SlUCA. 
fc&nsists ; afift, on account of its great abun- 
dance, it has been regarded as the primi- 
tive or elementary earth, the base of all the 
other earths. 
Silica fon '.s one of the constituent parts 
of most stony bodies ; but it exists in great- 
est abundance in agates, jasper, fliats, 
quartz, and rock crystal : in tlie latter it 
exists nearly in a state of purity. To ob- 
tain it [)erfectly pure, a quantity of quartz, 
or rock crystal, may be exposed to a red 
heat. When it is taken fro.m the fire, and 
wliile it is yet hot, it is suddenly immersed 
in cold water. It is then to be reduced to 
powder, and if transparent rock crystal 
has been employed, it is then in a state 
of tolerable purity. I'o have it perfectly 
pure, mix one part of the pounded stone 
with three/ parts of potash, and expose 
them in a crucible to a heat which is suffi- 
cient for the fusion of the mixture. The 
mass thus obtained is soluble in water. 
Add a sufficient quantity of water for its 
.solution, and drop in muriatic acid as long 
as there is any precipitate. Let this be 
repeatedly washed with water and dried. 
The substance thus obtained is pure silica. 
It is in the form of very fine white powder, 
which has neither taste nor smell. The 
particles are rough and harsh to the feel, 
as when they are nibbed between the fin- 
gers, or touched with the tongue. The spe- 
cific gravity is 2.6. Light has no action on 
silica, and it is one of the peculiar cha- 
racters of this earth, that it resists, un- 
changed, the greatest degree of heat. 
There is no action between silica and oxy- 
gen, azote, or hydrogen ; nor is it changed 
by exposure to the air. It is not acted 
upon by carbon, phosphorus, or sulphur. 
It is insoluble in water ; but, in a state of 
minute division, it absorbs a considerable 
portion, and forms with this liquid a trans- 
parent jelly. When it is exposed to the 
air, the whole of the moisture is evapo- 
rated. 
Silica is frequently found in nature in the 
crystallized form, and then it is distin- 
guished by the name of rock crystal. It is 
most commonly in hexagonal prisms, termi- 
nated by hexagonal pyramids. Crystals of 
silica have also been formed artificially. 
In a solution of silica in fluoric acid, which 
had remained at rest for two years, Berg- 
man found crystals, some of which were 
cubes, and some had truncated angles, at 
the bottom of the vessel. Crystals of silica 
have also been formed, by diluting largely 
with water the combination of silica and 
potash, and allowing it to remain for a long 
time. 
Silica is only acted on by a very few of 
the acids. These are the phosphoric and 
boracic, which combine with it by fusion, 
and the fluoric. Which dissolves silica either 
in the gaseous or liquid state. When silica 
is held in solution in water by means of an 
alkali, it is also dissolved by the muriatic- 
acid. The alkalies have a very powerful 
action on this earth. In the preparation of 
the pure earth, it was combined with pot- 
ash by means of fusion. Tliis Compound is 
dift'erent in its nature and properties, ac- 
cording to the proportions of the silica and 
tlie alkali. Two or three parts of potash, 
with one of silica, form a compound which 
is deliquescent in the air, and soluble in 
water. This was formerly distinguished by 
the name liquor silicum, or liquor of flints. 
It is now called silicated alkali. When 
this solution is long exposed to the air, the 
earth is deposited in a flaky gelatinous form. 
It is decomposed by acids, which combine 
with the alkali, and the pure earth falls to 
the bottom iii the state of tine powder. 
When the solution is largely diluted with 
water, and if a greater quantity of the acid 
be added than is sufficient to saturate the al- 
kali, the silica remains in solution. This is 
particularly the case when the muriatic acid 
is employed ; but when the silica is in 
greater proportion, a compound is formed 
which is possessed of very different pro- 
perties. The substance thus obtained is 
glass. This earth also enters into combi- 
nation with some of the earths. If to a 
solution of the liquor of flints lime-water be 
added, a precipitate is formed, which is 
found to be a compound of silica and lime. 
Silica also combines with lime by means of 
heat, and in certain proportions a glass is 
formed. Silicious earth eilt. rswith diffi- 
culty into combination with magnesia ; but 
if equal parts of silica and magnesia be 
exposed to very strong beat, they melt into 
a white enamel. But the most important 
compounds of the earths are those of silica 
and alumina. These earths may be com- 
bined together, as appears from the expe- 
riments of Guyton, in the humid way. He 
mixed together equal parts of alumina dis- 
solved by means of potash, and of silica 
held in solution by the same alkali. When 
the solutions came into contact, a brown 
zone was immediately formed, which spread, 
by agitation, through the whole mass, and 
communicated to it a yellowish colour. The 
mixture was no further changed during the 
