M I N £ R A L O G Y. 
with other earthy fubftances, would be melted, the earths 
being difpofed to vitrify when in contaft with each other 
at a much lower degree of heat than is requifite to fufe 
them feparately. If a mineral is capable of refilling a 
great degree of heat without fufion, it is laid to be re- 
fradory. 
The hardncfs of minerals has been expreffed either by a 
comparifon with each other, or their power ol fcratching 
glafs.; or by the effeft of the file upon them ; thofe which 
refift the file being the hardeft. 
By the pliofphorefcence of minerals is implied that faint 
light which they emit, either by expofure to fimple heat, 
or in confequence of fri&ion. 
The eledricity of a mineral is that property which, 
being excited either by fimple heat or by friftion, Ih'ows 
itlelf in the attraction or repulfion of other fubftances 
with which the mineral is brought nearly into contaft. 
It has been obferved that fome minerals acquire that kind 
of electricity which is called vitreous, as being peculiar to 
glafs and other polifhed earthy fubftances : this is alfo 
called their pofitive electricity. Others acquire that kind 
which is called rejinous , as being peculiar to refill and 
other inflammable fubftance : this is alfo called their ne¬ 
gative eletricity. Thofe which are in a metallic or near¬ 
ly a metallic ftate acquire neither the vitreous nor the refi- 
nous eletricity; but altogether tranfmit the eletric fluid. 
Some minerals are capable of acquiring both kinds, of 
eletricity; thefe are generally cryftallized ; and it is ob¬ 
ferved of them, that the parts which exhibit the different 
ftates of eletricity differ from each other with refpet to 
their form, although they are fimilarly fituated ; while in 
thofe cryflals that are not eletric the fimilarly-fituated 
parts correfpond alfo in form. If a cryftal, for inftance, 
confifts of a prifm terminated at each extremity by a py¬ 
ramid, and thefe pyramids differ as to the kind of eletri¬ 
city they are capable of acquiring ; it will be found that 
they alfo differ in their form ; one confifting of a greater 
number of furfaces than the other; and the part which 
has the greater number of faces acquires the vitreous, the 
other the refinous, eletricity. 
It is very generally known, that a ray of light, in pafling 
from one medium into another, as from air into water, or 
glafs, or any tranfparent fubftance, is turned more or lefts 
out of its original diretion ; and this alteration of courfe 
is called the refradiou of the ray. Some tranfparent fub¬ 
ftances feem to have the power of alfo fplitting the rays of 
light: this alteration of the courfe of the rays is called 
•their double refraction ; and minerals poft'effmg this 
power have the property of prefenting a double image of 
an obje& viewed through them. 
By the fpecijic gravity of a mineral is underftood the 
amount of its.weight when compared with the weight of 
a quantity of water of the fame bulk with itlelf. Thus, 
if a cubic inch of limeftone were found to be double the 
weight of a cubic inch of water, the limeftone would be 
fpecifically heavier than the water, in the proportion of 
two to one ; and its fpecific gravity would be fet down 
thus, 2. If its weight were found to be equal to the 
weight of two cubical inches and a half of water, it will 
be Ipecificaily heavier than the wafer, in the proportion 
of two and a half to one ; and its Ipecific gravity would 
be fet down thus 2'5; the fraCfional parts being exprefled 
by decimals. See Gravity, vol. viii. p. 809, 10. and the 
article Mechanics, vol. xiv. p. 701-10. 
The cryHalline forms under which individual minerals 
occur are various, and may be conlidered as one of their 
molt important characters. Every mineral fubftance, per¬ 
haps, if placed under particular circumftances is capable, 
by a peculiar arrangement of its particles, of affuming a 
regular and appropriate form ; and, as this eft’eCt was firlt 
obferved in the fubftance known by the name of ro.ck- 
cryjtal, the term cryjial was extended to every other fub¬ 
ftance poflefling an analogous regularity in its form ; even 
though it wanted that* icy appearance and tranfparency 
Vol. XV. No. 1 oyy. 
m? 
which belong to the rock-cryfta'l, and which give rife to 
the term itfelf. 
If minerals could always be made to cryftallize under 
precifely the fame circumftances, there is good ground for 
luppoling that thofe of the fame nature would always aft- 
fume the fame form ; for it is poftible in a great many in- 
ftances, by carefully placing different folutions of the 
lame fubftance in as fimilar circumftances as poifible, to 
obtain from all proportionally fimilar cryftals ; and, as the 
fame fimilarity continues in repeating the procefs what¬ 
ever number of times, it is clear that there is in thofe fub¬ 
ftances a difpofition to aflame one particular form in pre¬ 
ference to all others. But, as the leaft variation in" the 
circumftances will produce a correlponding variation in 
the form, it is alfo clear that the variation in the form of 
cryftals may be infinite. Hence there muft neceflariiy be 
a difficulty, where the fame fubftance prefents itlelf under 
a great variety of forms, of determining the particular 
form which the fubftance would affume when placed under 
the molt favourable circumftances for arranging its par¬ 
ticles according to the laws of its nature. 
This form, however, may' general^ be difeovered by 
mechanical divifion ; and in many, cryftallized fubftances 
the eye is capable of perceiving that a divilion may be ef¬ 
fected in fome directions much more eafily than in others. 
Suppofing therefore you were to take any one of the va¬ 
rieties of a cryftallized fubftance, and, forcing the edge of 
a knife in thofe directions where there was leaft refiftance, 
were to continue to make frelh and parallel lections, a 
folid figure would at laft be extracted of a different fliape 
from the original or any of the intermediate forms ; and 
this figure would remain unaltered, except with refpect 
to it* fize, though the divifion were carried to the furtheft 
point. In cutting off, for inftance, the folid angles of a 
cubic cryltal of fluor fpar, you firft obtain eight new tri¬ 
angular furfaces; and, if you continue the divifion in 
the parallel of thefe planes, you ultimately obtain an oc- 
tohedron ; which is the primitive form of fluor fpar. A 
fimilar nucleus may be extrafted from every cryftallized 
variety of the fame fubftance; and hence this is called its 
primitive form. 
Having arrived at the primitive form of any cryftal, iit 
molt inftances the divifion may be continued in the pa¬ 
rallels of the fame planes to any extent; but in no other 
direction : this divilion confequently does not alter the 
form. But fome primitive forms are divifible in planes 
that are not parallel to their furfaces ; and, when this is 
the cafe, a folid figure is extracted which differs frdrn the 
primitive form of the cryftal to which it belongs. M. Haiiy 
calls this the integrant molecule. 
The forms of the integrant molecule are three : the te¬ 
trahedron, the triangular prifm, and the parallelepiped ; 
and it leems probable that every cryftal is made up of a 
vaft number of thefe integrant molecules ; and that the 
different varieties of cryftals have been produced by a va¬ 
riation in the arrangement of thefe. This fuppofition 
may be illuftrated by taking a great number offmuil cubes 
for inftance, and lb arranging them as to form a larger 
cube, or an oftohedron, or any other form ; which may 
be ealily effected. Such a procefs may be traced by the 
eye in many cryftals ; particularly in thofe of fluor fpar. 
All thofe cryftals which are variations of the primitive 
form .are called Jecondary; but this with reference to 
form merely; for in their chemical compofition they are 
exadlly the lame. 
The term lecondary has however been applied to cer¬ 
tain cryftallizations poflefling a form not belonging to the 
nature of their fubftance. Thefe are alfo called, and with 
more propriety, pfeudo-viorphie cryftals; and appear to 
haye been formed by a depoiition of the particles of the 
pfeudo-cryftai, either immediately on the furface, or in 
the mould of the cryftal of fome other fubftance. Sup- 
pole, for inftance, a quantity of any plaftic fubftance 
were, to be moulded on a natural cryftal; and, the cryltal 
S 1 ' .being 
