
He thought alfo, with all other naturalifts, that the 
cryftals of the iron of the ifland of Elba were derived 
from the cubic form ; and he had referred to that form 
that of the cryftals of Framont in dodecahedra compoled 
of two right pyramids incomplete. He had always been 
ftruck, however, with a kind of Angularity pi-efented 
here by the cubic form which performed the function of 
the rhomboid ; that is to fay, that it was neceffary to 
fuppofe an axis paffing through the two oppofite folid 
angles, which were to be confidered as the fummits, 
and the laws of the decrement which took place around 
the fummits were diti'erent from thofe which related to 
the lateral angles. 
But he was (till more furprized, wjien, having lately 
tried to apply theory to a variety of the iron of Framont, 
which he had not before examined, he obferved that it 
was neceffary to fuppofe it to refult from a decrement 
by twenty rows on the inferior angles of the primitive 
cube, in order to have refults correfponding with obfer- 
vation. This law, though perfectly admiflible, deviated 
fo much from the ufual laws, that it infpired the author 
with fufpicions refpeCting the cubic form itfelf, and, by 
the Help of goniometry, he meaftired, for the firft time, 
on the cryftals of the ifland of Elba, the mutual inci¬ 
dence of the primitive faces; whereas he had before 
confined himfelf to meafuring that of the faces produced 
by the decrements, either among themfelves or on the 
primitive faces, having no idea that there could be any 
uncertainty in regard to a form which prefented in fo 
fenlible a manner the appearance of a cube; and the 
more fo as the facets by which it was modified prevented 
the difference from bemg obferved. He perceived that 
this form was a real rhomboid, fimilar tothat of volcanic 
•iron; and in that calc, this law, which had appeared fo 
lingular on the hypothelis of a cube, gave place to a 
fimple law, and every thing, by this means, was reduced 
into proper order. In regard to the varieties of the iron 
of Elba, he found no change to be made in the old laws ; 
becaufe the fecondary incidences which he had deter¬ 
mined in the fuppofition of a cube, differed only half a 
degree from thofe refulti.ng from the rhomboidal form. 
This, fays the author, is one of thofe cafes where a 
quantity very fenlible of itfelf is diminilhed by paffing 
into certain refults which depend on it. 
It arifes from thefe refearches, that all the iron ores 
which retain a metallic appearance may be reduced to 
two kinds, very diftindt from each other; one of which 
contains lubftances that cryftallize in regular odtahedra, 
fuch as the iron of Corlica; and the other, thofe having 
for their primitive form a rhomboid fomewhat acute, as 
the iron of the ifland of Elba, that of Framont, and 
that of volcanoes. The firft is diftinguifhed by the 
name of oxidulated iron, the fecond, oligijl iron ; that is to 
fay, little abundant in iron in the metallic ftate. It is 
here feen, that a greater quantity of oxygen imprints on 
the primitive form a character entirely peculiar, by 
making the regular odtahedron pafs to the rhomboid. 
Dr. Townfon, in his Philolophy of Mineralogy ob- 
ferves, that two laws appear to govern the mineral 
world : attraction of aggregation, and attraction of com¬ 
bination. To the firit, mineral bodies owe their exift. 
ence 
