CHAPTEVER 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.