cts upon the calcium carbonate. Some
limestone is decomposed by the heat, forming calcium oxide.
When treated with water the calcium oxide is changed into
hydroxide, and this prevents the water from decomposing the
insoluble calcium sulphide.

The crude product of the process is a hard black cake called
black ash. On digesting this mass with water the sodium
carbonate passes into solution. The pure carbonate is obtained
by evaporation of this solution, crystallizing from it in
crystals of the formula Na_{2}CO_{3}.10H_{2}O. Since over 60%
of this salt is water, the crystals are sometimes heated until
it is driven off. The product is called calcined soda, and is,
of course, more valuable than the crystallized salt.

2. _Solvay process._ This more modern process depends upon the reactions
represented in the equations

NaCl + NH_{4}HCO_{3} = NaHCO_{3} + NH_{4}Cl,

2NaHCO_{3} = Na_{2}CO_{3} + H_{2}O + CO_{2}.

The reason the first reaction takes place is that sodium hydrogen
carbonate is sparingly soluble in water, while the other compounds are
freely soluble. When strong solutions of sodium chloride and of ammonium
hydrogen carbonate are brought together the sparingly soluble sodium
hydrogen carbonate is precipitated. This is converted into the normal
carbonate by heating, the reaction being represented in the second
equation.

~Technical preparation.~ In the Solvay process a very
concentrated solution of salt is first saturated with ammonia
gas, and a current of carbon dioxide is then conducted into the
solution. In this way ammonium hydrogen carbonate is formed:

NH_{3} + H_{2}O + CO_{2} = NH_{4}HCO_{3}.

This enters into double decomposition with the salt, as shown
in the first equation under the Solvay process. After the
sodium hydrogen carbonate has been precipitated the mother
liquors containing ammonium chloride are treated with lime:

2NH_{4}Cl + CaO = CaCl_{2} + 2 NH_{3} + H_{2}O.

The lime is obtained by burning limestone:

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

The ammonia and carbon dioxide evolved in the latter two
reactions are used in the preparation of an additional quantity
of ammonium hydrogen carbonate. It will thus be seen that there
is no loss of ammonia. The only materials permanently used up
are calcium carbonate and salt, while the only waste produ