aste and is then poured into molds to
solidify. It is sold in the form of slender sticks.

(b) The newer methods depend upon the electrolysis of sodium chloride.
In the Castner process a solution of salt is electrolyzed, the reaction
being expressed as follows:

NaCl + H_{2}O = NaOH + H + Cl.

The chlorine escapes as a gas, and by an ingenious mechanical device the
sodium hydroxide is prevented from mixing with the salt in the solution.

In the Acker process the electrolyte is _fused_ sodium chloride. The
chlorine is evolved as a gas at the anode, while the sodium alloys with
the melted lead which forms the cathode. When this alloy is treated with
water the following reaction takes place:

Na + H_{2}O = NaOH + H.

[Illustration: Fig. 77]

~Technical process.~ A sketch of an Acker furnace is represented in Fig.
77. The furnace is an irregularly shaped cast-iron box, divided into
three compartments, A, B, and C. Compartment A is lined with
magnesia brick. Compartments B and C are filled with melted lead,
which also covers the bottom of A to a depth of about an inch. Above
this layer in A is fused salt, into which dip carbon anodes D. The
metallic box and melted lead is the cathode.

When the furnace is in operation chlorine is evolved at the
anodes, and is drawn away through a pipe (not represented) to
the bleaching-powder chambers. Sodium is set free at the
surface of the melted lead in A, and at once alloys with it.
Through the pipe E a powerful jet of steam is driven through
the lead in B upwards into the narrow tube F. This forces
the lead alloy up through the tube and over into the chamber
G.

In this process the steam is decomposed by the sodium in the
alloy, forming melted sodium hydroxide and hydrogen. The melted
lead and sodium hydroxide separate into two layers in G, and
the sodium hydroxide, being on top, overflows into tanks from
which it is drawn off and packed in metallic drums. The lead is
returned to the other compartments of the furnace by a pipe
leading from H to I. Compartment C serves merely as a
reservoir for excess of melted lead.

2. _Properties._ Sodium hydroxide is a white, crystalline, brittle
substance which rapidly absorbs water and carbon dioxide from the air.
As the name (caustic soda) indicates, it is a very corrosive substance,
having a disintegrating action on most animal and vegetable tiss