r metals of the family are of relatively small
importance.

The compounds of sodium and potassium are so similar in properties that
they can be used interchangeably for most purposes. Other things being
equal, the sodium compounds are prepared in preference to those of
potassium, since they are cheaper. When a given sodium compound is
deliquescent, or is so soluble that it is difficult to purify, the
corresponding potassium compound is prepared in its stead, provided its
properties are more desirable in these respects.

SODIUM

~Occurrence in nature.~ Large deposits of sodium chloride have been found
in various parts of the world, and the water of the ocean and of many
lakes and springs contains notable quantities of it. The element also
occurs as a constituent of many rocks and is therefore present in the
soil formed by their disintegration. The mineral cryolite
(Na_{3}AlF_{6}) is an important substance, and the nitrate, carbonate,
and borate also occur in nature.

~Preparation.~ In 1807 Sir Humphry Davy succeeded in preparing very small
quantities of metallic sodium by the electrolysis of the fused
hydroxide. On account of the cost of electrical energy it was for many
years found more economical to prepare it by reducing the carbonate with
carbon in accordance with the following equation:

Na_{2}CO_{3} + 2C = 2Na + 3CO.

The cost of generating the electric current has been diminished to such
an extent, however, that it is now more economical to prepare sodium by
Davy's original method, namely, by the electrolysis of the fused
hydroxide or chloride. When the chloride is used the process is
difficult to manage, owing to the higher temperature required to keep
the electrolyte fused, and because of the corroding action of the fused
chloride upon the containing vessel.

[Illustration: SIR HUMPHRY DAVY (English) (1778-1829)

Isolated sodium, lithium, potassium, barium, strontium, and calcium by
means of electrolysis; demonstrated the elementary nature of chlorine;
invented the safety lamp; discovered the stupefying effects of nitrous
oxide]

~Technical preparation.~ The sodium hydroxide is melted in a
cylindrical iron vessel (Fig. 76) through the bottom of which
rises the cathode K. The anodes A, several in number, are
suspended around the cathode from above. A cylindrical vessel
C floats in the fused alkali directly over the cathode, and
under this cap the sodium and hydrogen