The Screw.
The screw is an inclined plane wound round a cylinder; the height of all its revolutions round the cylinder taken together, compared with the space through which the power that turns it passes, is the measure of its mechanical advantage. Let the lever, used in the last experiment, be turned in such a manner as to reach from its gudgeon to the shaft of the Panorganon, guided by an attendant lever as before. (Plate 2. Fig. 8.) Let the wheel rest upon the lowest helix or thread of the screw: as the arms of the shaft are turned round, the wheel will ascend, and carry up the weight which is fastened to the lever. As the situation of the screw prevents the weight from being suspended exactly from the centre of the screw, proper allowance must be made for this in estimating the force of the screw, or determining the mechanical advantage gained by the lever: this can be done by measuring the perpendicular ascent of the weight, which in all cases is better, and more expeditious, than measuring the parts of a machine, and estimating its force by calculation; because the different diameters of ropes, and other small circumstances, are frequently mistaken in estimates.
The space passed through by the moving power, and by that which it moves, are infallible data for estimating the powers of engines. Two material subjects of experiments, yet remain for the Panorganon; friction, and wheels of carriages: but we have already extended this article far beyond its just proportion to similar chapters in this work. We repeat, that it is not intended in this, or in any other part of our design, to write treatises upon science; but merely to point out methods for initiating young people in the rudiments of knowledge, and of giving them a clear and distinct view of those principles upon which they are founded. No preceptor, who has had experience, will cavil at the superficial knowledge of a boy of twelve or thirteen upon these subjects; he will perceive, that the general view, which we wish to give our pupils of the useful arts and sciences, must certainly tend to form a taste for literature and investigation. The sciolist has learned only to talk — we wish to teach our pupils to think, upon the various objects of human speculation.
The Panorganon may be employed in trying the resistance of air and water; the force of different muscles; and in a great variety of amusing and useful experiments. In academies, and private families, it may be erected in the place allotted for amusement, where it will furnish entertainment for many a vacant hour. When it has lost its novelty, the shaft may from time to time be taken down, and a swing may be suspended in its place. It may be constructed at the expense of five or six pounds: that which stands before our window, was made for less than three guineas, as we had many of the materials beside us for other purposes.
When this question was sometime afterwards repeated to S —— , he observed, that the feather would throw down the castle, if its swiftness were so great as to make up for its want of weight.
Were it thought necessary to make these experiments perfectly accurate, a segment of a pulley, the radius of which is half the length of the scale-beam, should be attached to the end of the beam; upon which the cord may apply itself, and the pulley (P 3) should be raised or lowered, to bring the rope horizontally from the man’s shoulder when in the attitude of drawing.
The word power is here used in a popular sense, to denote the strength or efficacy that is employed to produce an effect by means of any engine.
In all these experiments with the skeleton-pulley, somebody must keep it in its proper direction; as from its structure, which is contrived for illustration, not for practical use, it cannot retain its proper situation without assistance.
In a loom this secondary lever is called a lamb, by mistake, for lam; from lamina, a slip of wood.
There should be three rollers used; one of them must be placed before the sledge, under which it will easily find its place, if the bottom of the sledge near the foremost end is a little sloped upwards. To retain this foremost roller in its place until the sledge meets it, it should be stuck lightly on the road with two small bits of wax or pitch.
Mechanical advantage is not a proper term, but our language is deficient in proper technical terms. The word power is used so indiscriminately, that it is scarcely possible to convey our meaning, without employing it more strictly.
In this experiment, the boy should pull as near as possible to the shaft, within a foot of it, for instance, else he will have such mechanical advantage as cannot be counterbalanced by any weight which the machine would be strong enough to bear.
CHAPTER XVIII. 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 t
he 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 g
as 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.
Complete Novels of Maria Edgeworth Page 730
