the addition of hydrochloric acid to a solution
of hydrogen sulphide. Hydrogen sulphide is a weak acid, and the
concentration of the hydrogen ions in its aqueous solutions is very
small. The equilibrium in such a solution may be represented as:

(!(Conc'n H^{+})^{2} x Conc'n S^{--})/Conc'n H_{2}S = Constant!, and a
marked increase in the concentration of the H^{+} ions, such as would
result from the addition of even a small amount of the highly ionized
hydrochloric acid, displaces the point of equilibrium and some of the
S^{--} ions unite with H^{+} ions to form undissociated H_{2}S. This
is of much importance in studying the reactions in which hydrogen
sulphide is employed, as in qualitative analysis. By a parallel course
of reasoning it will be seen that the addition of a salt of a weak
acid or base to solutions of that acid or base make it, in effect,
still weaker because they decrease its percentage ionization.

To understand the changes which occur when solids are dissolved where
chemical action is involved, it should be remembered that no substance
is completely insoluble in water, and that those products of a
chemical change which are least dissociated will first form. Consider,
for example, the action of hydrochloric acid upon magnesium hydroxide.
The minute quantity of dissolved hydroxide dissociates thus:
Mg(OH)_{2} <--> Mg^{++} + 2OH^{-}. When the acid is introduced,
the H^{+} ions of the acid unite with the OH^{-} ions to form
undissociated water. The concentration of the OH^{-} ions is thus
diminished, more Mg(OH)_{2} dissociates, the solution is no longer
saturated with the undissociated compound, and more of the solid
dissolves. This process repeats itself with great rapidity until, if
sufficient acid is present, the solid passes completely into solution.

Exactly the same sort of process takes place if calcium oxalate, for
example, is dissolved in hydrochloric acid. The C_{2}O_{4}^{--} ions
unite with the H^{+} ions to form undissociated oxalic acid, the acid
being less dissociated than normally in the presence of the H^{+} ions
from the hydrochloric acid (see statements regarding hydrogen sulphide
above). As the undissociated oxalic acid forms, the concentration of
the C_{2}O_{4}^{--} ions lessens and more CaC_{2}O_{4} dissolves,
as described for the Mg(OH)_{2} above. Numerous instances of the
applications of these principles are given in the Notes.

Water itself is slightly dissociated, and although