t it is a very usual thing
for us to live and move in the presence of things that are very common
to our everyday experience, and yet know scarcely anything about them,
beyond the fact that they in some way serve our purpose.

Salt is one of the commonest articles used in the preparation of our
food. It has been questioned by some people whether salt was a real
necessity as an animal food, or whether the taste for it is merely an
acquired one. All peoples in all ages seem to have used salt, and
reference to it is made in the earliest histories. Travelers tell us
that savage tribes, wherever they exist, are as much addicted to the use
of salt as civilized people. One of the early African travelers, Mungo
Park, tells us that the children of central Africa will suck a piece of
rock salt with the same avidity and seeming satisfaction as the
ordinary civilized child will a lump of sugar.

All animals seem to require salt, and it is claimed by those who have
tried the experiment that after one has refrained from the use of salt
for a certain length of time the craving for it becomes exceedingly
painful. It is most likely that the taste for salt is a natural craving.
In any event, whether it is a natural or an artificial taste, it has
become an article of the greatest importance in the preparation of food,
as well as on account of its use in the arts. Salt is a compound of
chlorine and sodium. In chemical language it is called sodium chloride.
The symbol is NaCl, which means that a molecule of salt is composed of
one atom of sodium and one of chlorine. Chlorine is an exceedingly
poisonous gas.

Formerly the chemist when he wished to obtain sodium extracted it from
common salt and discharged the chlorine gas into the air. It was found
that in establishments where the manufacture of sodium was conducted on
a large scale the destructive properties of the chlorine discharged into
the air was such that all vegetation was killed for some distance around
the manufactory. This came to be such a nuisance that the manufacturers
were either compelled to stop business or in some way take care of the
chlorine. This is done at the present day by uniting the chlorine gas
with common lime, forming a chloride of lime, which is used for
bleaching and purifying purposes.

Salt is found in great quantities as a natural product under the name of
rock salt. It is found in some parts of the world in great veins over
100 feet in thickness. In som