ct is
calcium chloride.

~Historical.~ In former times sodium carbonate was made by
burning seaweeds and extracting the carbonate from their ash.
On this account the salt was called _soda ash_, and the name is
still in common use. During the French Revolution this supply
was cut off, and in behalf of the French government Le Blanc
made a study of methods of preparing the carbonate directly
from salt. As a result he devised the method which bears his
name, and which was used exclusively for many years. It has
been replaced to a large extent by the Solvay process, which
has the advantage that the materials used are inexpensive, and
that the ammonium hydrogen carbonate used can be regenerated
from the products formed in the process. Much expense is also
saved in fuel, and the sodium hydrogen carbonate, which is the
first product of the process, has itself many commercial uses.
The Le Blanc process is still used, however, since the
hydrochloric acid generated is of value.

~By-products.~ The substances obtained in a given process, aside
from the main product, are called the by-products. The success
of many processes depends upon the value of the by-products
formed.

Thus hydrochloric acid, a by-product in the Le Blanc process,
is valuable enough to make the process pay, even though sodium
carbonate can be made cheaper in other ways.

~Properties of sodium carbonate.~ Sodium carbonate forms large crystals of
the formula Na_{2}CO_{3} . 10 H_{2}O. It has a mild alkaline reaction
and is used for laundry purposes under the name of washing soda. Mere
mention of the fact that it is used in the manufacture of glass, soap,
and many chemical reagents will indicate its importance in the
industries. It is one of the few soluble carbonates.

~Sodium hydrogen carbonate~ (_bicarbonate of soda_) (NaHCO_{3}). This
salt, commonly called bicarbonate of soda, or baking soda, is made by
the Solvay process, as explained above, or by passing carbon dioxide
into strong solutions of sodium carbonate:

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

The bicarbonate, being sparingly soluble, crystallizes out. A mixture of
the bicarbonate with some substance (the compound known as cream of
tartar is generally used) which slowly reacts with it, liberating carbon
dioxide, is used largely in baking. The carbon dioxide generat