CHEMISTRY.
In the first attempts to teach chemistry to children, objects should
be selected, the principal properties of which may be easily
discriminated by the senses of touch, taste or smell; and such terms
should be employed as do not require accurate definition.
When a child has been caught in a shower of snow, he goes to the fire
to warm and dry himself. After he has been before the fire for some
time, instead of becoming dry, he finds that he is wetter than he was
before: water drops from his hat and clothes, and the snow with which
he was covered disappears. If you ask him what has become of the snow,
and why he has become wetter, he cannot tell you. Give him a tea-cup
of snow, desire him to place it before the fire, he perceives that the
snow melts, that it becomes water. If he puts his finger into the
water, he finds that it is warmer than snow; he then perceives that
the fire which warmed him, warmed likewise the snow, which then
became water; or, in other words, he discovers, that the heat which
came from the fire goes into the snow and melts it: he thus acquires
the idea of the dissolution of snow by heat.
If the cup containing the water, or melted snow, be taken from the
fire, and put out of the window on a frosty day, he perceives, that in
time the water grows colder; that a thin, brittle skin spreads over
it; which grows thicker by degrees, till at length all the water
becomes ice; and if the cup be again put before the fire, the ice
returns to water. Thus he discovers, that by diminishing the heat of
water, it becomes ice; by adding heat to ice, it becomes water.
A child watches the drops of melted sealing-wax as they fall upon
paper. When he sees you stir the wax about, and perceives, that what
was formerly hard, now becomes soft and very hot, he will apply his
former knowledge of the effects of heat upon ice and snow, and he will
tell you that the heat of the candle melts the wax. By these means,
the principle of the solution of bodies by heat, will be imprinted
upon his memory; and you may now enlarge his ideas of solution.
When a lump of sugar is put into a dish of hot tea, a child sees that
it becomes less and less, till at last it disappears. What has become
of the sugar? Your pupil will say that it is melted by the heat of the
tea: but if it be put into cold tea, or cold water, he will find that
it dissolves, though more slowly. You should then show him some fine
sand, some clay, and chalk, thrown into water; and he will perceive
the difference between mechanical mixture and diffusion, or chemical
mixture. Chemical mixture, as that of sugar in water, depends upon the
attraction that subsists between the parts of the solid and fluid
which are combined. Mechanical mixture is only the suspension of the
parts of a solid in a fluid. When fine sand, chalk, or clay, are put
into water, the water continues for some time turbid or muddy; but by
degrees the sand, &c. falls to the bottom, and the water becomes
clear. In the chemical mixture of sugar and water, there is no
muddiness, the fluid is clear and transparent, even whilst it is
stirred, and when it is at rest, there is no sediment, the sugar is
joined with the water; a new, fluid substance, is formed out of the
two simple bodies sugar and water, and though the parts which compose
the mixture are not discernible to the eye, yet they are perceptible
by the taste.
After he has observed the mixture, the child should be asked, whether
he knows any method by which he can separate the sugar from the water.
In the boiling of a kettle of water, he has seen the steam which
issues from the mouth of the vessel; he knows that the steam is formed
by the heat from the fire, which joining with the water drives its
parts further asunder, and makes it take another form, that of vapour
or steam. He may apply this knowledge to the separation of the sugar
and water; he may turn the water into steam, and the sugar will be
left in the vessel in a solid form. If, instead of evaporating the
water, the boy had added a greater quantity of sugar to the mixture,
he would have seen, that after a certain time, the water would have
dissolved no more of the sugar; the superfluous sugar would fall to
the bottom of the vessel as the sand had done: the pupil should then
be told that the liquid is _saturated_ with the solid.
By these simple experiments, a child may acquire a general knowledge
of solution, evaporation, and saturation, without the formality of a
lecture, or the apparatus of a chemist. In all your attempts to
instruct him in chemistry, the greatest care should be taken that he
should completely understand one experiment, before you proceed to
another. The common metaphorical expression, that the mind should have
time to digest the food which it receives, is founded upon fact and
observation.
Our pupil should see the solution of a variety of substances in
fluids, as salt in water; marble, chalk, or alkalies, in acids; and
camphire in spirits of wine: this last experiment he may try by
himself, as it is not dangerous. Certainly many experiments are
dangerous, and therefore unfit for children; but others may be
selected, which they may safely try without any assistance; and the
dangerous experiments may, when they are necessary, be shown to them
by some careful person. Their first experiments should be such as they
can readily execute, and of which the result may probably be
successful: this success will please and interest the pupils, and will
encourage them to perseverance.
A child may have some spirit of wine and some camphire given to him;
the camphire will dissolve in the spirit of wine, till the spirit is
saturated; but then he will be at a loss how to separate them again.
To separate them, he must pour into the mixture a considerable
quantity of water; he will immediately see the liquor, which was
transparent, become muddy and white: this is owing to the separation
of the camphire from the spirit; the camphire falls to the bottom of
the vessel in the form of a curd. If the child had weighed the
camphire, both before and after its solution, he would have found the
result nearly the same. He should be informed, that this _chemical
operation_ (for technical terms should now be used) is called
_precipitation_: the substance that is separated from the mixture by
the introduction of another body, is cast down, or precipitated from
the mixture. In this instance, the spirit of wine attracted the
camphire, and therefore dissolved it. When the water was poured in,
the spirit of wine attracted the water more strongly than it did the
camphire; the camphire being let loose, fell to the bottom of the
vessel.
The pupil has now been shown two methods, by which a solid may be
separated from a fluid in which it has been dissolved.
A still should now be produced, and the pupil should be instructed in
the nature of distillation. By experiments he will learn the
difference between the _volatility_ of different bodies; or, in other
words, he will learn that some are made fluid, or are turned into
vapour, by a greater or less degree of heat than others. The degrees
of heat should be shown to him by the thermometer, and the use of the
thermometer, and its nature, should be explained. As the pupil already
knows that most bodies expand by heat, he will readily understand,
that an increase of heat extends the mercury in the bulb of the
thermometer, which, having no other space for its expansion, rises in
the small glass tube; and that the degree of heat to which it is
exposed, is marked by the figures on the scale of the instrument.
The business of distillation, is to separate the more volatile from
the less volatile of two bodies. The whole mixture is put into a
vessel, under which there is fire: the most volatile liquor begins
first to turn into vapour, and rises into a higher vessel, which,
being kept cold by water or snow, condenses the evaporated fluid;
after it has been condensed, it drops into another vessel. In the
experiment that the child has just tried, after having separated the
camphire from the spirit of wine by precipitation, he may separate the
spirit from the water by distillation. When the substance that rises,
or that is separated from other bodies by heat, is a solid, or when
what is collected after the operation, is solid, the process is not
called distillation, but sublimation.
Our pupil may next be made acquainted with the general qualities of
acids and alkalies. For instructing him in this part of chemistry,
definition should as much as possible be avoided; example, and occular
demonstration, should 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 when he takes hold of the bottle; nor will they
injure the table upon which he is at work. This custom, trivial as it
may seem, is of advantage, as it gives an appearance of order, and of
ease, and steadiness, which are all necessary in trying chemical
experiments. The little pupil may be told, that the custom which we
have just mentioned, is the constant practice of the great chemist,
Dr. Black.
We should take care how we first use the term _salt_ in speaking to
children, lest they should acquire indistinct ideas: he should be
told, that the kind of salt which he eats is not the only salt in the
world; he may be put in mind of the kind of salts which he has,
perhaps, smelt in smelling-bottles; and he should be further told,
that there are a number of earthy, alkaline, and metallic salts, with
which he will in time become acquainted.
When an acid is put upon an alkali, or upon limestone, chalk, or
marle, a bubbling may be observed, and a noise is heard; a child
should be told, that this is called _effervescence_. After some time
the effervescence ceases, and the limestone, &c. is dissolved in the
acid. This effervescence, the child should be informed, arises from
the escape of a considerable quantity of a particular sort of air,
called fixed air, or carbonic acid gas. In the solution of the lime in
the acid, the lime and acid have an attraction for one another; but as
the present mixture has no attraction for the gas, it escapes, and in
rising, forms the bubbling or effervescence. This may be proved to a
child, by showing him, that if an acid is poured upon caustic lime
(lime which has had this gas taken from it by fire) there will be no
effervescence.
There are various other chemical experiments with which children may
amuse themselves; they may be employed in analyzing marle, or clays;
they may be provided with materials for making ink or soap. It should
be pointed out to them, that the common domestic and culinary
operations of making butter and cheese, baking, brewing, &c. are all
chemical processes. We hope the reader will not imagine, that we have
in this slight sketch pretended to point out the _best_ experiments
which can be devised for children; we have only offered a few of the
simplest which occurred to us, that parents may not, at the conclusion
of this chapter, exclaim, "What is to be done? How are we to _begin_?
What experiments are suited to children? If we knew, our children
should try them."
It is of little consequence what particular experiment is selected for
the first; we only wish to show, that the minds of children may be
turned to this subject; and that, by accustoming them to observation,
we give them not only the power of learning what has been already
discovered, but of adding, as they grow older, something to the
general stock of human knowledge.