oms of hydrogen capable of being replaced by metals, and when
such hydrogen atoms are completely replaced by metals, there result
so-called neutral or normal salts, that is, neutral substances having no
action on litmus solution. These salts can also be produced by the union
of acids with equivalent quantities of certain metallic oxides or
hydroxides, called bases, of which those soluble in water are termed
alkalis. Alkalis have a caustic taste, and turn red litmus solution
blue.

In order to explain what is called the law of equivalence, I will remind
you of the experiment of the previous lecture, when a piece of bright
iron, being placed in a solution of copper sulphate, became coated with
metallic copper, an equivalent weight of iron meanwhile suffering
solution as sulphate of iron. According to the same law, a certain
weight of soda would always require a certain specific equivalent weight
of an acid, say hydrochloric acid, to neutralise its alkaline or basic
properties, producing a salt.

The specific gravities of acids and alkalis in solution are made use of
in works, etc., as a means of ascertaining their strengths and
commercial values. Tables have been carefully constructed, such that
for every degree of specific gravity a corresponding percentage strength
of acidity and alkalinity may be looked up. The best tables for this
purpose are given in Lunge and Hurter's _Alkali-Makers' Pocket-Book_,
but for ordinary purposes of calculation in the works or factory, a
convenient relationship exists in the case of hydrochloric acid between
specific gravity and percentage of real acid, such that specific gravity
as indicated by Twaddell's hydrometer directly represents percentage of
real acid in any sample of hydrochloric acid.

The point at which neutralisation of an acid by alkali or _vice versa_
just takes place is ascertained very accurately by the use of certain
sensitive colours. At first litmus and cochineal tinctures were used,
but in testing crude alkalis containing alumina and iron, it was found
that lakes were formed with these colours, and they become precipitated
in the solution, and so no longer sensitive. The chemist was then
obliged to resort to certain sensitive coal-tar colours, which did not,
as the dyer and printer knew, form lakes with alumina and iron, such as
methyl orange, fluorescein, Congo red, phenolphthalein, and so forth.
For determining the alkalimetric strength of commercial sodas, a known