a eens 
alkaline bodies, to be called silicic acid; and the 
many earthy combinations of it with potash and 
soda and lime and magnesia and other salifiable 
bases in rocks and soils and gems, must be re- 
garded as true salts or native silicates. Several 
of the varieties of silica itself, but especially all 
the most common salts of it, or most abundant 
native silicates, occur both naturally and artifi- 
cially, under modifications or in contact with 
substances or in exposure to agencies, which 
completely subdue its refractoriness and insolu- 
bility, and render it facilely and abundantly avail- 
able for absorption by plants. 
“ Quartz,” says Liebig, “forms a very pure 
variety of silica, and, in this condition, it is 
quite insoluble both in cold and in hot water, is 
without taste, and does not exert any action on 
vegetable colours. The principal property of 
silica in this state is, that it unites with alkalies, 
forming saline compounds, which are termed 
silicates, Window and plate glasses consist of 
mixtures of silicates of the alkaline bases, potash, 
soda, and lime. In such compounds, the alkali 
is generally completely neutralised. The pro- 
perty of neutralising metallic oxides and alkalies 
belongs only to acids; and it is owing to this 
that silica has received the name of silicic acid. 
Silica is a very feeble acid, for, in its crystallised 
form, it is destitute both of taste and of solubility 
in water; but it dissolves by continued digestion 
in alkaline leys. We may easily obtain com- 
pounds of silica with potash and soda, by melting 
it either with a pure alkali, or with an alkaline 
carbonate. By this treatment, white glasses are 
obtained, differing in properties according to 
their amount of soluble ingredients. When the 
glass contains 70 per cent. of silica and 30 per 
cent. of potash or soda, it becomes soluble in 
boiling water. Its solution may be spread over 
a surface of wood or of iron, and then dries into 
a vitreous substance, which has received the 
name of soluble glass. When there is a smaller 
proportion of alkali than the above quantity, or, 
in other words, when there is a larger proportion 
of silica, the resulting glass diminishes in solu- 
bility in a greater or less degree. All silicates 
soluble in water are decomposed by acids. If 
the solution of the silicate contains silica corre- 
sponding to more than zo the weight of the 
water, the addition of an acid causes the forma- 
tion of a precipitate of a very gelatinous appear- 
ance, This precipitate, being a compound of 
silica with water, is termed the hydrate of silica. 
But, if the solution contains less silica than the 
above proportion, no precipitate is formed on |! 
the addition of an acid, the whole remain- 
ing perfectly clear. This circumstance proves 
that silica, in the state in which it is pre- 
cipitated by an acid, possesses a certain de- 
gree of solubility in pure water. Indeed, by 
washing with water the gelatinous precipitate of 
silica, its volume diminishes, and silica may be 
detected in solution by evaporating the water 
SILICA. 
217 
which has passed through. From these facts we 
perceive, that silica possesses two distinct che- 
mical characters. In the form in which it is 
separated from a silicate, it possesses quite dif- 
ferent properties from those which it has when 
in the state of sand, quartz, or rock crystal. 
When sufficient water is present during its 
separation from a base, to effect its solution, the 
whole remains dissolved; in certain conditions, 
silica is more soluble in water than gypsum. 
On drying, silica loses almost completely its solu- 
bility in water. The solution of silica in acids 
acquires, at a certain degree of concentration 
after cooling, such a gelatinous consistence that 
the vessel containing it may be turned upside 
down without spilling a drop of the transparent 
jelly. By drying it still further, the water which 
retained it in the gelatinous condition, escapes | 
along with that which had served to hold it in 
solution. When the water has been once re- 
moved in this way, the silica is no longer soluble 
in water. But, although it has thus lost its 
solubility, it does not acquire all the properties 
of crystallised silica, such as sand and quartz, 
for it still possesses the power of dissolving in 
alkalies and alkaline carbonates at the ordinary 
temperature of the air, and this power it retains 
even when it has been heated to redness. There 
is scarcely any other mineral substance which | 
can be compared to silica for the possession of 
such remarkable properties as those now de- 
scribed. 
“Most of the insoluble silicates containing 
alkaline bases are decomposed by the action of 
hot water, particularly when that water contains 
an acid. In the middle of the last century, the 
ignorance of this fact led chemists to believe 
that water might be converted into an earth. 
When water is distilled in glass vessels, it is 
found to contain always a certain quantity of 
earthy substances, which may be detected by | 
evaporation, even if the water has been subjected 
to many repeated distillations. Lavoisier proved 
that part of the glass was dissolved in this opera- 
tion by the boiling water; and further, that the 
diminution in the weight of the glass vessel cor- 
responded exactly to the quantity of earthy resi- 
due left by the evaporation of the water. When 
the distillation of water is effected in metallic 
vessels, no such residue can be obtained. The 
action of water upon the silicates contained in 
glass may be observed in the opacity which gra- 
dually comes over the windows of hot-beds, these 
being exposed in a great degree to the influence 
of the air. This action is more marked in the 
windows of stables, where the carbonic acid 
formed by the processes of respiration of the ani- 
mals, and by the decay of animal matter, ac- 
celerates the decomposition. 
“ Silica being an acid of a very feeble charac- 
ter, the decomposition of the soluble silicates is 
effected even by carbonic acid. A solution of 
| soluble glass may be converted into a gelatinous 
a 
