is loosely
packed. A fire is built at the base of the stack, and when the
burning is complete it is allowed to die out and the lime is
removed from the kiln. The newer kilns are constructed as shown
in Fig. 80. A number of fire boxes are built around the lower
part of the kiln, one of which is shown at B. The fire is
built on the grate F and the hot products of combustion are
drawn up through the stack, decomposing the limestone. The kiln
is charged at C, and sometimes fuel is added with the
limestone to cause combustion throughout the contents of the
kiln. The burned lime is raked out through openings in the
bottom of the stack, one of which is shown at _D._ The
advantage of this kind of a kiln over the older form is that
the process is continuous, limestone being charged in at the
top as fast as the lime is removed at the bottom.

[Illustration: Fig. 80]

~Calcium hydroxide ~ (_slaked lime_) (Ca(OH)_{2}). Pure calcium hydroxide
is a light white powder. It is sparingly soluble in water, forming a
solution called _limewater_, which is often used in medicine as a mild
alkali. Chemically, calcium hydroxide is a moderately strong base,
though not so strong as sodium hydroxide. Owing to its cheapness it is
much used in the industries whenever an alkali is desired. A number of
its uses have already been mentioned. It is used in the preparation of
ammonia, bleaching powder, and potassium hydroxide. It is also used to
remove carbon dioxide and sulphur compounds from coal gas, to remove the
hair from hides in the tanneries (this recalls the caustic or corrosive
properties of sodium hydroxide), and for making mortar.

~Mortar~ is a mixture of calcium hydroxide and sand. When it is exposed to
the air or spread upon porous materials moisture is removed from it
partly by absorption in the porous materials and partly by evaporation,
and the mortar becomes firm, or _sets_. At the same time carbon dioxide
is slowly absorbed from the air, forming hard calcium carbonate:

Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.

By this combined action the mortar becomes very hard and adheres firmly
to the surface upon which it is spread. The sand serves to give body to
the mortar and makes it porous, so that the change into carbonate can
take place throughout the mass. It also prevents too much shrinkage.

~Cement.~ When limestone to which clay and sand have been added in