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.
CHAPTER XIX.
ON PUBLIC AND PRIVATE EDUCATION.