its name would indicate, HCl, and an addition even of excess of such a
base as caustic soda (written NaOH) would only yield what is known as
common salt or chloride of sodium (NaCl), in which the metal sodium (Na)
has replaced the hydrogen (H) of the hydrochloric acid. Now chloride of
sodium when dissolved in water will turn litmus neither blue nor red; it
is therefore neutral. Such simple, neutral, monobasic salts are mostly
very stable. By "stable" we mean they possess considerable resistance to
agencies, that, in the case of other salts, effect decompositions of
those salts. Such other salts which are decomposed more or less readily
are termed "unstable," but the terms are of course only comparative.

Now let us consider a di- or bi-basic acid. Such an one is vitriol or
sulphuric acid (H_{2}SO_{4}). The hydrogen atoms are in this case an
index of the basicity of the acid, and accordingly the fully saturated
sodium salt is Na_{2}SO_{4} or neutral, or better normal, sulphate of
soda. In like manner the fully saturated salt of the dibasic acid,
carbonic acid (H_{2}CO_{3}), is Na_{2}CO_{3}, ordinary or normal
carbonate of soda. But we must observe that with these dibasic acids it
is possible, by adding insufficient alkali to completely saturate them,
to obtain salts in which only one hydrogen atom of the acid is replaced
by the metal of the base. Thus sulphuric and carbonic acids yield
NaHSO_{4}, acid sulphate or bisulphate of soda, and NaHCO_{3},
bicarbonate of soda, respectively. An example of a tribasic acid is
phosphoric acid, H_{3}PO_{4}, and here we may have three different
classes of salts of three various degrees of basicity or
base-saturation. We may have the first step of basicity due to
combination with soda, NaH_{2}PO_{4}, or monosodium phosphate, the
second step, Na_{3}HPO_{4}, or disodium phosphate, and the third, and
final step, Na_{3}PO_{4}, or trisodium phosphate. Now let us turn to the
varying degrees of acidity, or rather the proportions of acid radicals