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 versâ_