by the power of Niagara Falls. It is curious
to note that a substance so useful and so harmless as common salt should
be made out of two such refractory and dangerous elements as chlorine
and sodium. Both of these elements, standing by themselves, seem to be
out of harmony with nature, but when combined there are few substances
that serve a better purpose.

These great salt beds that are found to exist in England and America and
other parts of the world were undoubtedly deposited from the water of
the ocean at some stage in the formation of the earth's crust. It is
well known that sea water is exceedingly saline; 300 gallons of sea
water will produce a bushel of salt. Undoubtedly beds of salt are also
formed by inland lakes, such as the Great Salt Lake in Utah. Only about
2.7 per cent. of ocean water is salt, while the water of the Great Salt
Lake of Utah contains about 17 per cent. When there is so much salt in
water that it is called a saturated solution, salt crystals will form
and drop to the bottom, which process will in time build up under a
large body of salt water a great bed of rock salt.

The water in all rivers and springs contains salt to a certain degree,
and where it runs into a basin like that of a lake with no outlet,
through the process of evaporation pure water is being constantly
carried off, leaving the salt behind. It is easy to see that if this
process is kept up long enough the water will become in time a saturated
solution, when crystallization sets in and precipitation follows,
accounting for the deposits of rock salt.

AIR.

CHAPTER VI.

THE ATMOSPHERE.

Meteorology is a science that at one time included astronomy, but now it
is restricted to the weather, seasons, and all phenomena that are
manifested in the atmosphere in its relation to heat, electricity, and
moisture, as well as the laws that govern the ever-varying conditions of
the circumambient air of our globe. The air is made up chiefly of oxygen
and nitrogen, in the proportions of about twenty-one parts of oxygen and
seventy-nine parts nitrogen by volume, and by weight about twenty-three
parts oxygen and seventy-seven of nitrogen. These gases exist in the air
as free gases and not chemically combined. The air is simply a mixture
of these two gases.

There is a difference between a mixture and a compound. In a mixture
there is no chemical change in the molecules of the substances mixed. In
a compound there has been