ents are very
similar to each other and show a regular variation in properties in
passing from calcium to strontium and from strontium to barium. This is
seen in the solubility of the sulphate and hydroxide, and in the ease of
decomposition of the carbonates, as given in the table. Unlike the
alkali metals, their normal carbonates and phosphates are insoluble in
water.

CALCIUM

~Occurrence.~ The compounds of calcium are very abundant in nature, so
that the total amount of calcium in the earth's crust is very large. A
great many different compounds containing the clement are known, the
most important of which are the following:

Calcite (marble) CaCO_{3}.
Phosphorite Ca_{3}(PO_{4})_{2}.
Fluorspar CaF_{2}.
Wollastonite CaSiO_{3}.
Gypsum CaSO_{4}.2H_{2}O.
Anhydrite CaSO_{4}.

~Preparation.~ Calcium is now prepared by the electrolysis of the melted
chloride, the metal depositing in solid condition on the cathode. It is
a gray metal, considerably heavier and harder than sodium. It acts upon
water, forming calcium hydroxide and hydrogen, but the action does not
evolve sufficient heat to melt the metal. It promises to become a useful
substance, though no commercial applications for it have as yet been
found.

~Calcium oxide~ (_lime, quicklime_) (CaO). Lime is prepared by strongly
heating calcium carbonate (limestone) in large furnaces called kilns:

CaCO_{3} = CaO + CO_{2}.

When pure, lime is a white amorphous substance. Heated intensely, as in
the oxyhydrogen flame, it gives a brilliant light called the lime light.
Although it is a very difficultly fusible substance, yet in the electric
furnace it can be made to melt and even boil. Water acts upon lime with
the evolution of a great deal of heat,--hence the name quicklime, or
live lime,--the process being called slaking. The equation is

CaO + H_{2}O = Ca(OH)_{2}.

Lime readily absorbs moisture from the air, and is used to dry moist
gases, especially ammonia, which cannot be dried by the usual
desiccating agents. It also absorbs carbon dioxide, forming the
carbonate

CaO + CO_{2} = CaCO_{3}.

Lime exposed to air is therefore gradually converted into hydroxide and
carbonate, and will no longer slake with water. It is then said to be
air-slaked.

~Limekilns.~ The older kiln, still in common use, consists of a
large cylindrical stack in which the limestone