It is a very good amusement for ingenious boys at home, in the long winter evenings, to construct such philosophical instruments, or perform such experiments, as are practicable, with such materials and means as are within their reach. It is true, that this may sometimes make parents or an older sister some trouble, but with proper care on the part of the young philosophers, this trouble will not be great, and parents will generally be willing to submit to it for the sake of having their children engaged in an entertaining and instructive employment. We shall, therefore, give our readers such lessons in practical philosophy, as we suppose may be of use. In this article we will show them how they may, with few materials and ordinary ingenuity, construct an Air thermometer.

The materials which will be wanted are these:—a glass phial,—one that is broad at the base in proportion to its height, so as to stand firm, is desirable,—a glass tube of small bore, six or eight inches long,—a cork to fit the mouth of the phial,—a little sealing-wax, a lamp, and a small pitcher of water. The work may be safely done upon the parlour-table, provided that the materials are all placed upon a large tea-tray, with an old newspaper, or a sheet of wrapping-paper spread over it. The paper will then intercept any drops of hot sealing-wax which may chance to fall, and which might otherwise injure the tray, and the tray itself will receive whatever may be spilt.

The only article of the above materials in regard to which the reader will have any difficulty, is the tube. Such a tube, however, can usually be procured at an apothecary’s, at a very trifling expense. One about the dimensions of a pipe-stem will be best. In constructing the instrument, this tube is to be passed down through the cord, which is to be placed in the neck of the phial, the lower end to go below the surface of a little water, which is to be put in the bottom of the phial.

The appearance of the instrument, when completed and fitted with a scale, as will be explained hereafter, is represented in the annexed wood-cut. In constructing the instrument, the operations, or the manipulations, as the philosophers call them, are,

First, to bore a hole through the cork, to receive the tube.

Second, to cement the tube into the cork.

Third, to cement the cork into the phial.

1. The first thing is to bore a hole through the cork, and this must be just large enough to admit the glass tube.—After turning in the gimlet a little way, it should be drawn out straight, by which means the chips will be drawn out, and then it should be put in again. For a gimlet will not clear its own way in cork, as it will in wood. By drawing it out, however, in the manner above described, taking care to operate gently, so as not to split the cork, and to guide the gimlet straight through the centre of the cork, the hole may be bored without much difficulty. If the hole is not quite large enough, it may be widened by a penknife which has a narrow blade, or it may be burnt out to a proper size with a hot knitting-needle, or a piece of iron wire. And thus the hole is bored through the cork.

2. The next thing is to cement the tube to the cork. In order to do this, the water is first to be poured into the phial. About one quarter or one fifth as much as the phial will contain, will be sufficient. When this is done, the neck of the phial inside should be wiped dry, for the cork is to be sealed into it, and unless the glass is dry, the sealing-wax will not adhere. Then the tube is to be passed through the cork, and the cork put into its place, and the tube slipped down until the lower end reaches below the surface of the water, and nearly touches the bottom of the phial. Observe, then, at what part of the tube the cork comes, for this part is to be heated, and covered with sealing-wax, in order to seal it into the cork. It may be marked with a touch of ink from a pen, at a point just above where it issues from the cork. Then take out the cork with the tube from the phial, and slip the cork along down towards the lower end of the tube, so that you can put the wax upon the glass.

In order to cover the part of the tube, which is to pass through the cork, with sealing-wax, it must be heated; for sealing-wax will not adhere to glass, or any other smooth or hard substance, if it is cold. To heat glass requires some care. It must be heated gradually, and one part must not be made very hot, while the adjoining parts remain cold; for glass will not bear sudden changes of temperature, or a great difference of temperature in contiguous parts. Therefore, in heating the glass, you must proceed gradually. Hold the part over the flame of a lamp, but not so as to touch the flame, and move it backward and forward, so as to warm a portion of one or two inches in length, equally. Then you can hold it more steadily, in such a manner as to heat the central portion. As you do this, hold a stick of sealing-wax, so as to touch the hottest part of the glass with it occasionally, that is, the part immediately below the ink mark. When the glass is hot enough to melt the sealing-wax, the glass will coat itself with the wax. After holding it a moment over the flame, turning it round and round, so as to melt all parts of the wax equally, the cork is to be slipped back again over it into its place, where it will become firmly fixed, as the work cools. Thus the tube will be cemented into the cork.

3. Nothing now remains but to cement the cork into the neck of the phial. The cork ought to be of such a size, that it will go well down into the neck of the phial, so as to have the top of it a little below the upper part of the neck. For the whole of the upper part of the cork ought to be covered with sealing wax, in order to make it air tight, and this can be best done if the glass rises a little above the top of the cork. If necessary, therefore, the upper part of the cork must be carefully removed with a penknife, and then, when it is properly fitted, the sides may be covered with sealing wax, by heating the wax in the lamp and rubbing it on all around. When it is covered with a thin coat of wax, it should be held over the lamp a moment, turning it round and round, until it is melted in every part; and the neck of the phial should be heated in the same gradual and cautious manner recommended in the case of the tube. When both are of the proper temperature, the cork must be pressed down into its place. Before the wax cools, see that the bottom of the tube does not quite touch the bottom of the phial, and observe also that the tube stands perpendicular. If it does not, it may be gently pressed to one side or the other, as may be required, and held so until the wax has cooled, when it will retain its position. The top of the cork must then be covered with sealing wax, and the surface smoothed by holding it over a lamp until its inequalities run together. Thus the cork will be cemented into the phial, and the air thermometer completed, with the exception of the scale. And the following experiments can be performed with it:—

Exp. 1. On examining the instrument, it will be observed that there is a portion of air closely confined in the upper part of the phial. It cannot escape up the tube, for the water covers the lower end of the tube. If now the instrument is put into a warm place, so as to expand this body of air within the phial, the force of the expansion will press against the water, and cause a portion of it to rise in the tube. When so much water has thus ascended as is necessary to allow such a degree of additional space within, as will enable the expansive force of the air within exactly to balance the pressure in the top of the tube from without, the water will remain at rest. If now the air is warmed still more, the expansion will cause the water to rise still higher, until the two forces are again in equilibrium.

Exp. 2. When the water has been forced into the tube as high as it will rise under the greatest heat to which it is safe to expose it, it may be carried again into a cool place. The heat which was in the air and the glass will now pass off, and the air within will lose some of its expansive force, and will evince a tendency to return to its former dimensions. This will be shown by the subsidence of the water in the tube. Thus by carrying the instrument successively into warm and cold places, the surface of the water in the tube will be found to rise and fall, thus indicating, by the level at which it stands, the temperature of the air around it, at its several places of exposure. A scale for this thermometer may be formed of pasteboard, and fastened to the tube by threads or slits in the pasteboard, or, in any other convenient manner.

Exp. 3. When the instrument is cooled, the water in the tube does not simply fall by its own weight. It is forced down by the pressure of the outward atmosphere. For although the expansive force of the air within is diminished by the cold, there is still force left, far more than sufficient to counteract the weight of the water. So that the water descends, not by its own weight, but by the pressure of the atmosphere without, acting upon the surface of the water in the tube. This may be proved in the following manner. Raise the water in the tube as high as possible, by placing the instrument before the fire, and then stop the upper end of the tube with the thumb, or a little hot sealing-wax. Now, if the thermometer be taken to a cool place, it will be found that the water will not fall. The pressure from above in the tube being taken off the water is kept up by the expansive force which still remains in the air within. When the stopper is removed from the tube, so as to allow the external atmosphere to press upon the water again, it will immediately subside.

Exp. 4. Whatever may have been the temperature of the room where the thermometer was made, the water in the tube will be, when at that temperature, just level with the water in the phial; and of course, when it begins to rise, it will be some little time before it gets up above the neck of the phial. Now as it can be seen better above the neck of the phial than below, it is convenient to have the instrument so adjusted, as to have the surface of the water in the tube always kept above. This can be effected by forcing a little more air into the phial, thus increasing the expansive force within. A few bubbles of air may be blown in with the breath, by applying the mouth to the top of the tube. This will add to the force within, so that, even when the thermometer is cold, the water will stand in the tube above the neck of the phial, and all the changes that take place will be above that level, where they can be easily seen.

Exp. 5. Place the mouth at the top of the tube, and blow down into it as long and as hard as you can. By this means you will force air in, until the expansive force within is increased so much that you cannot any longer overcome it. Then, before taking the mouth away, stop the upper part of the tube with the thumb. You will now have so great a quantity of air within, that it will probably have force enough to raise the water higher than the top of the tube. If so, on taking away the thumb the water will spout out at the top, in a jet,—doing no harm, however, except to sprinkle the spectators. After a few such experiments, you will find that the water has spouted itself all out, and you will be sadly puzzled to know how to get more in without taking out the cork. There is a very easy way, if you only had philosophical knowledge enough to discover it.

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