ues. It
is a strong base. It is used in a great many chemical industries, and
under the name of lye is employed to a small extent as a cleansing agent
for household purposes.

~Sodium chloride~ (_common salt_) (NaCl). 1. _Preparation._ Sodium
chloride, or common salt, is very widely distributed in nature. Thick
strata, evidently deposited at one time by the evaporation of salt
water, are found in many places. In the United States the most important
localities for salt are New York, Michigan, Ohio, and Kansas. Sometimes
the salt is mined, especially if it is in the pure form called rock
salt. More frequently a strong brine is pumped from deep wells sunk into
the salt deposit, and is then evaporated in large pans until the salt
crystallizes out. The crystals are in the form of small cubes and
contain no water of crystallization; some water is, however, held in
cavities in the crystals and causes the salt to decrepitate when heated.

2. _Uses._ Since salt is so abundant in nature it forms the starting
point in the preparation of all compounds containing either sodium or
chlorine. This includes many substances of the highest importance to
civilization, such as soap, glass, hydrochloric acid, soda, and
bleaching powder. Enormous quantities of salt are therefore produced
each year. Small quantities are essential to the life of man and
animals. Pure salt does not absorb moisture; the fact that ordinary salt
becomes moist in air is not due to a property of the salt, but to
impurities commonly occurring in it, especially calcium and magnesium
chlorides.

~Sodium sulphate~ (_Glauber's salt_) (Na_{2}SO_{4}.10H_{2}O). This salt is
prepared by the action of sulphuric acid upon sodium chloride,
hydrochloric acid being formed at the same time:

2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.

Some sodium sulphate is prepared by the reaction represented in the
equation

MgSO_{4} + 2NaCl = Na_{2}SO_{4} + MgCl_{2}.

The magnesium sulphate required for this reaction is obtained in large
quantities in the manufacture of potassium chloride, and being of little
value for any other purpose is used in this way. The reaction depends
upon the fact that sodium sulphate is the least soluble of any of the
four factors in the equation, and therefore crystallizes out when hot,
saturated solutions of magnesium sulphate and sodium chloride are mixed
together and the resulting mixture cooled.

Sodium sulphate forms large efflorescent crysta