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 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 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: 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 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 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 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. |