causticising, that is, treatment with caustic lime or quicklime.

It will have been noticed that one of the chief reagents in the Leblanc
process is the sulphur used in the form of brimstone or as pyrites for
making vitriol in the first stage; this sulphur goes through the entire
process; from the vitriol it goes to form a constituent of the
salt-cake, and afterwards of the calcium sulphide contained in the black
ash. This calcium sulphide remains as an insoluble mass when the
carbonate of soda is extracted from the black ash, and forms the chief
constituent of the alkali waste, which until the year 1880 could be seen
in large heaps around chemical works. Now, however, by means of
treatment with kiln gases containing carbonic acid, the sulphur is
extracted from the waste in the form of hydrogen sulphide, which is
burnt to form vitriol, or is used for making pure sulphur; and so what
was once waste is now a source of profit.

_Ammonia-Soda Process of Alkali Manufacture._--This process depends
upon the fact that when carbonic acid is forced, under pressure, into a
saturated solution of ammonia and common salt, sodium bicarbonate is
precipitated, whilst ammonium chloride or "sal-ammoniac" remains
dissolved in the solution. The reaction was discovered in 1836 by a
Scotch chemist named John Thom, and small quantities of ammonia-soda
were made at that time by the firm of McNaughton & Thom. The successful
carrying out of the process on the large scale depends principally upon
the complete recovery of the expensive reagent, ammonia, and this
problem was only solved within comparatively recent years by Solvay. The
process has been perfected and worked with great success in England by
Messrs. Brunner, Mond, & Co., and has proved a successful rival to the
Leblanc process.

Alkali is also produced to some extent by electrolytic processes,
depending upon the splitting up of a solution of common salt into
caustic soda and chlorine by the use of an electric current.

LECTURE VI

BORIC ACID, BORAX, SOAP

_Boric Acid._--At ordinary temperatures and under ordinary conditions
boric acid is a very weak acid, but like silicic and some other acids,
its relative powers of affinity and combination become very much changed
at high temperatures; thus, fused and strongly heated boric acid can
decompose carbonates and even sulphates, and yet a current of so weak an
acid as hydrogen sulphide, passed through a strong solution of borax,