SI L 
81 1 
S I L 
m 
Soft those in return, after the particular ships, 
vice versa, when the object is to convey any 
intelligence from the latter to the admiral. 
Besides the signals above-mentioned, there 
*re others for different ranks of officers ; as 
Signals. 
A short roll, 
A flam, 
To arms. 
The march. 
The quick march. 
The point of war. 
The retreat, 
Drum ceasing. 
Two short roils, 
The dragoon march. 
The grenadier march. 
The troop. 
The long roll. 
The grenadier march. 
The preparative. 
Hie general, 
Two long rolis, 
SIGNATURE, in printing, is a letter put 
! at the bottom of the first page at least, in 
each sheet, as a direction to the binder, in 
folding, gathering, and collating them. The 
signatures consist of the capital letters of the 
alphabet, which change in every sheet: if 
there are more sheets than letters in the al- 
phabet, to the capital letter is added a smalt 
one of the same sort, as Aa, Bb ; which are 
repeated as often as necessary. In large vo- 
lumes it is usual to distinguish the number of 
alphabets, after the first three or four, by 
placing a figure before the signature, as 5B, 
<}B, &c. 
SIGNET, one of the king’s seals, made 
j use of in sealing his private letters, and all 
grants that pass by bill signed under his raa- 
! jesty’s hand: it is always in the custody of 
the secretaries of state. 
SIGNIFICAVIT, in law, a writ which 
i issues out of the court of chancery, on a certi- 
ficate given by the ordinary of a person’s 
standing excommunicated forty days, in or- 
| tier to have him imprisoned till he submits to 
' the authority of the church. 
SILENE, cat clifly, or viscous campion, a 
genus of plants belonging to the class of de- 
candria, and order of trigynia ; and in the 
natural system arranged under the 22 d or- 
I der, caryophylleae. The calyx is vefttricose; 
] the petals are five in number, bifid and un- 
guiculated, and crowned by a nectarium; 
the capsule is cylindrical, covered, and tri- 
locular. There are 66 species, of which se- 
\ ven are natives of Britain and Ireland. 
SILICA. There is a very hard white 
: stone, known by the name of quartz, very 
; common in almost every part of the world. 
Sometimes it is transparent and crystallized, 
; and then is called rock crystal. Very fre- 
; quently it is in the form of sand. As this 
stone, and several others which resemble it, 
1 as flint, agate, calcedony, &c. have the pro- 
'! perty of melting into a glass when heated 
| along with fixed alkali, they were classed to- 
gether by mineralogists under the name of 
- vitrifiable stones. Mr. Pott, who first de- 
scribed their properties in 1746, gave them 
' the name of siliceous stones, on the supposi* 
for eaptaini, lieutenants, masters, &e. er for 1 
any or those officers of a peculiar ship. 
Signals b>) the drum, made use of, in the 
exercise of the army, instead of the word ol 
command, viz. 
Operations. 
To caution. 
To perform any distinct thing. 
To form the line or battalion. 
( To advance, except when intended for a sa- 
l lute. ‘ 
To advance quick. 
To march and charge. 
To retreat. 
To halt. 
To perform the flank-firing. 
To open the battalion. 
To form the column. 
To double divisions. 
To form the square. 
To reduce the square to the column. 
To make ready and fire. 
To cease firing. 
To bring or lodge the colours. 
tion that they were all chiefly composed of a 
peculiar earth called siliceous earth or silica. 
This earth was known to Glauber, who de- 
scribes the method of obtaining it: but it was 
long before its properties were accurately 
ascertained. Geoffroy endeavoured to prove 
that it might be converted into lime, and Pott 
and Beaume that it might be converted into 
alumina: but these assertions were refuted 
by Cartheuser, Scheele, and Bergman. To 
this last chemist we are indebted for the first 
accurate detail of the properties of silica. 
1. Silica may be obtained pure by the fol- 
lowing process : Mix together, in a crucible, 
one part of pounded flint or quartz, and 
three parts of potass, and apply a heat suffi- 
cient to melt the mixture completely. Dis- 
solve the mass formed in-water, saturate the 
potass with muriatic acid, and evaporate to 
dryness. Towards the end of the evaporation 
the liquid assumes the form of a jelly; and 
when all the moisture is evaporated, a white 
mass remains behind. This mass is to be 
washed in a large quantity of water, and dried ; 
it is then silica in a state of purity. 
2. Silica, thus obtained, is a fine white 
powder, without either taste or smell. Its 
particles have a harsh feel, as if they consisted 
of very minute grains of sand. Its specific 
gravity is 2.66. 
It maybe subjected to a very violent heat 
without undergoing any change. Lavoisier 
and Morveau exposed it to the action of a 
fire maintained by oxygen gas without any 
alteration. Saussure, indeed, has succeeded 
in fusing, by means of the blowpipe, a por* 
tion of it so extremely minute as scarcely to 
be perceptible without a gla s. According to 
the calculation of this philosopher, the tem- 
perature necessary for producing this effect is 
equal to 4043° Wedgewood. 
3. It is insoluble in water except when 
newly precipitated, and then one part of it is 
soluble in 1000 parts of water. It has no ef- 
fect on vegetable colours. 
It is capable of absorbing about one-fourth 
of its weight of water, without letting any 
drop from it; but on exposure to the air, the 
water evaporates very readily. When pre- 
4P 2 
cipitated from potass by means of muriatic 
acid and slow evaporation, it retains a consi- 
derable portion of water, and forms with it a 
transparent jelly ; but the moisture gradually 
evaporates on exposure to the air. 
Silica may be formed into a paste with a 
small quantity of water: this paste has not the. 
smallest ductility, and when dried forms a 
loose, triable, and incoherent mass. 
Silica is capable of assuming a crystalline 
form. Crystals of it are found in many parts 
ot the world. They are known by the name 
of rock crystal. When pure they are trans- 
parent and colourless like glass: they assume 
various forms; the most usual is a hexagonal 
prism, surmounted with hexagonal pyramids 
on one or both ends, the angles of the prism 
corresponding with those of the pyramids. 
Their hardness is very great, amounting to 
1 1 . Their specific gravity is 2.633. 
4. Silica neither combines with oxygen, 
with the simple combustibles, nor with me- 
tals; but it combines with many of the me- 
tallic oxides by fusion, and forms various co- 
loured glasses and enamels, 
5. Azote has no action on' silica, neither 
has muriatic acid when the silica is in a solid 
state; but when the silica is combined with 
an excess of alkali, muriatic acid dissolves the 
compound, and forms a permanent solution. 
By concentrating this solution, the silica se- 
parates from it in the form of a jelly. 
6. Thare is a strong affinity between silica 
and fixed alkalies. It may be combined 
with them either by fusing them along with it 
in a crucible, or by boiling the liquid alkalies 
over it. When the potass exceeds the silica 
considerably, the compound is soluble in wa- 
ter, and constitutes what was formerly called 
liquor silicum, and now sometimes silicated 
potass or soda. When the silica exceeds, the 
compound is transparent and colourless like 
rock crystal, and is neither acted on by wa- 
ter, air, nor (excepting one) by acids. This 
is the substance so well known under the 
name of glass. See G lass. 
Silica is not acted on by ammonia, whether 
in the gaseous or liquid state. 
7. There is a strong affinity between ba- 
rytes and silica. When barytes water is 
poured into a solution of silica in potass, a 
precipitate appears, which is considered by 
Morveau as the two earths in a state of com- 
bination. Barytes and silica may tfo com- 
bined by means of heat. The compound is 
of a greenish colour, and coheres but im- 
perfectly. The effect of heat on various 
mixtures of barytes and silica will appear 
from the following experiments of Mr. Kii- 
wan : 
Proportions. 
80 Silica 
20 Barytes 
75 Silica 
20 Barytes 
66 Silica 
33 Barytes 
Heat. 
155° Wedg. 
150 
150 
Effect. 
A white brittle 
mass. 
A brittle hard 
mass, semitrans- 
parent at the 
edges. 
Melted into a 
hard somewhat 
porous porceiaia 
mass. 
* 
