hould be pursued. Who would begin to explain by words
the difference between an acid and an alkali, when these can be shown
by experiments upon the substances themselves? The first great
difference which is perceptible between an acid and an alkali, is
their taste. Let a child have a distinct perception of the difference
of their tastes; let him be able to distinguish them when his eyes
are shut; let him taste the strongest of each so as not to hurt him,
and when he has once acquired distinct notions of the pungent taste of
an alkali, and of the sour taste of an acid, he will never forget the
difference. He must afterwards see the effects of an acid and alkali
on the blue colour of vegetables at _separate times_, and not on the
same day; by these means he will more easily remember the experiments,
and he will not confound their different results. The blue colour of
vegetables is turned red by acids, and green by alkalies. Let your
pupil take a radish, and scrape off the blue part into water; it
should be left for some time, until the water becomes of a blue
colour: let him pour some of this liquor into two glasses; add vinegar
or lemon juice to one of them, and the liquor will become red;
dissolve some alkali in water, and pour this into the other glass, and
the dissolved radish will become green. If into the red mixture alkali
be poured, the colour will change into green; and if into the liquor
which was made green, acid be poured, the colour will change to red:
thus alternately you may pour acid or alkali, and produce a red or
green colour successively. Paper stained with the blue colour of
vegetables, is called _test_ paper; this is changed by the least
powerful of the acids or alkalies, and will, therefore, be peculiarly
useful in the first experiments of our young pupils. A child should
for safety use the weakest acids in his first trials, but he should be
shown that the effects are similar, whatever acids we employ; only the
colour will be darker when we make use of the strong, than when we use
the weak acids. By degrees the pupil should be accustomed to employ
the strong acids; such as the vitriolic, the nitric, and the muriatic,
which three are called fossil acids, to distinguish them from the
vegetable, or weaker acids. We may be permitted to advise the young
chemist to acquire the habit of wiping the neck of the vessel out of
which he pours any strong acid, as the drops of the liquor will not
then burn his hand