S, 1/2 g. charcoal. Pile the mixture on a brick, and apply a
lighted match. The adhering product can be removed by soaking in
water.

AMMONIUM COMPOUNDS.

248. Read the chapter on NH3. Also, review the experiments on
bases. Examine NH4Cl, NH4NO3, (NH4)2SO4, (NH4)2CO3.

Ammonium, NH4, is too unstable to exist alone, but it forms salts
similar to those of K and Na. NH3 dissolved in water forms NH4OH.

The food of plants, as well as that of animals, must contain N.
It has not yet been shown that they can make use of that
contained in the air, but they do absorb its compounds from the
soil. All fertilizers and manures contain a soluble compound of
NH4. All NH4 compounds are now obtained either from coal, in
making illuminating-gas, or from bones, by distillation.

Suppose the product obtained from the gas-house to be NH4OH, how
would NH4Cl be made? (NH4)2SO4? NH4NO3? Write the reactions.
(NH4)2CO3 is made by heating NH4Cl with CaCO3. Give the reaction.

Chapter XLVI.

CALCIUM COMPOUNDS.

Examine CaCO3--marble, limestone, chalk, not crayon,--CaSO4 --
gypsum or selenite--CaCl2, CaO.

249. Occurrence.--The above are the chief compounds of Ca. The
element itself is not found uncombined, is very difficult to
reduce (page 141), is a yellow metal, and has no use. Its most
abundant compound is CaCO3. Shells of oysters, clams, snails,
etc., are mainly CaCO3, and coral reefs, sometimes extending
thousands of miles in the ocean, are the same. CaCO3 dissolves in
water holding CO2, and thence these marine animals obtain it and
therefrom secrete their bony framework. All mountains were first
laid down on the sea bottom layer by layer, and afterwards lifted
up by pressure. Rocks and mountains of CaCO3 were formed by
marine animals, and all large masses of CaCO3 are thought to have
been at one time the framework of animals. Marble is
crystallized, transformed limestone. The process, called
metamorphism, took place in the depths of the earth, where the
heat is greater than at the surface.

250. Lime.--If CaCO3 be roasted with C, CO2 escapes and CaO is
left. CaCO3 - CO2 = ? This is called burning lime, and is a large
industry in limestone countries. CaO is unslaked lime, quicklime
or calcium oxide. It may be slaked either by exposure to the
air, air-slaking, when it gradually takes up H2O and CO2; or by
mixing with H2O, water-slaking. Ca0 + H2O = Ca(OH)2.

Great heat is generated in the latter case, though not so much as
in the formation of KOH and NaOH. Like them, Ca(OH)2 dissolves in
water, forming lime-water. Milk of lime, cream of lime, etc.,
consist of particles of Ca(OH)2 suspended in H2O.

251. Uses of Lime--CaO is infusible at the highest temperatures.
If it be introduced into the oxy-hydrogen blow-pipe (page 28), a
brilliant light, second only to the electric, is produced. Mortar
is made by mixing CaO, H2O, and Si02. It hardens by evaporating
the extra H2O, absorbing CO2 from the air, and uniting with Si02
to form calcium silicate. It often continues to absorb CO2 for
hundreds or thousands of years before being saturated, as is
found in the Egyptian pyramids. Hence the tenacity of old mortar.
Hydraulic mortar contains silicates of Al and Ca, and is not
affected by water. What are the uses of mortar? Being the
important constituent of mortar and plaster, lime is the most
useful of the bases.

252. Hard Water.--Review Experiment 76. The solubility of CaCO3
in water that contains CO2 leads to important results. Much
dissolves in the waters of all limestone countries; and the
water, though perfectly transparent, is hard; i.e. soap has
little action on it. See page 187. Such water may be softened by
boiling, a deposit of CaCO3 being formed as a crust on the
kettle. Such water is called water of temporary hardness. MgCO3
produces a similar effect, and water containing it is softened in
the same way. Permanently hard waters contain the sulphates of Ca
and Mg, which cannot be removed by boiling, but may be by adding
(NH4)2CO3. 253. The Formation of Caves in limestone rocks is due
also to the solubility of CaCO3. Water collects on the mountains
and trickles down through crevices, dissolving, if it contains
CO2, some of the CaCO3, and thus making a wider opening, and
forcing its way along fissures and lines of least resistance into
the interior of the earth, or out at the base of the mountain.
Its channel widens as it dissolves the rock, and the stream
enlarges until in the course of ages an immense cavern may be
formed, with labyrinths extending for miles, from the entrance of
which a river often issues. In the long ages which elapsed during
the slow formation of Mammoth Cave its denizens lost many of the
characters of their ancestors, and eyeless fish and also eyeless
insects now abound there.

254. Reverse Action.--Drops of water on the roofs of these
caverns lose their CO2, and deposit CaCO3. Thus long, pendant
masses of limestone, called stalactites, are slowly formed on the
roofs like icicles. From these, water charged with CaCO3 drops to
the bottom, loses CO2 and deposits CaCO3, which forms an upward-
growing mass, called stalagmite. In time it may meet the
stalactite and form a pillar. Notice that the same action which
formed the cave is filling it up; i.e. the solubility of CaCO3 in
water charged with CO2.

255. Famous Marbles.--The marble from Carrara, Italy, is most
esteemed on account of a pinkish tint given by a trace of oxide
of iron. The best of Grecian marble was from Paros, one of the
Cyclades. The isles of the Mediterranean are of limestone, or of
volcanic, origin, often of both. 256. Calcium Sulphate occurs in
two forms, (1) with water of crystallization--gypsum, CaSO4 + 2
H2O, --(2) without it--anhydrite, CaSO4. The former, on being
strongly heated, gives up its water, and is reduced to a powder--
plaster of Paris. This, on being mixed with water, again takes up
2 H2O, and hardens, or sets, without crystallizing. If once more
heated to expel water, it will not again absorb it. When plaster
of Paris sets, it expands slightly, and on this account is
admirable for taking casts.

257. Uses.--Gypsum finds use as a fertilizer and as an adulterant
in coloring-materials, etc. CaSO4 is employed in making casts,
molds, statuettes, wall-plaster, crayons, etc.

How can CaCl2 be made?  What is its use?  See page 27. What else
is used for a similar purpose?

Symbolize and name the acid represented by Ca(ClO)2, and name
this salt (page 107). It is one of the constituents of bleaching-
powder, the symbol of which, though still under discussion, may
be considered Ca(ClO)2 + CaCl2. This is made by passing Cl over
Ca(OH)2 2 Ca(OH)2 + 4 Cl = Ca(ClO)2 + CaCl2 + 2 H2O.

CHAPTER XLVII.

MAGNESIUM, ALUMINIUM, AND ZINC.

MAGNESIUM AND ITS COMPOUNDS.

Examine magnesite, dolomite, talc, serpentine, hornblende,
meerschaum, magnesium ribbon, magnesia alba, Epsom salt.

258. Occurrence and Preparation.--Mg is very widely distributed,
but does not occur uncombined. Its salts are found in rocks and
soils, in sea water and in the water of some springs, to which
they impart a brackish taste.

The most common minerals containing Mg are magnesite, MgCO3,
dolomite, MgCO3 + CaCO3, and talc, serpentine, hornblende, and
meerschaum. The last four are silicates, and often are unctious
to the touch. What proportion of the earth's crust is composed of
Mg?  See page 173.

259. Metallic Mg is prepared by fusing MgCl2 with Na. Why is the
process expensive?  Write the reaction.

Experiment 120.--With forceps hold a short strip of Mg ribbon in
a flame. Note the brilliancy of the light, and give the reaction.
Examine and name the product.

Photographs of the interior of caverns, where sunlight does not
penetrate, are taken by Mg light. Gun-cotton sprinkled with
powdered Mg has recently been employed for that purpose. Mg
tarnishes slightly in moist air. Compounds of Mg.--MgO, magnesia,
like CaO, is very infusible, and is used for crucibles. Magnesia
alba, a variable mixture of MgCO2 and Mg(OH)2, is employed in
medicine, as is also Epsom salt, MgSO4 + 7 H2O.

ALUMINIUM AND ITS COMPOUNDS.

Examine aluminium, aluminium bronze, corundum, emery, feldspar,
argillite, clay. Note especially the color, luster, specific
gravity and flexibility of Al.

What elements are more common in the earth than Al? What metals?
Compare the abundance of Al with that of Fe.

260. Compounds of Al.--Al occurs only in combination with other
elements. Feldspar, mica, slate, and clay are silicates of it. It
occurs in all rocks except CaCO3 and SiO2, and in nearly 200
minerals. Though found in all soils, its compounds are not taken
up by plants, except by a few cryptogams. Corundum, Al2O3, is the
richest of its ores. Compute its percent of Al. Compounds of Al
are very infusible and difficult of reduction.

261. Reduction.--Like most other metals not easily reducible by C
or H, it was originally obtained by electrolysis, but more
recently from its chloride, by the reducing action of strongly
heated K or Na. Al2Cl6 + 6 Na = 6 NaCl + 2 Al.

What is the chief use of Na? As it takes three pounds of Na to
make one pound of Al, the cost of the latter has been fifteen
dollars or more per pound. Its use has thus been restricted to
light apparatus and aluminium bronze, an alloy of Cu 90, Al 10,
which is not unlike gold in appearance.

Al2O3 has lately been reduced by C. Higher temperatures than have
heretofore been known are obtained by means of the electric arc
and large dynamo machines. Afurnace made of graphite, because
fire-clay melts like wax at such a high temperature, is filled
with Al2O3--corundum, --C, and Cu. In the midst of this are
embedded large carbon terminals, connected with dynamos. The
reduction takes several hours.

The following reaction takes place: Al2O3 + 3 C = 2 Al + 3 CO. Cu
is also added, and an alloy of Al and Cu is thus formed. This
alloy is not easily separable into its elements. Explain the
action of the C. CO escapes through perforations in the top of
the furnace, burning there to CO2. Only alloys of Al have yet

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