e cases the rock salt is mined, when it has
to be purified for commercial purposes. The common mode of obtaining
salt, however, is by pumping the solution from these great beds where it
is mingled with water--salt water; the water is then evaporated, and
when it reaches a certain stage of evaporation the salt crystallizes and
falls to the bottom.

Different substances crystallize in different forms. The crystallization
of water when it freezes, as we shall see hereafter, arranges its
molecules in such a form as to make a lump of ice of given dimensions
lighter than the same dimensions of water would be. Salt in
crystallizing does not follow the same law; the salt crystal is in the
shape of a cube and is denser in its crystalline form than in solution,
hence it is heavier and falls to the bottom.

It is said that there is a deposit of rock salt in Galicia, Austria,
covering an area of 10,000 square miles. There are also very large
deposits in England, the mining of which has become a great industry.
There are also great beds of salt in various parts of the United
States, notably near Syracuse, N. Y., where large salt deposits were
exposed in an old river bed formed in preglacial times. The common mode
of preparing salt for domestic purposes is by the process of evaporation
from brine that has been pumped from salt wells. The quality of the salt
is determined largely by the temperature at the time of evaporating the
water from it. Ordinary coarse salt, such as is used for preserving meat
or fish, is made at a temperature of about 110 degrees; what is known as
common salt is made at a temperature of about 175 degrees; while common
fine or table salt is made at a temperature of 220 degrees. Thus it will
be seen that the process of granulation with reference to its fineness
is determined by the rapidity of evaporation. Salt is one of the
principal agents in preserving all kinds of meats against putrefaction.
It will also preserve wood against dry rot. Vessel builders make use of
this fact to preserve the timbers used in the construction of the
vessels.

Salt at the present day is very cheap, but at the beginning of the
present century it was worth from $60 to $70 per ton. The methods of
decomposing salt to obtain its constituents, which are used in various
other compounds, are very simple to-day as compared with the processes
that prevailed in the days before the advent of electricity in large
volume, such as is produced