CHAPTER XXII

SCIENCE FOR THE PLAY-HOUR

$A Home-Made Electrical Machine.$--To make a really first-class machine of the modern type would require a good deal of mechanical skill, even supposing my readers to be the happy possessors of the necessary tools and materials; but the older type of machine--though of course not so powerful--will probably do quite well enough for most of their purposes.

I will, therefore, describe one of the simplest forms of these machines, such as any one, with a little care and patience, can make for himself.

The first thing to do is to get a general idea of what you are going to construct, which may be had from the illustration, and from the actual machines you may sometimes see in a shop window or in a scientific collection, like the Science Departments of the South Kensington Museum. It is the making of the cylinder machine we are going to work out, and, therefore, to begin with, the glass cylinder must be procured. This can be had from a dealer in chemical apparatus and costs only a few pence for the smaller size--about 3 inches by 6 inches. At the same time purchase a round glass rod, 3/8 inch diameter by 5 inches long; a sheet or two of tinfoil, and sixpennyworth of amalgam. From a carpenter or timber-merchant you will require a base-board for the machine, say 13 inches by 8 inches, by 1 inch thick, and of heavy wood; also two uprights, which are to stand on the base-board to support the cylinder. These may be 6 inches tall, by 2 inches by 3/4 inch.

Having now the principal parts of the frame, the work of fitting together can be begun by making a circular hole (centre about 1-1/4 inches from the end) in one wooden upright, to take easily one of the projecting glass pieces, or pivots, at the ends of the cylinder--probably 3/4 inch diameter will do. This hole may be made with a brace and suitable bit, or failing that, with a round chisel--taking care not to split the wood. In one end of the other upright cut a slot of same width as the hole, the bottoms of both being on the same level. Then rest the two glass pivots in the hole and slot, holding the uprights vertically on the base-board, when the cylinder should be quite horizontal. If it is not so, deepen the slot, or shorten either upright, as required. Drill a hole through the two sides of the slot at the top, and insert a round nail to keep the pivot from having too much play.

It will next be necessary to secure these supports to the board, which may be done by driving stout screws from below, together with the aid of some strong glue. If you have the skill it will be better to sink the supports 1/2 inch into the surface.

The position should be such, that the cylinder is not quite over the middle of the board. (See illustrations.) Next remove the cylinder by a little side working, and screw a piece of wood, 1-1/2 inches by 1/4 inch by about 7 inches, to the supports and base. This is to act as a brace to the supports, and also for holding tightening screws for the rubber.

We now come to the preparation of the rubber, which is an important detail. Get a wooden block 1-1/4 inches by 1/2 inch and 1 inch shorter than the cylinder. Smoothe off all the corners, and glue on one long edge, a piece of thin leather (chamois will do); fold over the flat side, and then glue it again at the other long edge; double it back _loosely_, and glue again in original place. This should make a sort of bag on one side of the block, which should now be stuffed with _dry_ wool or hemp, and the two ends fastened down. A piece of black silk, about 5 inches by 9 inches, must be attached to the bottom edge.

Now place the cylinder on its bearings, and press the rubber against the middle of one side, which will show what length to make the rubber stand. The thickness may be 1/4 inch, and the breadth 2 inches; one end being screwed to the rubber block at the back, and the other resting on the base-board, but attached to the brace piece by two bolts with adjustable nuts. These you can get at an ironmonger's--thumb nuts are preferable, as they can be tightened up without pliers.

As this board will be on a slope, the cushion block must be bevelled off with a chisel, so that it may rest "squarely" against the glass. The adjusting screws will enable the pressure on the glass to be regulated. Be careful to see that the silk flap (attached to the bottom edge of the rubber) comes _between_ the leather and the cylinder, and then folds over the cylinder to about the middle of the opposite side.

We next come to the "prime conductor," which is a piece of rounded wood, 2 inches in diameter and 1 inch less than the length of the cylinder. The end corners must be made round with a knife and sandpaper, so the whole surface may be quite smooth. Then lay on _evenly_ with paste, a sheet of tinfoil, notching it so that it may fold nicely over the spherical ends, and take out any ridges by rubbing with the knife handle.

An insulating support must be given to the conductor, as it is to hold the accumulated electric energy, and for this the glass rod above mentioned is required. Make a suitable hole in one side of the conductor, and in it fix one end of the rod with cement. The other end can be fixed to the base-board in the same way; or a separate stand may be used; but before doing this, drive a horizontal row of strong pins along a side of the conductor, at right angles with the rod. These should be 1/4 inch apart, starting and finishing 1/4 inch from where the surface becomes spherical at the two ends; the heads should be cut off previously with pliers, and the external length, when driven into the wood, should not exceed 1/2 inch. Now erect the conductor, and see that the rod brings it level or thereabouts with the centre line of the cylinder-side. The points should not quite touch the latter; and the silk flap must not hang down far enough to come between.

There remains now but one piece of mechanism to construct--the handle. This is apt to give trouble at first, but with care may be successfully completed. A short piece of hard wood (say 2-1/2 inches long), half of circular and half of square section, must be procured, and the rounded half cemented into one of the glass pivots. This must be done with good cement and both the glass and wood warmed, and cleaned first of all. Be careful not to crack the glass by too rapid heating. A thin layer of cement is best, while, of course, the wooden rod ought to fit closely. The square end now projecting must be provided with a handle, the making of which will serve to pass the time during which the cement is drying. Cut a square hole to fit the end in a piece of wood say 1/4 inch by 2-1/2 inches by 1 inch, which is the handle shaft. Pass a bolt through the lower part and secure the handle-bobbin itself by a nut. If nothing else can be got, a cotton-reel makes a fair handle when the flanges are cut off. If the nut works loose, pinch the threads at the end of the screw, or add a "lock nut"--_i.e._ an extra nut. Dry the cylinder and put a wooden stopper in the other glass pivot to keep out damp.

Take care to have the handle on the right-hand end of the machine when the rubber is closest to you and the conductor opposite; notice also that of the supports the _slotted_ one should now be on the _left_-hand side.

All the woodwork, as well as the ends and pivots of the cylinder, and the glass rod should be painted with shellac varnish, which may generally be had ready mixed from paint merchants, or may be made at home by dissolving shellac in methylated spirits. A stick of red sealing-wax gives a more pleasing colour for the glass work if added to the shellac solution.

All through the construction of the machine must be borne in mind the fact that rough edges or points "attract" away the electricity, and, therefore, all the edges and corners must be well rounded off and smoothed with sandpaper, and everything must be kept clean and free from dust.

When the shellac is dry, let all the parts, especially the rubber, cylinder, and rod, have a good warming before the fire. Then fixing the cylinder in its place, press the rubber firmly against it by means of the adjusting screws. After turning for a few minutes, the handle should become stiffer, and a small spark be obtained on touching the conductor. If not, tighten up the screws a little more. It is also advisable to lay a little amalgam with tallow on the rubber, _between_ the silk and the leather: a piece of tinfoil is also said to be of advantage when amalgam is not handy. Sometimes, too, a wire connection from the back of the cushion to a neighbouring gas or water pipe helps the success of the machine, but if proper attention be paid to warming and cleaning and the avoiding of edges and corners, success is almost certain after a short time. A delicate test is to observe whether a thread is attracted by the conductor, and if so, a spark may be soon looked for.

An iron clamp or two will be found of great assistance for holding down the base-board to the corner of a table.

$The Indestructible Candles.$--When a candle burns, the matter of which the candle is composed, is not lost nor destroyed. It is simply changing its form, and every part of it may be accounted for.

If we take a cold clean tumbler and hold it over the flame of the candle (Fig. 1) we shall see that the inside becomes moist with water, and on touching it our fingers are made wet. On the tumbler becoming warm, this moisture disappears. If we could surround the tumbler with an ice jacket, we should see the water from the flame of the candle dripping down, and if this were caught in a vessel we could obtain from an ordinary candle about a wine-glassful of water. We may therefore produce water from a burning candle. The cause of the water being formed is that there is in the fat of the candle, as one of its constituents, hydrogen, and as the candle burns, this unites with the oxygen of the air to form water. Wherever water is found it always consists of hydrogen and oxygen in combination, and of nothing else.

$Presence of Hydrogen Proved.$--We may prove the presence of hydrogen gas by bringing a lighted taper within two or three inches of the wick of a candle just after it has been extinguished. On holding the lighted taper in the stream of smoke coming from the wick, we shall see a tiny flame run down the smoke and re-light the candle. The hydrogen gas coming from the hot fat is being carried off in the smoke. It is very inflammable, and the flame from the taper ignites it, and in turn rekindles the candle. When the stream of smoke has ceased, it does not matter how near we hold the taper to the wick without actually touching, it will not be re-lighted. (See Fig. 2.)

$The Hydrogen Located.$--A still better way of showing the presence of this gas is by bending a piece of glass tubing of small-bore, into the shape shown in Fig. 4.

Glass tubing may be bent easily to any shape by holding it in the flame of an ordinary gas burner. The tube becomes covered with soot, and this prevents its getting hot too rapidly, and so enables the tube to bend easily and evenly. The bending must never be forced, but very gently done as the glass softens. (See Fig. 3.) A little practice will enable any boy to make a first-rate bend.

On carefully observing the flame of the candle we shall see that it really consists of three distinct parts. Round the wick it looks black, this is really a hollow chamber filled with unconsumed hydrogen. Next to this is a bright luminous cone, and outside of that is an almost invisible covering of blue flame. In the black space gas is unconsumed, in the luminous part the combustion is only partial, but outside of all, where there is most oxygen, the combustion is complete, and the flame can hardly be discovered. Now when the flame is quite steady the tube must be gently inserted at an angle into the black cone; after a few minutes, on applying a light at the end of the tube, although the candle is still burning, we shall see that this free hydrogen will burn there too with a small bluish flame.

$The Candle's Carbon.$--As the candle burns, another part of its constituents is passing off into the air as soot or carbon, and this can be shown by holding a sheet of white paper or cardboard in the top of the flame, or better still, a cold saucer, on which there will be a copious deposit of black soot. This is another proof that as a candle burns it is not destroying matter, but only changing its form; from the white fat of the candle, black sooty carbon is liberated by the process of incomplete combustion that is going on. (See Fig. 5.)

$Carbonic Acid Gas.$--When substances containing carbon are burnt, one of the products is an invisible gas, commonly called carbonic acid gas. After an explosion in a mine, all the workings are filled with a deadly gas, which often kills more men than the explosion. This is called choke damp, and is the same as carbonic acid gas. Whenever a fire burns--gas, lamp, coal fire, or candle, this gas is one of the products. Let us fasten a piece of wire round our candle, and, after lighting it, lower it down into a glass bottle with a wide mouth. At first the candle burns dimly, and then, when a current of air is established, brightens. Now cover the mouth of the jar with a piece of card or the hand, and we shall see that the candle again burns dimly and quickly goes out. The jar now contains a considerable quantity of this carbonic acid gas. We may prove its presence by pouring into the jar a little clear lime-water and shaking it up. The carbonic acid gas will turn the lime-water milky. (See Fig. 6.)

Lime-water can be purchased at any chemist's very cheaply, or it can be made by pouring water on a piece of quicklime, well shaking it, and then allowing it to settle. The clear lime-water may then be poured off. The lime may be used again and again until it is all dissolved.

$Our Use of Oxygen.$--We are breathing out carbonic acid gas; and on breathing through a piece of glass tubing into some of the clear lime-water we shall see that it will be turned milky in just the same way as when the candle burned. We are using up oxygen to support life, the candle uses up oxygen to support life, and in both cases the product is carbonic acid gas, as we have proved by means of the lime-water test. (See Fig. 7.)

$Convincing Proof.$--All that we have done up to the present supports our statement that the matter of the candle is not destroyed. In fact we have accounted for all its parts excepting that of a little mineral ash which will be left after the candle has burned away. We may, however, show in a very convincing way that our contention is true. An ordinary gas chimney is obtained, and at about three inches from one end a piece of wire gauze is placed, and the open end filled up with quicklime, at the lower end a cork is fixed upon which a short piece of candle is placed. There must also be a hole in the cork for the admission of air; when all is ready, carefully counterpoise the scales. Then remove the cork and light the candle and quickly replace. After burning a short time it will be found that the chimney glass bears down the beam because of increased weight. The products of the burning candle have united with the oxygen of the air, and these products, consisting chiefly of carbonic acid gas and water, have been caught by the quicklime. Because of the added oxygen they are heavier than the original candle. (See Fig. 8.)

$Capillary Attraction.$--There is still one interesting thing to illustrate about the burning candle, and that is the way in which the particles of fat ascend the wick to reach the flame. This is accomplished by what is known as capillary attraction. A very good illustration of this is afforded by a piece of salt standing upon a plate, on which is poured some salt water coloured blue with indigo or ink. The liquid will rise up the pillar of salt, and eventually reach the top. It rises by the force of capillary attraction. Let the pillar of salt represent the wick of the candle, and the coloured water, the fat, and the illustration is complete.

$Analysis of Candle Flame.$--Our candle can still give us some useful and suggestive illustrations of flame and combustion. We have seen that unconsumed gaseous vapours can be obtained from the flame by means of a bent glass tube. In the candle flame (Fig. 9) we see that this is because of the way the flame is built. The part marked _o_ is the gaseous chamber, _i_ is the luminous part, and _e_ is where combustion is complete. On taking a sheet of clean white paper and pressing it down on the candle flame for a moment or two we shall get the fact of this hollow chamber demonstrated by the smoke ring upon the paper, which will appear thus--

The paper is left clean at the hollow chamber, but marked with smoke at the luminous part of the flame. (See Fig. 10.)

Now we must find the differences between the non-luminous outer flame and the luminous inner flame. To do this thoroughly we must have a Bunsen burner to afford the best illustration. This is not an expensive item. A cheap and simple form of it can be obtained for 1s. 3d. To understand the nature of the flame we must first understand the principles of the Bunsen. It is a burner in which a mixture of air and gas is consumed. A is a brass tube, mounted on a solid foot K, with a small tube C to admit the gas. There are two holes at the bottom of the brass tube to admit air in the direction of the arrows, and a movable brass collar fits over these holes, so that the air can be admitted or excluded at will. On igniting the gas, with the holes of the Bunsen open, we shall see that it burns with a non-luminous but exceedingly hot flame. On closing the holes we shall notice that the flame becomes luminous, much more languid, and does not give off nearly so much heat. (See Fig. 11.)

We must ask ourselves the question, What is the cause of this difference? The answer is a simple but very instructive one. Coal-gas, like the fat of the candle, contains carbon, and in the luminous flame, owing to the limited supply of oxygen, these particles of carbon are made white hot, and so emit light, but are not entirely consumed till they reach the outer edge of the flame, where combustion is more complete, owing to the contact of the flame with the air, and even then many of them escape; and so where gas is burnt the ceilings after a time become blackened.

In the non-luminous flame, owing to the air being admitted and mixed with the gas, the increased supply of oxygen renders combustion more complete, greatly increases the heat of the flame, but renders it incapable of giving light. Now, the reasons for the differences of the two flames are made clear.

A very clever modification of this principle has been utilised in what is known as the Argand burner, in which the gas and air are not mixed as in a Bunsen, but the burner is made circular, and the air is made to pass up the centre of the flame, so that it gets its supply of oxygen, burns steadily, and presents a very large surface of luminous flame. (See Fig. 12.)

$A Pretty Experiment.$--Let us now go back to our candle flame. We see that it gives light, emits smoke, and does not yield a very large amount of heat. We have learnt that it gives light because the particles of carbon are heated to a white heat, but not entirely consumed. These particles in the flame are held very closely together, and so present a continuous surface. If we could get inside the flame and scatter them we should have a pretty shower of glowing sparks.

We can illustrate this by the following experiment. Take as much gunpowder as will rest on a sixpence, and a like quantity of iron filings, mix them together on a small tin dish. (See Fig. 13.)

This must be done carefully and without friction. Then ignite with a taper. The gunpowder burns, makes the particles of iron red hot, and scatters them in a beautiful shower of glowing sparks. This is a fair representation of pulling a candle flame to pieces, the only difference is that the glowing particles are of iron instead of carbon.

$Artificial Lightning.$--This may be further illustrated by putting a flame together. We may accomplish this by passing any very fine particles of carbonaceous matter through a non-luminous flame, and we shall see that whilst these particles pass through the flame it will give light owing to their presence.

We require a little lycopodium, a piece of glass tubing one foot long, and about a quarter-inch bore, and the non-luminous flame of the Bunsen burner or a spirit lamp. Insert into one end of the tube a little of the lycopodium powder, and then, pea-shooter fashion, apply the mouth to the other end of the tube, and blow the contents into the flame. There will be a great flash of light whilst these infinitely small particles are passing through the flame, thus establishing the fact that luminosity is due to the presence of unconsumed solid matter in the flame. This experiment is sometimes called "making artificial lightning," and in a dark room it is very effective. (See Fig. 14.)

$Flames that Laugh.$--What makes the candle flame burn steadily is the next problem before us, and we shall see that it is very simple and at the same time most philosophical. It tells us the reason why candles are made round, and not square. The section of a candle being circular, with the wick in the centre, it can, as it burns, get its supply of oxygen from all directions at an equal distance; thus it burns regularly and steadily. If the candle were square, the four corners being at a greater distance from the wick than the sides, we should have four columns of fat standing up at the corners, and as the air rushed in to feed the flame it would come into contact with these, and so the current would be broken and the flame would become unsteady. We can show this by placing some cotton wool on tin dishes, and saturating it with methylated spirits and igniting it. This will give us what are known as laughing flames, because they burn so unsteadily. The air rushing in to feed the flame comes into contact with the wool, which impedes it, and so the flame has a dancing or laughing appearance. This experiment may be made very pretty by rendering the flames coloured. To do this add to the cotton wool, before pouring on the methylated spirit, chloride of copper; this will give a green flame; to another, chloride of strontium; this will colour the flame red; to another, common salt; this will give a yellow coloration. All these should be shown in a dark room.

$The Importance of Oxygen.$--By previous experiment we have seen that oxygen is necessary to a flame, and our ingenious readers may now make a piece of apparatus to prove this. (See Fig. 15.) It consists of two pieces of glass tube standing upright near the two ends of a board, in which there is a covered channel communicating with the two. A small candle is lighted and placed in one of the tubes. The air heated by the flame rises in the tube and causes a corresponding descent of cold air down the other tube. This gives us a good illustration of ventilation produced by artificial heat. So great is the down draught, that if we hold a lighted taper over the mouth of the cold tube the smoke and the flame will be carried down, with the result that the candle is soon extinguished. The reason for this is that the smoke and burnt air from the taper contain insufficient oxygen to feed the candle flame, and it dies. To make this apparatus, obtain a piece of deal board about ten inches long and four inches wide, cut along the middle a groove about three quarters of an inch deep, and about the same width, leaving about half an inch at each end uncut. Cover this groove with a tightly-fitting slip. Over the two ends of the groove are fastened two small blocks of cork pierced with apertures, into which fit the vertical glass tubes; these should be about ten inches high and about three-quarters of an inch bore. Fig. 16, which is a section of one end of the apparatus, shows how a small candle like those used on Christmas trees is held erect by a wooden socket at the end of the groove so as not to impede the current of fresh air.

$Rates of Combustion.$--We must remember that all things do not burn at the same rate. Iron rust is a product of very slow combustion. In using up food to maintain the heat of the body, combustion goes on more quickly than in rusting iron, the candle burns more quickly still, gas still faster, the Bunsen burner faster still. We may get an idea of the different rates of combustion by the two following experiments. On a tin dish place half a thimbleful of gunpowder and lay on it a tiny piece of gun-cotton. Ignite the gun-cotton; it burns so fast that it has no time to set fire to the gunpowder, which may now be ignited in its turn by the taper. Another example is the laying of two long trains of gunpowder, one fine grain and the other coarse. It will be found that the two flames travel at very different rates along the same path.

$The Egg and Bottle Trick.$--An ordinary water-bottle, a hard-boiled egg, divested of its shell, and a piece of thin paper are all that is requisite. How can we make this egg get inside the bottle? Light the paper, quickly thrust it into the bottle, and immediately place the egg over the mouth of the bottle, gently pressing it closely down to the glass. The burning paper consumes some of the air, a partial vacuum is formed, and air pressure will force the egg into the bottle with a loud detonation. (See Fig. 17.)

$Making Water Boil by Means of Coldness.$--Heat some water to boiling in a glass flask over a spirit lamp. After the water has boiled for a minute or two, quickly insert a well-fitting cork, and remove the flask from the flame. Wrap a duster or towel round the neck of the flask, and, holding it over a basin (in case of breakage), pour gently a stream of cold water on to the flask. The steam inside is condensed, a partial vacuum is formed, and as long as any heat remains in the water, it will boil, whilst the stream of cold water is continued on the outside. When ebullition no longer occurs, it will be found that the cork is held in so tightly by air pressure that it is very difficult to draw it. (See Fig. 18.)

$Fire Designs.$--This is very simple, amusing, and effective. Make a saturated solution of nitrate of potash (common nitre or saltpetre), by dissolving the substance in warm water, until no more will dissolve; then draw with a smooth stick of wood any design or wording on sheets of white tissue paper, let it thoroughly dry, and the drawing will become invisible. By means of a spark from a smouldering match ignite the potassium nitrate at any part of the drawing, first laying the paper on a plate or tray in a darkened room. The fire will smoulder along the line of the invisible drawing until the design is complete. (See Fig. 19.)

$The Magic Wine Glass.$--The holding of a wine-glass to a substance mouth upwards without its falling off, may be accomplished thus. Obtain a wine-glass with a very even edge (this may be done by grinding on a flat stone), a square of blotting-paper, and a piece of glass. About half fill the glass with water, place upon its rim the blotting paper, and then the piece of glass. Whilst pressing them closely down invert the glass. The blotting-paper absorbs some of the water, a partial vacuum is formed, and on holding the sheet of glass, the wine-glass will remain suspended, being held on by atmospheric pressure. (See Fig. 20.)

$The Floating Needle.$--The idea of making a needle float upon water at first sight seems an impossibility but it can be done, and that with comparative ease. Take a fine needle, and rub the fingers over it gently to grease it. Now lay it very carefully on a piece of thin tissue paper on the surface of the water, as shown. Presently the paper will sink, and leave the needle floating on the water. The thin coating of grease serves to protect the needle from actual contact with the water, and thus enables it to float. (See Fig. 21.)

$A Glass of Water Turned Upside Down.$--A tumbler is filled with water, a piece of paper laid on, and the surface and the tumbler deftly inverted, the atmospheric pressure being unable to enter the glass, the water is kept in, so long as the paper holds. The effect of the experiment is very greatly increased, if, instead of using paper, a piece of thin mica, cut to the size of the glass, is used. The audience cannot then discover what prevents the water from running out. Any gasfitter will supply a piece of mica.

$The Inexhaustible Bottle.$--This wonderful bottle, from which five separate liquids can be poured, owes its marvellous qualities to the application of the simple law of atmospheric pressure. It is made of tin, and encloses five internal cylinders, each of which has a tube from the upper end running into the neck of the bottle, and another tube from the lower end opening into the side. The cylinders are filled with different liquids--water, milk, tea, coffee, lemonade. Whilst the fingers are kept over the holes the bottle may be inverted, and nothing will run out. On opening the holes one by one the liquid may be poured out, according to the wishes of the audience, and greatly to their astonishment. (See Fig. 22.)

$The Magic Writing.$--Fill a deep tumbler with water, and add a few crystals of iodide of potassium and a few drops of sulphuric acid. The liquid will remain perfectly clear like water. On some strips of white cardboard write various names with starch paste; when dry these will be invisible. On dipping the cardboard into the liquid the name will appear in blue writing, owing to the formation of starch iodide, which is blue. By previously preparing the names of those present at the experiment, by a little manipulation you can, to the astonishment of the audience, produce any name called for.

$Producing Smoke at Will.$--Two glass cylinders are the best for this, but ordinary tumblers will do. With a separate feather make the inside of each tumbler quite wet, one with hydrochloric acid, and the other with liquid ammonia. Both glasses appear to be quite empty, and nothing occurs. But on bringing the mouths of the two vessels together, a thick white smoke is at once developed. The hydrochloric acid gas and the ammonia gas unite chemically, and form the solid white powder known as sal-ammoniac. (See Fig. 23.)

$A Novel Fountain.$--This is a pretty experiment, and owes its action to the fact that ammonia gas is very soluble in water. In a basin place some water. Fit up a flask with a small-bore glass tube, about eighteen inches long, as shown. The end entering the flask should be drawn out so that there is only a small opening. In the flask place about a teaspoonful of liquid ammonia, and heat it over a spirit lamp. As soon as the liquid boils a large amount of ammonia gas is disengaged, and fills the flask and the tube. Now close the tube by means of the finger, and invert the flask over the basin of water. When the end of the tube is under the water remove your finger, and then, as the water dissolves the gas, it will rise in the tube, and will presently play into the flask like a fountain until the flask is full. (See Fig. 24.)

$To Boil Water in a Paper Bag.$--"Here is a sheet of note-paper; can you boil me a little water in it?" This would appear to be a thorough puzzler, yet it is exceedingly easy to do. Fold a piece of paper so that it will hold water, now suspend it above the flame of a lamp. The water will so readily take up all the heat that there is none left with which to burn the paper, and presently it will bubble and give off steam. (See Fig. 25.)

$Illuminated Water.$--Wet a lump of loaf sugar with phosphorized ether, and throw it into a basin of water in a dark room. The surface of the water will become luminous. Blow on the water, and you will have phosphorescent waves, and the air, too, will be illuminated. In winter the water should be warmed a little. If the phosphorized ether be applied to the hand or to other warm bodies these will become luminous. The ether will not injure the hand.

$Brilliant Crystals.$--Spread upon a plate of glass or upon a smooth slate, a few drops of nitrate of silver, previously diluted with double its quantity of soft water. Place at the bottom of it, flat upon the glass, and in contact with the fluid, a copper or zinc wire, bent to any figure, and let the whole remain undisturbed in a horizontal position. In a few hours a brilliant crystallization of metallic silver will make its appearance around the wire upon the glass, and this arrangement of crystals will extend gradually till the whole quantity of fluid has been acted on by the wire.

$A Well of Fire.$--Add gradually one ounce, by measure, of sulphuric acid, to five or six ounces of water in an earthenware basin; and add to it also, gradually, about three quarters of an ounce of granulated zinc. A rapid production of hydrogen gas will instantly take place. Then add, from time to time, a few pieces of phosphorus of the size of a pea. A multitude of gas bubbles will be produced, which will fire on the surface of the effervescing liquid; the whole surface of the liquid will become luminous, and fire balls, with jets of fire, will dart from the bottom through the fluid with great rapidity and a hissing noise.

$The Writing on the Wall.$--Take a piece of phosphorus from the bottle in which it is kept, and, while the room is lighted write upon a whitewashed wall any word or sentence, or draw any object. Now put out the light, and the writing will appear in illuminated letters. Care must be taken to dip the pencil of phosphorus in cold water frequently while you are using it. Otherwise it will burn.

$To Make a Ghost.$--Put one part of phosphorus into six of olive oil, and let it dissolve in a slightly warm place. Shut your eyes tightly and rub the mixture upon your face. In the dark your face will be luminous, your eyes and mouth like dark spots. Altogether you will have a very ghastly appearance. There is no danger in the experiment, and the effect might be useful in charades or home theatricals.

$A Seeming Conflagration.$--Take half an ounce of sal-ammoniac, one ounce of camphor, and two ounces of aqua vitae. Put them into an earthen vessel that is small at the top. Set fire to the contents, and the room will seem to be on fire.

$Three Haloes.$--One of the pleasing experiments of Dr. Brewster was to take a saturated solution of alum, and having spread a few drops of it over a plate of glass, it will crystallize rapidly though the crystals are so small you may scarcely see them. When this plate of glass is held between you and the sun or artificial light, with the eyes very near to the smooth side of the glass, there will be seen three beautiful haloes of light.

$Beautiful Crystals.$--Pour three ounces of diluted nitric acid into a glass vessel, and add gradually to it two ounces of bismuth, broken by a hammer into small pieces. The metal will be attacked with great energy, and nitrate of bismuth will be formed. Crystallize the solution by a gentle heat, and preserve the crystals, which possess great beauty, under a glass.

$The Centre of Gravity.$--A shilling may be made to balance on the point of a needle with very simple apparatus. Put a bottle on the table with a cork in its neck; into the cork stick a middle-sized needle in an upright position. In another cork cut a slit, and insert the shilling, then into this cork stick a couple of forks, one on each side, with the handles inclining outwards. Now poise the rim of the shilling upon the point of the needle, and it will rotate without falling. So long as the centre of gravity is kept within the points of support of a body it cannot fall. The balancing shilling may be transposed to the edge of a bottle, and it will still perform, even as the bottle is being tilted.

$What a Vacuum Can Do.$--Take a new or nearly new penny and rub it briskly upon your coat sleeve until it is warm. Then slide it up and down upon a door panel, pressing it closely to the wood. Now hold it in one place for a few seconds and you will find it will stick there, because between the penny and the surface of the door there is a layer of air which was slightly heated. As it became cool a partial vacuum was formed, and the pressure of the outer air held the penny to the door.

$An Experiment in Leverage.$--It would seem almost impossible that a column of iron or a plank or a spar of any kind could be so placed that one end of the spar needs support only, whilst the other end would extend from, say the edge of a precipice, horizontally into space; but that such can be done is very easily demonstrated, by very simple materials almost always at hand. By adopting the principle we may easily perform an interesting scientific parlour experiment, which always causes difficulty to the non-studious section of humanity, until the apparent mystery is explained.

In illustrating this experiment the prongs of two ordinary table forks are fastened together, one over the other--net fashion--thus causing the handles of the forks to form the termini of an angle of about 45 degrees. Now take an ordinary lucifer match and place one end between the network of the prongs firmly. Then place the other end of the match upon the edge of an elevation, such as a tumbler or cup, when the match, acting as a lever, with the forks giving a hundred or a thousand times additional weight to the lever, will rest (or apparently float in the air) without further support.

Ask your friends to try the experiment, after placing the materials before them, and find how many can perform it without guidance.

$Coloured Fires.$--It is perilous to make some coloured fires, especially those in which there is sulphur, and even if they do not explode their fumes are harmful, so that their use in the house for charades or other home purposes is objectionable and at times positively dangerous. We give, however, a number of coloured fires that are free from these drawbacks, though all the same it is wiser to reduce the ingredients to powder quite separately before they are mixed, and if a pestle and mortar are used all traces of one powder should be removed before another is introduced. Each ingredient should be reduced to a fine powder.

RED FIRE. Parts. Strontia 18 Shellac 4 Chlorate of Potash 5 Charcoal 4

GREEN FIRE.

Nitrate of Barytes 18 Shellac 4 Calomel (Chloride of Mercury) 4 Chlorate of Potash 2

GREEN FIRE.

Nitrate of Barytes 9 Shellac 3 Chlorate of Potash 12 Charcoal 4

BLUE FIRE. Parts. Chlorate of Potash 14 Salpetre 6 Ammonia Sulphate of Copper 6 Arsenite of Copper 6 Shellac 2

BLUE FIRE.

Ammonia Sulphate of Copper 8 Chlorate of Potash 6 Shellac 1 Charcoal 2

RED FIRE.

Nitrate of Strontia 9 Shellac 3 Chlorate of Potash 1-1/2 Charcoal 4