tes the presence of alkali. We have, therefore, in bicarbonate of
soda a salt which behaves as an acid to phenolphthalein and as an alkali
to litmus. Another extremely sensitive indicator is the coal-tar
dyestuff known as "Congo red"; the colour changes produced by it are
exactly the inverse of those produced in the case of litmus, that is, it
gives a blue colour with acids and a red colour with alkalis.

We have now learned that acids are as the antipodes to alkalis or bases,
and that the two may combine to form products which may be neutral or
may have a preponderance either of acidity or of basicity--in short,
they may yield neutral, acid, or basic salts. I must try to give you a
yet clearer idea of these three classes of salts. Now acids in general
have, as we have seen, what we may call a "chemical appetite," and each
acid in particular has a "specific chemical appetite" for bases, that
is, each acid is capable of combining with a definite quantity of an
individual base. The terms "chemical appetite" and "specific chemical
appetite" are names I have coined for your present benefit, but for
which chemists would use the words "affinity" and "valency"
respectively. Now some acids have a moderate specific appetite, whilst
others possess a large one, and the same may be said of bases, and thus
as an example we may have mono-, di-, and tri-acid salts, or mono-, di-,
and tri-basic salts. In a tri-acid salt a certain voracity of the base
is indicated, and in a tri-basic salt, of the acid. Again, with a base
capable of absorbing and combining with its compound atom or molecule
several compound atoms or molecules of an acid, we have the possibility
of partial saturation, and, perhaps, of several degrees of it, and also
of full saturation, which means combination to the full extent of the
powers of the base in question. Also, with an acid capable of, or
possessing a similar large absorptive faculty for bases, we have
possibilities of the formation of salts of various degrees of basicity,
according to the smaller or larger degree of satisfaction given to the
molecule of such acid by the addition of a base. We will now take as a
simple case that of hydrochloric acid (spirits of salt), which is a
monobasic acid, that is, its molecule is capable of combining with only
one molecule of a monoacid base. Hydrochloric acid may be written, as
its name would indicate, HCl, and an addition even of excess of such a
base as caustic soda (wr