ecipitate
all the acid carbonates. After a short time the sodium
carbonate is added to precipitate the other soluble salts of
calcium and magnesium, together with any excess of calcium
hydroxide which may have been added. The quantity of calcium
hydroxide and sodium carbonate required is calculated from a
chemical analysis of the water. It will be noticed that the
water softened in this way will contain sodium sulphate and
chloride, but the presence of these salts is not objectionable.

~Calcium carbide~ (CaC_{2}). This substance is made by heating well-dried
coke and lime in an electrical furnace. The equation is

CaO + 3C = CaC_{2} + CO.

The pure carbide is a colorless, transparent, crystalline substance. In
contact with water it is decomposed with the evolution of pure acetylene
gas, having a pleasant ethereal odor. The commercial article is a dull
gray porous substance which contains many impurities. The acetylene
prepared from this substance has a very characteristic odor due to
impurities, the chief of these being phosphine. It is used in
considerable quantities as a source of acetylene gas for illuminating
purposes.

~Technical preparation.~ Fig. 81 represents a recent type of a
carbide furnace. The base of the furnace is provided with a
large block of carbon A, which serves as one of the
electrodes. The other electrodes B, several in number, are
arranged horizontally at some distance above this. A mixture of
coal and lime is fed into the furnace through the trap top C,
and in the lower part of the furnace this mixture becomes
intensely heated, forming liquid carbide. This is drawn off
through the taphole D.

The carbon monoxide formed in the reaction escapes through the
pipes E and is led back into the furnace. The pipes F
supply air, so that the monoxide burns as it reenters the
furnace and assists in heating the charge. The carbon dioxide
so formed, together with the nitrogen entering as air, escape
at G. An alternating current is used.

[Illustration: Fig. 81]

~Calcium phosphate~ (Ca_{3}(PO_{4})_{2}). This important substance
occurs abundantly in nature as a constituent of apatite
(3Ca_{3}(PO_{4})_{2}.CaF_{2}), in phosphate rock, and as the chief
mineral constituent of bones. Bone ash is therefore nearly pure calcium
phosphate. It is a white powder, insoluble in water, although it