contents of the stomach are colloidal.

216. Silicates.--Si is a tetrad. SiO2 + 2 H2O =? Si02 + H2O =? In
either case the product is called silicic acid. Replace all the H
with Na, and name the product. Replace it with K; Mg; Fe; Ph; Ca.
Na4SiO4 and Na2SiO3 are typical silicates of Na, but others
exist.

217. Formation of SiO2 from Sodium Silicate. Experiment 117.--To
5cc.Na4SiO4 in au evaporating-dish add 5cc. HCl. Describe the
effect. Pour away any extra HCl. Heat the residue gently, above a
flame, till it becomes white, then cool it and add water. In a
few minutes taste a drop of the water, then pour it off, leaving
the residue. Crush a little in the fingers, and compare it with
white sand, SiO2. Apply to the experiment these equations: -
Na4SiO4 + 4 HCl = 4 NaCl + H4SiO4. H4SiO4 + 2 H2O = Si02. Why was
H4Si04 heated? Why was water finally added?

Water glass, sodium or potassium silicate, used somewhat for
making artificial stone, is made by fusing SiO2 with Na2CO3 or
K2CO3, and dissolving in water. Silicic acid forms the basis of a
very important series of compounds, - the silicates. The above
two are the only soluble ones, and may be called liquid glass.

Chapter XLII.

GLASS AND POTTERY.

Examine white sand, calcium carbonate, sodium carbonate, smalt;
bottle, window, Bohemian and flint glass.

218. Glass is an Artificial Silicate.--Si02 alone is almost
infusible, as is also Ca0; but mixed and heated the two readily
fuse, forming calcium silicate. Ca0 + SiO2 = ? Notice that Si02
is the basis of an acid, while CaO is essentially a base, and the
union of the two forms a salt. There are four principal kinds of
glass: (1) Bohemian, a silicate of K and Ca, not easily fused,
and hence used for chemical apparatus where high temperatures are
required; (2) window or plate glass, a silicate of Na and Ca; (3)
bottle glass, a silicate of Na, Ca, Al, Fe, etc., a variety which
is impure, and is tinged green by salts of Fe; (4) flint glass, a
silicate of K and Pb, used for lenses in optical instruments, cut
glass ware, and, with B added, for paste, or imitation diamonds,
etc. Pb gives to glass high refracting power, which is a valuable
property of diamonds, as well as of lenses.

219. Manufacture.--Pure white sand, Si02, is mixed with CaCO3 and
Na2CO3, some old glass - cullet - is added, and the mixture is
fused in fire-clay crucibles. For flint glass, Pb304, red lead,
is employed. If color is desired, mineral coloring matter is also
added, but not always at this stage. CoO, or smalt, gives blue;
uranium oxide, green; a mixture of Au and Sn of uncertain
composition, called the "purple of Cassius," gives purple. MnO2
is used to correct the green tint caused by FeO, which it is
supposed to oxidize. Opacity, or enamel, as in lamp-shades, is
produced by adding As2O3, Sb2O3, SnO2, cryolite, etc. The glass-
worker dips his blowpipe--a hollow iron rod five or six feet
long--into the fused mass of glass, removes a small portion,
rolls it on a smooth surface, swings it round in the air, blowing
meanwhile through the rod, and thus fashions it as desired, into
bottles, flasks, etc. For some wares, e.g. common goblets, the
glass is run into molds and stamped; for others it is blown and
welded. All glass must be annealed, i.e. cooled slowly, for
several days. The molecules thus arrange themselves naturally. If
not annealed, it breaks very easily. It may be greatly toughened
by dipping, when nearly red-hot, into hot oil. Cut glass is
prepared at great expense by subsequent grinding. Glass may be
rendered semi-opaque by etching either with HF, or with a blast
of sand.

220. Importance.--Few manufactured articles have more importance
than glass. Without it the sciences of chemistry, physics,
astronomy, microscopic anatomy, zoology, and botany, not to
mention its domestic uses, would be almost impossible.

221. Porcelain and Pottery.--Genuine porcelain and china-ware are
made of a fine clay, kaolin, which results from the
disintegration of feldspathic rocks. Bricks are baked clay. The
FeO in common clay is oxidized to Fe2O3, on heating, a process
which gives their red color. Some clay, having no Fe, is white;
this is used for fire-bricks and clay pipes. That containing Fe
is too fusible for fire-clay, which must also have much SiO2. The
electric arc, however, will melt even this, and the most
refractory vessels are of calcium oxide or of graphite. Pottery
is clay, molded, baked, and either glazed, like crockery, or
unglazed, like flower-pots. Jugs and coarse earthenware are
glazed by volatilizing NaCl in an oven which holds the porous
material. This coats the ware with sodium silicate. To glaze
china, it is dipped into a powder of feldspar and SiO2 suspended
in water and vinegar, and then fused. If the ware and glaze
expand uniformly with heat, the latter does not crack.

Chapter XLIII.

METALS AND THEIR ALLOYS.

222. Comparison of Metals and Non-Metals.--The majority of
elements are metals, only about a dozen being non-metallic in
their properties. The division line between the two classes is
not very well defined; e.g. As has certain properties which ally
it to metals; it has other properties which are non-metallic. H
occupies a place between the two classes. The following are the
more marked characteristics of each group: -

METALS.

1. Metals are solid at ordinary temperatures, and usually of high
specific gravity.

Exceptions: Hg is liquid above -39.5 degees; Li is the lightest
solid known; Na and K will float on water.

2. Metals reflect light in a way peculiar to themselves. They
have what is called a metallic luster.

3. They are white or gray. Exceptions: Au, Ca, Sr are yellow; Cu
is red.

4. In general they conduct heat and electricity well.

NON-METALS. 1. Non-metals are either gaseous or solid at ordinary
temperatures, and of low specific gravity. Exceptions: Br is a
liquid; I has the heaviest known vapor.

2. Non-metallic solids have different lusters, as glassy,
resinous- silky, etc. Exceptions: I, B, and C have metallic
luster.

3. Non-metals have no characteristic color.

4. They are non-conductors of heat and electricity. Exceptions: C
and some others are conductors. 5. They are usually malleable and
ductile.

6. They form alloys, or "chemical mixtures," with one another,
similar to other solutions. Exceptions: Some, as Ph and Zn, will
not alloy with one another.

7. Metals are electro-positive elements, and unite with O and H
to form bases. Exceptions: Some of the less electro-positive
metals, with a large quantity of O, form acids, as Cr, As, etc.

Numbers 2, 6, and 7 are the most characteristic and important
properties.

5. They are deficient in malleability and ductility.

6. They often form liquid solutions, similar to alloys in metals.

7. Non-metals are electronegative, and with H, or with H and O,
form acids.

Examine brass, bronze, bell-metal, pewter, German silver, solder,
type-metal.

223. Alloys.-An alloy is not usually a definite chemical
compound, but rather a mixture of two or more metals which are
melted together. One metal may be said to dissolve in the other,
as sugar dissolves in water. The alloy has, however, different
properties from those of its elements. For example, plumber's
solder melts at a lower temperature than either Ph or Sn, of
which it is composed. Some metals can alloy in any proportions.
Solder may have two parts of Sn to one of Pb, two of Pb to one of
Sn, or equal parts of each, or the two elements may alloy in
other proportions. Not all metals can be thus fused together
indefinitely; e.g., Zn and Pb. Nickel and silver coins are
alloyed with Cu, gold coins with Cu and Ag.

Gun-metal, bell-metal, and speculum-metal are each alloys of Cu
and Sn. Speculum-metal, used for reflectors in telescopes, has
relatively more Sn than either of the others; gun-metal has the
least. An alloy of Sb and Pb is employed for type-metal as it
expands at the instant of solidification. Pewter is composed of
Sn and Pb; brass, of Cu and Zn; German silver, of brass and Ni;
bronze, of Cu, Sn, and Zn; aluminium bronze, of Cu and Al.

224. Low Fusibility is a feature of many alloys. Wood's metal,
composed of Pb eight parts, Bi fifteen, Sn four, Cd three, melts
at just above 60 degrees, or far below the boiling-point of
water. By varying the proportions, different fusing-points are
obtained. This principle is applied in automatic fire alarms, and
in safety plugs for boilers and fire extinguishers. Water pipes
extend along the ceiling of a building and are fitted with plugs
of some fusible alloy, at short distances apart. When, in case of
fire, the heat becomes sufficiently intense, these plugs melt and
the water flows out.

225. Amalgams.--An amalgam is an alloy of Hg and another metal.
Mirrors are "silvered" with an amalgam of Sn. Tin-foil is spread
on a smooth surface and covered with Hg, and the glass is pressed
thereon.

Various amalgams are employed for filling teeth, a common one
being composed of Hg, Ag, and Sn. Au or Ag, with Hg, forms an
amalgam used for plating. Articles of gold and silver should
never be brought in contact with Hg. If a thin amalgam cover the
surface of a gold ring or coin, Hg can be removed with HNO3, as
Au is not attacked by it. Would this acid do in case of silver
amalgam? Heat will also quickly cause Hg to evaporate from Au.

CHAPTER XLIV.

SODIUM AND ITS COMPOUNDS.

Examine NaCl, Na2SO4, Na2CO3, Na, NaOH, HNaCO3, NaNO3.

226. Order of Derivation.--Though K is more metallic, or electro-
positive, than Na, the compounds of Na are more important, and
will be considered first. The only two compounds of Na which
occur extensively in nature are NaCl and NaNO3. Almost all others
are obtained from NaCl, as shown by this table, which should be
memorized and frequently recalled.


                     ) Na
NaCl ) Na2SO4) Na2CO3) NaOH
NaNO3)       ) 	     ) HNaCO3



From what is Na2SO4 prepared, as shown by the table? Na2CO3? Na?

227. Occurrence and Preparation of NaCl.--NaCl occurs in sea
water, of which it constitutes about three per cent, in salt

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