litmus are termed indicators. A substance called phenolphthalein, a
coal-tar product, is a very delicate indicator; it is more sensitive to
acids than litmus is. Now there are some salts which contain a
preponderance of acid in their composition, _i.e._ in which the acid has
not been fully neutralised by the base; such salts are termed acid
salts. Bicarbonate of soda is one of these acid salts, but so feeble is
carbonic acid in its acid properties and practical evidences, that we
shall see both monocarbonate or "neutral" carbonate of soda and
bicarbonate or "acid" carbonate of soda show evidences of, or, as
chemists say, react with alkalinity towards litmus. However,
phenolphthalein, though reacting alkaline with monocarbonate of soda,
indicates the acidity of the bicarbonate of soda, a thing which, as I
have just said, litmus will not do. We will take two jars containing
solution of monocarbonate of soda, and in the first we will put some
phenolphthalein solution, and in the second, some litmus tincture. The
solution in the first jar turns rose coloured, and in the second, blue,
indicating in each case that the solution is alkaline. If now, however,
carbonic acid be blown into the two solutions, that in the first jar,
containing the phenolphthalein, becomes colourless as soon as the
monocarbonate of soda is converted into bicarbonate, and this
disappearance of the rose colour indicates acidity; the blue solution in
the jar containing litmus, on the other hand, is not altered by blowing
in carbonic acid. Furthermore, if to the colourless solution containing
phenolphthalein, and which is acid towards that reagent, a little
reddened litmus is added, this is still turned blue, and so still
indicates 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